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Currently, design of tall wood buildings is generally accomplished in the USA through the so-called alternate means process, with requires extensive testing, engineering analysis, and a stringent peer review process. As it pertains to cross-laminated timber (CLT), it is critical to develop effective performance prediction models, through laboratory testing elaborating on material behaviors (e.g. hygrothermal, vibrational, etc.) as well as monitoring data on the mid- to long-term performance of timber structures in situ. This paper presents the scope and preliminary outcomes of a project aiming to cross reference laboratory research and in-situ monitoring to establish a holistic performance-monitoring protocol for mass timber buildings; this protocol can later serve to define standards for mid- to long-term monitoring as well as to develop guidelines for the design of mass timber structures.
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41rpee | Série III | n.º 7 | julho de 2018
How monitoring CLT buildings can remove market barriers
and support designers in North America: an introduction
to preliminary environmental studies
Como a monitorização de edifícios CLT pode remover barreiras de mercado e apoiar
o seu dimensionamento na América do Norte: resultados preliminares de um
estudo de impacto ambiental
Evan L. Schmidt
Mariapaola Riggio
Paul F. Laleicke
Andre R. Barbosa
Kevin van den Wymelenberg
Resumo
Nos Estados Unidos da América, bem como em muitos outros países
do globo, uma vez que não existem regras de dimensionamento
regulamentadas que permitam o dimensionamento de edifícios
altos em madeira, o processo de dimensionamento é realizado
com auxilio a ensaios em laboratório, modelos computacionais
avançados, e um rigoroso processo de revisão de projeto por peritos
externos. No que diz respeito a edifícios que usem “Cross-laminated
Timber” (CLT), para além dos ensaios laboratoriais, ainda existe um
numero limitado de edifícios monitorizados in-situ que permitam a
caracterização do comportamento higrotérmico e vibracional deste
tipo de edifícios. Este artigo apresenta resultados preliminares de
um projeto de investigação que visa cruzar ensaios laboratoriais com
dados recolhidos num programa de monitorização in-situ de médio
e longo prazo, a fim de estabelecer um protocolo de monitorização
do desempenho de edifícios altos em madeira e apresentar diretrizes
para o projeto no futuro.
Abstract
Currently, design of tall wood buildings is generally accomplished
in the USA through the so-called alternate means process, with
requires extensive testing, engineering analysis, and a stringent
peer review process. As it pertains to cross-laminated timber (CLT),
it is critical to develop effective performance prediction models,
through laboratory testing elaborating on material behaviors (e.g.
hygrothermal, vibrational, etc.) as well as monitoring data on the
mid- to long-term performance of timber structures in situ. This
paper presents the scope and preliminary outcomes of a project
aiming to cross reference laboratory research and in-situ monitoring
to establish a holistic performance-monitoring protocol for mass
timber buildings; this protocol can later serve to define standards for
mid- to long-term monitoring as well as to develop guidelines for
the design of mass timber structures.
Palavras-chave: Monitoramento / Comportamento higrotérmico / CLT / Relação
madeira-água
Keywords: CLT / Cross laminated timber / Hygrothermal performance / Monitoring
/ Structural health / Wood-water relationship
42
How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
Aviso legal
As opiniões manifestadas na Revista Portuguesa de Engenharia de
Estruturas são da exclusiva responsabilidade dos seus autores.
Legal notice
The views expressed in the Portuguese Journal of Structural Engineering
are the sole responsibility of the authors.
SCHMIDT, E. L. [et al.] How monitoring CLT buildings can
remove market barriers and support designers in North America:
an introduction to preliminary environmental studies. Revista
Portuguesa de Engenharia de Estruturas. Ed. LNEC. Série III. n.º 7.
ISSN 2183-8488. (julho 2018) 41-48.
Evan L. Schmidt
MS Student, Department of Wood Science & Engineering
Oregon State University, Corvallis, OR - USA
evan.schmidt@oregonstate.edu
Mariapaola Riggio
Assistant Professor, Department of Wood Science & Engineering
Oregon State University, Corvallis, OR - USA
mariapaola.riggio@oregonstate.edu
Paul F. Laleicke
Assistant Professor and Wood Products Extension Specialist
Wood Products Extension, Department of Forest Biomaterials
North Carolina State University
frederik.laleicke@ncsu.edu
Andre R. Barbosa
Assistant Professor, School of Civil and Construction Engineering
Oregon State University, Corvallis, OR - USA
andre.barbosa@oregonstate.edu
Kevin van den Wymelenberg
Associate Professor, Department of Architecture, Director Energy
Studies in Buildings Laboratory
University of Oregon, Eugene, OR - USA
kevinvdw@uoregon.edu
1 Introduction
Engineered wood products are increasingly incorporated as
structural elements into mid- and high- rise construction in
Europe and North America as incentives and initiatives align with
technology and awareness. Specifically, cross-laminated timber
(CLT) has gained traction over the last few decades, primarily in
Europe, as its use in wall, floor, and roof assemblies has allowed for
the scale and size of mass timber buildings to increase. In North
America, Federal initiatives and incentives are emerging to support
research for the use of CLT and mass timber products, but tall wood
building construction is still inhibited by a general lack of awareness,
understanding, acceptance, and coherent incorporation of design
standards into the building code.
Currently a body of mixed research is emerging on CLT and mass
timber performance that is elucidating important design parameters,
including those pertaining to engineering mechanics, connection
and fastener behavior, moisture adsorption/desorption, fire, and
vibration performance [e.g. 1, 2]. Valuable information that informs
design standards and practices is gained both from laboratory
testing of materials and systems, as well as from measuring as-
built performance of structures. These two forms of analysis are
complementary, as controlled experimentation forms the basis for
element analysis and modeling, while in situ analysis provides data
on the actual performance of these elements and systems within
the context of a complex global structure and relative environment
over time. Due to the complexity of building systems at the global
scale, and the dynamic nature of behavior of wood in situ/over time,
further development of research at the building scale is necessary
to complement and augment laboratory research. Structural health
monitoring, via continuous sensor output, can efficiently give reliable
real-time performance data on various engineering metrics in timber
structures, while simultaneously allowing for a more comprehensive
assessment of various parameters and their interactions studied at
the laboratory level. The acquisition of mid- to long-range data sets
can serve to directly validate design assumptions or give important
cues as to why assumptions are violated. In addition to providing
research-oriented data to support design standard development
and numerical model optimization, continuous monitoring has
pragmatic maintenance, service, and rating applications. Structural
health monitoring can also contribute to the safety and service
life of a building by serving as an early indicator for dangerous
service conditions such as localized high moisture contents. In situ
inspections and maintenance efforts can thus be coordinated with
performance values and early warning indicators.
Recently, research and educational institutes have initiated
programs to promote the use of innovative and sustainable timber
structures, which include the construction of new facilities made
of mass timber and the development of research programs for
monitoring the structural performance and the indoor climatic
conditions of these buildings. Among these monitored structures,
we can cite the extension of the ESB – École Supérieure du Bois, in
Nantes [3,4], the House of Natural Resources at the Swiss Federal
Institute of Technology in Zurich (ETHZ), Switzerland [5], the Wood
Innovation and Design Centre at the University of Northern British
Columbia (UNBC), Prince George, Canada [6], the Brock commons
43
How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
at the University of British Columbia (UBC) in Vancouver, Canada
[7], and the Arts and Media building in Nelson, New Zealand (NZ)
[8]. In all cited cases, the monitoring program plays a central role
in promoting the constructive systems adopted and aims to analyze
these systems for their effectiveness in the mid- to long-term.
Construction and in-service risks of mass timber buildings include
water events, which can severely affect durability and serviceability
performance of the timber systems [9, 10]. Thus, in all monitoring
plans of mass timber buildings, control of moisture content (MC) in
CLT panels and other structural timber elements is mandatory.
Fortunately, modern sensor technology increasingly allows for
efficient and reliable quantification and correlation of environmental
conditions and wood MC over time and subsequently for correlation
to other critical design parameters [11].
To date, only a small number of research projects have been
conducted directly on CLT samples or in situ, as it pertains to wetting/
drying potential, stability, and crack formation [e.g. 12]. In addition
to elaborating on these important material properties, laboratory
work can help develop and implement strategies for collecting rich
data derivable from monitoring CLT in situ.
In this paper, we present results of a preliminary laboratory
campaign, finalized to define a strategy for the control of MC-related
parameters on site.
2 The Smart-CLT project
The SMART-CLT research project, whose formal title is “Structural
Health Monitoring and Post-Occupancy Performance of Mass Timber
Buildings”, aims to measure various performance indicators of CLT
assemblies, both within a controlled laboratory setting and within
selected case-study buildings. Through measurement of structural
efficiency and serviceability, durability and maintainability, and
thermal performance, the goal is to identify the interdependence of
various indicators in an effort to generate monitoring protocols for
this building type, and ultimately define performance standards for
the CLT systems. Using physical sensor measurement of vibration,
moisture content, ambient and material temperature, relative
humidity (RH), air velocity, and thermal resistance, the project aims
to collect significant performance data and use these to track design
outcomes and define principles for future design iterations.
The following sections describe a preliminary laboratory activity,
whose aim is twofold: (1) begin collecting observational data
on various moisture-related performance parameters of CLT
(adsorption/desorption, stability, checking), and (2) define a
methodology for onsite monitoring.
2.1 Materials and methods
Accelerated weathering tests were carried out to evaluate the
hygrothermal performance of CLT panels. To this end, the Multi-
Chamber Modular Environmental Conditioning (MCMEC) System
at the Green Building Materials Lab, Oregon State University, was
used. The MCMEC consists of three (3) separate chambers, which
can be set to individual environmental conditions. The temperature
range is –30 to 40°C (-22 to 104°F) and the relative humidity range
is defined by -20°C dew point and up to 95%. A mobile spray rack
can be positioned in each of the chambers to simulate rain at a
spray rate of up to 5 liters per minute. A two (2) kilowatt metal-
halide lamp solar array can be used to simulate sun exposure up to
1200 W/m2.
In this study, the samples were exposed to two (2) wetting/drying
cycles over the course of 52 days, as described in Figure 1. The first
cycle consisted of: two (2) days spray-wetting, at 95% RH followed
by two days no-wetting at 95% RH and finally by thirteen days
dry at 30% RH. Spray-wetting consisted of two overhead emitters
spraying at a rate of (2.2 L/min) for two (2) hours at a time, four
(4) times a day. The second cycle consisted of two (2) days wetting
at 95% RH followed by seventeen days (17) dry at 30% RH and
another fourteen (14) days dry at 45/65% RH. The temperature was
kept constant at 20°C for the two cycles, and slightly lowered (18°C)
during the last fourteen (14) days of the second cycle.
The test material described in this paper consisted of two CLT
specimen types (sealed and unsealed) made of five (5) plies of
mixed-species woods (Pseudotsuga menziesii, Abies concolor, Pinus
ponderosa). These samples utilized a water and weather resistant
melamine resin (MF) adhesive. The panels, conditioned at 20°C,
60% RH were of approximately the same volume and mass
(90 cm X 30 cm X 18 cm and 20 kg). Specimen “A” was left unsealed
and specimen “B” had all its edges with exposed end-grain sealed
with putty and marine grade epoxy resin to prevent adsorption
through end-grain in order that diffusion through plies was more
clearly delineated.
Continuous material moisture data were collected during the cycles
using a resistance-type moisture monitoring device from Scanntronik
[13]. The system additionally measures and stores climate data such
as relative humidity, room temperature and material temperature
at the location of moisture measurement. Insulated electrodes
were placed towards the center of each specimen and moisture
content (MC) readings were conducted in three (3) different plies:
the bottom-most ply (PLY 1), and the two plies above it (PLY 2 and
PLY 3).
Figure 1 Climate conditions over the course of the 52-day
experiment
In addition to collecting continuous MC readings during these
cycles, the samples were removed from the chamber on a biweekly
44
How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
basis and measured for weight, and dimensional change, as well as
photo-scanned for surface cracking. While MC readings were carried
out along the entire duration of the cycles (52 days), the other
measurements were terminated at the 41st day of the test.
2.2 Results and discussion
Results related to sorption/desorption cycles and dimensional
changes consequent to the accelerated weathering tests are
presented in this section. Due to the quantity of dimensional data,
only select (and representative) surfaces were chosen for discussion.
Figures 2 and 3 below show MC curves of the unsealed and sealed
panels, respectively. MC readings in specimen B (sealed) indicate
that, in exposure conditions simulating wetting of a roof or floor
CLT panel from above (long faces), the interior plies have a very low
wetting potential. Conversely, without a sealant (specimen A) the
wetting potential is higher for the interior. Furthermore, moisture
collected from a few days wetting takes a few months to dry out
(an estimated two months at dry conditions to equilibrate to pre-
wetting levels). This signifies that proximity of moisture sources
to an edge, and edge/face ratio could strongly affect interior
wetting and drying potential. It was also found that adsorption/
desorption rates were exaggerated during the 2nd cycle, indicating
that exposure and environmental conditions can potentially affect
the behavior of this material over the short and long term, due to
different factors affecting the peculiar sorption hysteresis behavior
of this material [10].
Figure 4 above shows mass variation of the two specimens along the
first 41 days of exposure. Mass change is an indirect indicator of MC
variations in the specimens; Figure 3 confirms a more pronounced
absorption and desorption phenomenon during the second cycle.
Figures 5 and 6 below (for specimens A and B, respectively) each
display climate-dependent dimensional variations of two surfaces
as measured by 3 points along each surface (one point at each end
of the surface and one in the center). The "lower face" graph for
each specimen indicates the % change of width across the bottom
most facial surface, while the consecutive "end condition" graph for
each indicates the % change in thickness across an edge surface.
These chosen surfaces are representative of behavior for analogous
surfaces, i.e. thickness change was similar across all edge surfaces,
while width and length changes were similar across all surfaces. These
results verify that the thickness of the specimens (out of plane) was
the least dimensionally stable and exhibited an average deformation
during the 2nd cycle of about 2% in the unsealed specimen and a
more subdued value of about 0.5% for the sealed specimen. The max
change in width at the surface was close to 0.5% for both specimens,
and the change in length was on average less than 0.025%. These
results confirm in field observations of the monitored CLT floor slabs
in the Wood Innovation and Design Centre, as reported by Wang
et al. 2016 [6]. Wood, in fact, is generally stable only longitudinally
(along the grain) and has significantly higher deformation rates
across the grain. By merit of the fact that CLT is comprised of
layers of length-wise members laminated orthogonally, moisture-
dependent deformation in CLT is limited in the planar directions by
the restraining action of consecutive plies. Dimensional stability is
also increased as the cross-section increases in a wooden member
[14,15]. This is related to the restraining action of the stable core,
or “passive” zone [16] that is less prone to environmental flux (MC
variation), as confirmed by readings in the different plies in the two
specimens (Fig.2). The resultant MC “lag”, or insulatory effect of the
interior makes it difficult for CLT to gain and lose moisture deep
within [17, 18, 19], and when combined with hysteresis/desorption
behaviors [10], possibly more difficult to lose. Work by Alsayegh
indicates that moisture uptake values (A-values) through the cross
section of CLT panels are smaller than for standard lumber, due to
moisture inhibition at lamination lines [14]. This insulatory effect
means that, like for all wooden members (if not more), CLT is most
susceptible to environmental flux and resultant deformations at the
surfaces/surface plies.
.
Figure 2 Moisture curves in the bottom-most ply (PLY 1), and
the two plies above it (PLY 2 and PLY 3, respectively)
of specimen A-unsealed over the duration of 52 days.
Note: values that deviate from 20°C are not temperature-
corrected; anomalous spikes in MC can be directly
correlated to temperature spikes in Figure 1
Figure 3 Moisture curves of specimen B-sealed over the duration
of 52 days
Figure 4 above shows mass variation of the two specimens along the
first 41 days of exposure. Mass change is an indirect indicator of MC
variations in the specimens; Figure 3 confirms a more pronounced
absorption and desorption phenomenon during the second cycle.
45
How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
Figure 4 Mass of specimens over the duration of the first 41 days
Figures 5 and 6 below (for specimens A and B, respectively) each
display climate-dependent dimensional variations of two surfaces
as measured by 3 points along each surface (one point at each end
of the surface and one in the center). The "lower face" graph for
each specimen indicates the % change of width across the bottom
most facial surface, while the consecutive "end condition" graph for
each indicates the % change in thickness across an edge surface.
These chosen surfaces are representative of behavior for analogous
surfaces, i.e. thickness change was similar across all edge surfaces,
while width and length changes were similar across all surfaces. These
results verify that the thickness of the specimens (out of plane) was
the least dimensionally stable and exhibited an average deformation
during the 2nd cycle of about 2% in the unsealed specimen and a
more subdued value of about 0.5% for the sealed specimen. The max
change in width at the surface was close to 0.5% for both specimens,
and the change in length was on average less than 0.025%. These
results confirm in field observations of the monitored CLT floor slabs
in the Wood Innovation and Design Centre, as reported by Wang
et al. 2016 [6]. Wood, in fact, is generally stable only longitudinally
(along the grain) and has significantly higher deformation rates
across the grain. By merit of the fact that CLT is comprised of
layers of length-wise members laminated orthogonally, moisture-
dependent deformation in CLT is limited in the planar directions by
the restraining action of consecutive plies. Dimensional stability is
also increased as the cross-section increases in a wooden member
[14,15]. This is related to the restraining action of the stable core,
or “passive” zone [16] that is less prone to environmental flux (MC
variation), as confirmed by readings in the different plies in the two
specimens (Fig.2). The resultant MC “lag”, or insulatory effect of the
interior makes it difficult for CLT to gain and lose moisture deep
within [17, 18, 19], and when combined with hysteresis/desorption
behaviors [10], possibly more difficult to lose. Work by Alsayegh
indicates that moisture uptake values (A-values) through the cross
section of CLT panels are smaller than for standard lumber, due to
moisture inhibition at lamination lines [14]. This insulatory effect
means that, like for all wooden members (if not more), CLT is most
susceptible to environmental flux and resultant deformations at the
surfaces/surface plies.
Because moisture uptake is more pronounced in end grain and
longitudinally than in the transverse directions [10], and because
consecutive plies will moisture-dependently-deform at varying rates
relative to one another, internal stresses are generated between
plies and within dimension lumber elements. In our experiment,
adjacent plies were measured for width at the ends in the unsealed
specimen and compared: it was found that the ply containing end
grain swelled at a maximum of about 0.6% whereas the adjacent
lengthwise ply swelled at each end by only 0.1-0.3% during the same
period. By running one’s hands down the corner of the specimen,
one could feel this differential in the form of a sinusoidal pattern.
Figure 5 Specimen A % dimensional change in width and
thickness
Figure 6 Specimen B % dimensional change in width and
thickness
Figure 7 Checking, gap widening, material defect, and
delamination in edge condition Specimen A, day 42
46
How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
This stress resulted in pronounced checking in specimen A (Figure 7
above) and sheared epoxy resin in specimen B. Thus, the same
phenomenon that stabilizes CLT results in large boundary stresses
across the surface, and in two directions (two grain orientations).
This cross-lamination effect is similar to the effect of restraining
deformation at connections described in Dietsch and Tannert [11].
Over time these checking discontinuities can extend further into the
interior of the CLT: as stresses develop, relax, and cycle, a “zipper”
effect will lengthen the checks to equilibrate stresses [20].
Delamination that occurred in specimen A (e.g. Figure 7) was less
than the 5% maximum acceptable delamination of total lamination
length on sawn faces as specified in the Cyclic Delamination Test
protocol for glulam AITC, 2007 [21] – which, should be noted,
is not truly applicable in this case, as our materials and methods
deviate from the standard. Other ostensible delaminations in these
specimens are in actuality splits in the wood adjacent to bond lines,
induced by hygrothermal stresses acting perpendicular to grain.
Stress induced checking and delaminations, as well as gaps resulting
from lack of edge-gluing, manufacturing errors, and imperfections in
elements (e.g. rounded corners) (Figure 6) are potentially significant
access routes into CLT’s interior for air and water, and may worsen
over time with environmental fluctuation. McClung [19] observes
that tests on smaller CLT samples minimize understanding of the
potential for these cracks to increase water uptake into the interior,
while Lepage [17] confirms preliminarily that discontinuities and
checking do affect sorption behavior. Furthermore, Wang [18]
confirms observationally that the resultant gaps from various
manufacturing practices of CLT members allowed water to penetrate
the edges and into the interior during his experiments.
Our own observations lead us to suspect that there is a correlation
between checking and the wetting potential of CLT’s interior. This
was illustrated by the fact that the mass, dimensions and interior
ply MCs all increased more significantly for the unsealed specimen
during the second cycle than during the first, despite an identical
wetting exposure and even fewer high RH days (while the exterior
ply – ply 1 – gained a similar amount of moisture during each cycle).
Specifically, relative (comparing wetting cycles) mass increase was
more than double for each specimen (Figure 3), relative dimensional
deformation in depth, width and length at the surfaces nearly
doubled (Figures 4-5) and relative MC increase of the interior
plies more than doubled (while rate of MC change increased too)
(Figure 2). There also is the possibility that defects in lamination
were caused or exacerbated by cyclic environmental change [20]
that allowed for higher rates of diffusion between plies (this, as well
as micro-cracks in the edge sealant, could account for the higher
mass gain in specimen B during the second cycle). Because checking
is associated with moisture gradients from exposure, especially
from the amount and rate of drying (exacerbated by rapid drying)
[22] understanding exposure effects over the short and long term
are important (i.e. exposure during construction through post-
occupancy) to understanding sorption behaviors as well.
It is important to emphasize that checking can happen at any depth
within CLT [23], as swelling/shrinking can elicit checking within the
interior as well as the exterior [11]. This was possibly confirmed in
our experiment, as output from one of the sensors was suddenly
lost during the drying cycle and was regained during wetting.
This is suggestive of an internal check that developed between
electrodes as the wood shrank, and its subsequent closure as it
swelled. This phenomenon reoccurred during the second drying
cycle and a subsequent exposure to high RH. It should be noted that
this, amongst other unpredictable interior phenomena (including
anatomical anomalies such as knots) are challenges related to
monitoring CLT with resistance-based electrodes.
3 Recommendations and conclusions
Results from our preliminary experimental campaign indicate that
there are strong and interesting correlations between climate cycles,
sorption/desorption rates, mass and dimensional changes, and
checking in CLT panels. The effects will be further investigated onsite
in full-size elements and assemblies. Also, the influence of these
interrelated phenomena on other relevant performance indicators
will be studied in the frame of the SMART-CLT project, specifically
to analyze how (and if) the hygrothermal behavior can affect the
dynamic properties of CLT panels (relevant for serviceability/
vibrational performance of floors) and the thermal properties of CLT
assemblies.
Our preliminary observations and literature confirm that
environmental parameters can differently affect hygrothermal
performance of CLT panels, depending on the exposed surfaces (i.e.
end-grain, long face); the initial geometrical features of the panel
(thickness – number of plies; planar extension; presence of gaps
between dimensional lumber and in the glue lines, etc.).
It is also evident, that since the edges of CLT are the most sensitive
to climatic flux and integral to interior wetting potential (and
the resultant consequences), and at the same time are present in
the most critical places (e.g. building envelope, connections and
apertures), they require further attention, both in research and
design.
Although testing of connections and joints was not specific to our
preliminary research, their analysis associated to the analysis of the
hygrothermal behavior of the panel is a point of importance and
interest with regard to CLT. Connections are an important source
of continuity and ductility in timber structures, and a natural point
of sensitivity to deterioration due to cyclic loading (e.g. wind and
seismic) [5] and moisture trapping. Koch, for example, found that
an expansion joint of a mass-timber-element-end was prone to
moisture trapping in a study on a mass timber bridge in Cologne,
Germany, and exhibited higher than acceptable MC for serviceability
[23]. Wherever CLT will be joined to another element, and wherever
CLT will be opened with an aperture (e.g. a window or door), its
edges will be exposed, and it (and importantly its interior) will be
more vulnerable to climatic fluctuations, leaks, moisture trapping,
and the resultant risks that are associated (decay, dimensional
change, strength loss, etc.). This will be exacerbated by existing
gaps from non-edge-glued panels, imperfections in layup and the
47
How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
tendency for an untreated end to check under climatic fluctuation.
Edge treatment is a possible solution to reducing these effects and
wetting potential to the interior.
Practically speaking, flaws in manufacturing, design and construction
are inevitable and robust safety measures should be incorporated to
account for this. Structural health monitoring can offer increased
safety through continuous material observation, while a deeper
understanding of long term material behavior at the global scale
can be achieved, improving building safety and efficiency through
performance assessment and design modifications.
Acknowledgements
The SMART-CLT project is conducted through the TallWood Design
Institute and funded by the U.S. Department of Agriculture’s
Agricultural Research Service. The material presented in this
contribution is also based upon work that is supported by the
National Institute of Food and Agriculture, U.S. Department of
Agriculture, McIntire Stennis project under 1009740.
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How monitoring CLT buildings can remove market barriers and support designers in North America: an introduction to preliminary environmental studies
Evan L. Schmidt , Mariapaola Riggio, Paul F. Laleicke, Andre R. Barbosa, Kevin van den Wymelenberg
rpee | Série III | n.º 7 | julho de 2018
... We have yet to extensively study the effects of similar flooding/immersion on MTP's [10,11], but it is recognized that drying regimes must be modified to limit the potential for physical deformation. Preliminary assessments under controlled laboratory conditions as well as case studies of moisture intrusion during construction are beginning to illustrate the magnitude of moisture changes possible, but again these studies have been limited to temperate applications [12][13][14]. ...
... The absence of definitive data on the rates of moisture intrusion into MTP's and its possible effects have also limited the potential for developing appropriate standards and protocols for moisture management in timber buildings during construction [15]. Relationships between climate cycles, sorption/desorption rates, dimensional and weight changes, and checking/splitting of panels remain poorly understood as do the effects of those changes on performance and expected service life (especially in the joints and connections) [12]. Most moisture-related studies have concentrated on the risk of fungal damage, but insects, especially termites, become a much greater concern in Australia and with the exception for a few early studies in the Southeastern U.S., remain largely unexplored (Mankowski et al. 2020 AWPA proceedings). ...
... This study indicated the need for moisture-rated standard development for timber panels used in the structural industry [72]. Similar work under the project title ''SMART-CLT" focused on the structural effectiveness, serviceability, durability and maintainability of CLT panels (5 plies of mixed species including: Pseudotsuga menziesii, Abies concolor, Pinus ponderosa) during and post occupancy [12]. The study concluded that the thickness dimension of the specimen (out of plane) had the lowest dimensional stability with 2% deformation on average during the second weathering cycle. ...
Article
This paper summarises the existing literature on water ingress and egress concerns in mass timber products. The main emphasis of this review paper is to identify and present the existing gap in knowledge regarding mass timber products’ water absorption and desorption characteristics in comparison with the information available for solid wood. The paper also focuses on highlighting the need for further studies on climate effects on mass timber products, particularly in relation to hotter and more humid environments. The paper concludes that existing knowledge in this field mainly concentrates on wood species from North America and Europe under temperate conditions, while major climate differences in Australia and other similar climatic regions, could have dissimilar effects on the in-service performance of mass timber products. This review includes details of testing experiments and techniques used to model and monitor water gain in mass timber products and identifies an existing gap in unified methods for testing and sampling. The review also outlines a plan for defining water ingress and egress characteristics of mass timber products in the Australian climate. This review is a part of a larger project focusing on water ingress and egress issues in mass timber buildings in tropical and sub-tropical climates of Australia as part of the National Centre for Timber Durability and Design Life (NCTDDL) research program.
... Few studies, however, have explored the implications of moisture exposure of mass timber during construction or for accidental wetting in service e.g., [10,11]. Checking, interfacial shearing/delamination, and warpage-all observed on CLT exposed to environmental simulations within a laboratory [10][11][12] and in computer simulations [13]-are examples of moisture-induced damage that can occur as a result of heavy environmental fluctuations, particularly when involving direct exposure to liquid water. Moisture induced discontinuities, which can form already during construction, can affect various physical and mechanical properties. ...
... As an example of magnitude, Brock Commons (18 stories) has over 300 sensors installed (192 MC sensors in CLT), while Carbon12 (eight stories) has 82 MC sensors (43 in CLT). These studies are also paralleled by a growing body of environmental laboratory simulations and modeling efforts, aiming at assessing the wetting-and drying-performance of CLT in various exposure, sealant, and assembly schemes [11,12,33,[35][36][37][38][39]. Research focusing specifically on overhead wetting of larger CLT specimens [11,12,33,37,39] have varying conditions, results, and insights, but generally have one common observation: CLT, even when heavily wetted, is capable of drying to acceptable MCs (e.g., <16%) in less than a half year (and in many cases less), except where certain moisture trapping conditions prevail (e.g., addition of floor/wall assembly layers such as impermeable/semi-permeable membranes and a concrete screed, wood-on-wood connections, etc.). ...
... These studies are also paralleled by a growing body of environmental laboratory simulations and modeling efforts, aiming at assessing the wetting-and drying-performance of CLT in various exposure, sealant, and assembly schemes [11,12,33,[35][36][37][38][39]. Research focusing specifically on overhead wetting of larger CLT specimens [11,12,33,37,39] have varying conditions, results, and insights, but generally have one common observation: CLT, even when heavily wetted, is capable of drying to acceptable MCs (e.g., <16%) in less than a half year (and in many cases less), except where certain moisture trapping conditions prevail (e.g., addition of floor/wall assembly layers such as impermeable/semi-permeable membranes and a concrete screed, wood-on-wood connections, etc.). Data from the SHM study at Brock Commons-which began construction summer of 2016-showed that after one year the vast majority of floors-which were topped with a concrete screed-were in-between 16%-20% MC (averaging about 16%), and after the second year the majority of these had dried to below 16% (averaging about 13%) [32]. ...
Article
Full-text available
There are currently no standards regulating water management for mass timber elements during construction, little knowledge of impacts of moisture exposure (wetting and drying performance, dimensional stability, checking), and few precedents serving as guidelines for monitoring moisture response of mass timber. To address these gaps, a hygrothermal monitoring study was devised to track moisture performance of U.S. made cross laminated timber (CLT) and glulam at a three-story mass timber building. This paper discusses moisture measurements that were collected during the first six months of construction at a CLT rocking shear wall and a timber floor connection. Despite the limited number of structural systems monitored during construction, the distribution and number of sensors in these elements allow to draw some important conclusions. The data confirmed that moisture distribution and wetting/drying rates varied based on local conditions and details (aspect, coatings, connections, etc.), with measurements at an uncoated, north-facing area showing the highest moisture levels (reaching fiber saturation at multiple ply depths and locations). Most locations rarely exceeded 16% moisture content for more than a few months. Certain moisture-trapping details consistently showed higher moisture levels (i.e., above 16%) and poorer drying. Some interior plies continued to show slow increases in MC even after months of drying conditions. These observations suggest preventative approaches implementable in the design (e.g., avoiding moisture trapping details), during fabrication (e.g., localized coating), and construction (e.g., sequencing installation to minimize exposure and allow drying).
... McClung et al. [19], Wang [20], Lepage et al. [21], Kordziel [22], and Schmidt et al. [23,24], conducted laboratory scale experiments emphasizing monitoring of wetting and drying behaviors of large CLT specimens. McClung et al. [19] found that 5-ply CLT specimens wetted by submersion for one week with epoxy-sealed edges and subsequently built into assemblies of varying permeability were generally within acceptable MC ranges (< 26%) after one month and were mostly dry after 4-6 months. ...
... In addition, Kordziel monitored a CLT roof under construction and observed that impermeable roof membranes could strongly retard drying, with some locations remaining above 20% MC for over a year. Schmidt et al. [23] compared dimensional stability, checking, mass, and MC gradients of an edge sealed and non-edge-sealed CLT sample during wetting, finding that edges and gaps played a strong role in the general hygrothermal behavior. These studies [19][20][21][22][23] generally indicated that CLT is capable of drying to safe levels after a few weeks or less of wetting exposure, but is susceptible to long periods of moisture stagnation under certain moisture trapping conditions. ...
... Schmidt et al. [23] compared dimensional stability, checking, mass, and MC gradients of an edge sealed and non-edge-sealed CLT sample during wetting, finding that edges and gaps played a strong role in the general hygrothermal behavior. These studies [19][20][21][22][23] generally indicated that CLT is capable of drying to safe levels after a few weeks or less of wetting exposure, but is susceptible to long periods of moisture stagnation under certain moisture trapping conditions. The variety of conditions that affect hygrothermal performance (e.g., weather patterns, detailing and material quality) makes it critical to conduct additional tests to better understand moisture behavior of CLT. ...
... In addition, timber structures represent a growing market for different types of contemporary construction, i.e. large-span structures and multistorey buildings. The wood construction industry is advancing in response to increased performance demands (Schmidt, Riggio, Laleicke, Barbosa, & van den Wymelenberg, 2018). Therefore, it is important to characterize the as-built behaviour of timber structures and provide benchmark data for future designs. ...
... As a result, more new timber structures have been monitored to respond to increased performance demands and to provide data for alternate design paths (Schmidt et al., 2018). Twenty-two projects in this survey are mass timber buildings (either engineered timber frame or panelized construction) featuring different types of EWPs. ...
Article
In recent years, multiple historic and contemporary timber buildings have been instrumented with sensors to monitor the performance of wood products and novel engineering systems. This paper presents the results of a literature survey focused on timber structural health monitoring (SHM) projects. This survey was aimed at investigating how the scopes of monitoring projects reported in the literature are technically addressed and who are the primary users of these data. The main contribution of this study is the definition of a general taxonomy to describe timber SHM projects, their scope, approaches and potential outcomes. This taxonomy aids readers in identifying ways of using information from SHM data. The results of this survey can be used to develop strategies allowing for data-supported decision-making for the preservation of historic buildings, the design of new structures and the service life management of built facilities.
... While testing specific material parameters under defined artificial circumstances is important to gain inputs for element analysis and modeling, additional monitoring can provide data on the actual performance within a complex environment over time. Therefore, taking into account the dynamics of natural wood behavior and the complexity of building systems, further development of in-situ research at the building level is required [1][2][3]. By collecting extended sets of in-situ measurements regarding the interaction of indoor air quality, hygrothermal material parameters, user comfort and energy consumption, this monitoring project aims to contribute to a broad spectrum of topics in timber construction. ...
Conference Paper
Full-text available
The use of solid timber for building construction is regulated - amongst others - by various standards regarding the physical properties of wood. In this long-term monitoring project of an office building in Austria, the relationship between the currently required theoretical target for heat transmission based on national code and the actual state of the construction is derived. The transient hygrothermal behavior of its exterior walls as well as of the different space-enclosing surfaces made of cross laminated timber (CLT) is being monitored and analyzed on the basis of everyday conditions. The focus of this paper is on the transmission heat loss through the nine-layer CLT construction which does not have any additional insulation layers. The results after one year of monitoring indicate a measured U-value that is lower than the one calculated according to current code. Particular attention was paid to the selection of the data in order to achieve the most accurate results possible. This shows the importance of sensor positioning, monitoring of moisture content and fairly long, undisturbed test periods with significant temperature differences between inside and outside. This paper therefore serves two purposes: It supports the proposal for a reduction in the current coefficient of thermal conductivity for CLT, and provides helpful information for future monitoring projects in order to further examine our findings.
... As noted in the previous section, adequate protection strategies are needed to enhance MT structures' performance in service, especially in areas with high moisture exposure and degradation potential [23,51]. The sensitivity of MT connections to deterioration due to moisture ingress and cyclic loading has also been reported by the same authors. ...
Article
Mass timber (MT), a group of large engineered structural wooden panels such as cross-laminated timber (CLT), glue-laminated timber (Glulam), laminated veneer lumber (LVL), etc., is becoming increasingly popular due to sustainable construction. Despite the numerous benefits of MT-based buildings, such as low-carbon emission, short construction time, and cost-effectiveness, the concerns regarding the durability of MT may limit their market acceptance. In this review, we discuss the advantages and opportunities of applying MT in tall buildings, as well as the durability issues associated with MT application. We examine the traditional wood protection techniques including, preservative treatment, thermal and chemical modification, and discuss the potential of applying these techniques for MT protection. We survey the recent studies on MT durability evaluation, as well as the recent progress in MT structure protection through a moisture control strategy. Finally, we highlight the MT protection strategies through the preservative, thermal, and chemical treatment approaches, review the effects of these treatment methods on the properties of MT such as wettability, glue penetration, bonding strength, etc., and discuss the future of the field.
... Like most mass timber products, CLT wets and dries more slowly than dimension lumber. CLT panels can be vulnerable to moisture accumulation at water-trapping interfaces such as connections (Schmidt, Riggio, Laleicke, Barbosa, & van den Wymelenberg, 2018, 2019. Details within and between assemblies are therefore crucial. ...
Article
Mass timber construction systems, such as cross-laminated timber (CLT), which originated in Europe, are gaining acceptance within North America as a viable alternative to multi-story steel-frame or concrete structures. The purpose of this study is to analyze types of innovation in the design of the building enclosure precipitated by the introduction of CLT in the North American market. Additionally, knowledge management and transfer mechanisms supporting the use of CLT in the field are investigated through the analysis of case examples. A number of factors are identified that characterize the implementation of innovation in mass timber building enclosure, among which is the interplay of structural and enclosure design in those elements at higher risk of moisture intrusion. These system innovations required concurrent design collaboration dynamics and effective knowledge management among the different project’s stakeholders and partners. This analysis can contribute to a better understanding of the current state of practice in architectural design, as well as in the mechanisms supporting knowledge management and R&D initiative in an emerging construction market.
... It will enable field testing, monitoring and sharing of data on the in-service performance of engineered wood products and advanced, earthquake-resistant timber construction systems. While the original purpose of the instrumentation was to provide data to experts and interested investors [70], the Living Lab also presents a unique opportunity to create a highly specialized and, publicly accessible learning environment. ...
Article
Full-text available
Real time data associated with the Building Information Model plays a critical role in the interpretation of the built environment, which is particularly relevant as an increasing number of education facilities and institutions promote sustainable engineering practices and monitoring data available to the public. However, it is challenging for non-technical audiences to fully comprehend or use information concealed in scientific data related to the performance of structures and materials. It is especially difficult for them to connect these concepts to physical contexts and phenomena. In this paper, we present how cross-reality paradigms in Architecture, Engineering, and Construction, coupled with multimodal representation techniques, enhance data literacy in both professionals and laypeople alike. In particular, we present the design of a learning environment where cutting-edge holographic interfaces and display technologies are combined with sonified and visual data to create a more immersive environment for data analysis and exploration, empowering users with situated data awareness and new ways of understanding real-time data.
Article
Full-text available
Cross-laminated timber (CLT) has been widely discussed as a relevant industrialized construction solution. Numerous publications have considered CLT as a structural wood-based panel, but other documents have mentioned it as a building or even a construction system. Many authors address its application in multistory buildings, although single-family houses and lower building applications have become desirable topics as well. Given these gaps, this review study addresses a systematic method to evince the functions of cross-laminated timber in construction. The elucidation and discussion were led by technical and scientific contents through publications present in scientific websites and the Google web search engine. Intricate perceptions about the knowledge and reference of CLT functions were identified. From prospections, it was possible to state that CLT is a timber-forest product created in Europe, whose function acts as a structural composite panel of the engineered wood product category. However, CLT has been mentioned by many publications as a building or a construction system. Suggestions were raised to clarify to all readers with respect to misconceptions, and elucidate the construction systems capable of using it as the main resource. Discussions evinced the characteristics and potentials of this wood product. Even with its increasing application in tall buildings, the commercial application of CLT in low-rise buildings may be boosted by the possibility of large-scale production of industrialized houses.
Article
This study examines how cross-laminated timber (CLT) constructions, including joints, connections and attachment points, are affected by precipitation during construction. The case studies are based on moisture content measurements and material sampling as well as microbiological analysis during the structure’s construction stage. The study does not include remediation control. The field measurements show microbiological growth in all buildings and almost all floor structures that were investigated. Of a total of 200 measuring points analysed, half had mould growth and around a third had moderate or extensive growth. The moisture content measurements for one of the locations with the largest percentage of elevated or high moisture content was at the top of the floor structure in the bottom gap between timbers in the CLT top layer. This is one example of several materials or construction components where there is limited possibility of dry out. Based on the outcome, it would appear difficult, or impossible, to avoid the appearance of microbial growth during construction with CLT without weather protection. Previous studies indicate that microbiological analysis of CLT is extremely rare in both laboratory and field studies, which implies that there are obvious shortcomings in the scientific work. The fact that mould growth is often invisible needs to be disseminated, especially in practical studies. However, there seems to be a good level of awareness in the literature that theoretical studies often conduct mould growth risk evaluations. There do not appear to be any moisture safety assembly methods or solutions for CLT construction that do not have weather protection or a declaration of the critical moisture conditions for CLT products.
Conference Paper
Full-text available
The hygroscopic behaviour of wood leads to changes in the physical and mechanical properties. The correct estimation of the moisture content is important for the design and life cycle of timber structures. Therefore experimental test series were performed to determine the moisture content distribution over the cross section as a function of loading duration, respecting the glue lines of glulam and block glued cross sections. Alongside, numerical simulations of the moisture diffusion process were set up and validated with the experimental test series. The correlation of the numerical simulation with the reality offers guidance in the design of timber structures.
Conference Paper
Full-text available
Durability and long-term behaviour of timber structures are strongly influenced by environmental and in-service conditions. Measuring the real in-service behaviour of a full-scale timber structure and its environment can help collecting experience to validate existing models and design assumptions. To this purpose, a long-term monitoring system has been installed on a three-floor structure composed by wooden trusses, composite timber-concrete slabs and timber-framed walls. The structure is located in Nantes, France, and it is an extension of the Wood Science and Technology Academy (ESB). The paper will show, through the available measurements, some of the phenomena that are strongly influenced by humidity variations. These analyses will lead to develop a numerical model to study the shrinkage effect on a structural element. The shrinkage deformation will be finally filtered from the global measured deformation to put into evidence the mechanical effects, such as mechanosorpive behaviour, creep effect, and damage impact, at the structural scale.
Article
Full-text available
Cross laminated timber (CLT) has become a well-known engineered timber product of global interest. The orthogonal, laminar structure allows its application as a full-size wall and floor element as well as a linear timber member, able to bear loads in- and out-of-plane. This article provides a state-of-the-art report on some selected topics related to CLT, in particular production and technology, characteristic material properties, design and connections. Making use of general information concerning the product’s development and global market, the state of knowledge is briefly outlined, including the newest findings and related references for background information. In view of ongoing global activities, a significant rise in production volume within the next decade is expected. Prerequisites for the establishment of a solid timber construction system using CLT are (1) standards comprising the product, testing and design, (2) harmonized load-bearing models for calculating CLT properties based on the properties of the base material board, enabling relatively fast use of local timber species and qualities, and (3) the development of CLT adequate connection systems for economic assembling and an increasing degree of utilization regarding the load-bearing potential of CLT elements in the joints. The establishment of a worldwide harmonized package of standards is recommended as this would broaden the fields of application for timber engineering and strengthen CLT in competition with solid-mineral based building materials.
Chapter
Full-text available
Cross-laminated timber (CLT) was developed in Europe for the prefabricated construction of wall, roof, and flooring elements. Adaptation of CLT for use in the United States requires consideration of the different climates, building codes, and construction methods in this country. Building enclosure design has important implications for the energy performance and durability of the structure as well as indoor air quality. The key performance requirements of the enclosure discussed in this Chapter are prevention of water intrusion and control of heat flow, air flow, and moisture flow. The use of prefabricated CLT panels does not change the basic heat, air, and moisture control design principles for an exterior wall or roof assembly. However, the design of CLT assemblies requires attention to the unique characteristics of this product. CLT panels are massive solid wood elements and therefore provide some level of thermal insulation, thermal mass, and airtightness (a separate continuous air barrier system is nevertheless recommended). CLT panels have a relatively high capacity to store moisture but relatively low vapor permeability. If exposed to excessive wetting during transport, storage on the jobsite, construction, or in building service, the panels may absorb a large amount of moisture, and the subsequent drying may be slower than it is for lightweight wood-frame construction. This Chapter provides guidance on heat, air, and moisture control in wall and roof assemblies that utilize CLT panels in U.S. climate zones. The overarching strategies are to prevent wetting of CLT panels by using drained wall systems, to control airflow using an air barrier on the exterior of the CLT panels, to place rigid insulation to the exterior of the panels, to prevent moisture from accumulating within the panels, and to allow the panels to dry should they get wet. In certain climates, preservative treatment of CLT is recommended to provide additional protection against potential hazards such as decay and termites. It is intended that these guidelines should assist practitioners in adapting CLT construction to U.S. conditions and ensuring a long life for their buildings. However, these guidelines are not intended to substitute for the input of a professional building scientist. This may be required in some jurisdictions and is recommended in all areas at least until such time as CLT construction becomes common practice.
Article
Full-text available
Timber structures represent nowadays a significant part of our cultural heritage as well as an increasing market for individual buildings and large architectural and engineering projects. A lot of theoretical and experimental experiences exist at the material level, but a huge amount of parameters have to be investigated due to the singularity of wood behaviour. Therefore, some simplified behaviour models are usually proposed and validated through laboratory experiences, to be considered in design standards, but usually these studies remain at the material scale. The paper will first present and comment some experiences at the material scale to focus on those parameters having the greatest influence at the global structural level. The presence in European standards of coefficients or assumptions taking into account the described parameters will be underlined, when existent. In addition, material parameters as well as connection behaviour between structural elements and other materials have also a great impact on the global response. The gap between broadly available scientific results and the available information in European standards needs to be overcome with the integration of non-destructive testing (NDT)/monitoring techniques to measure the structural behaviour under in-service conditions and to understand how the material parameters influence the global response. Some case studies on NDT/monitoring of laboratory and in-field timber structures will be finally presented to show that data interpretation can easily become very difficult depending on the structural complexity, on the phenomenon to investigate as well as on the number of parameters to be measured.
Conference Paper
Although not yet seen as common practice, building with cross laminated timber (CLT) is gaining momentum in North America. Behind the scenes of the widely publicized project initiatives such as the Wood Innovation Design Centre Building in Canada and the recent U.S. Tall Wood Building Competition, substantial research, engineering, and development has been completed or is underway to enable the adoption of this innovative building system. This paper presents a brief overview of the current status of CLT building development in North America, highlighting some recent U.S. and Canadian research efforts related to CLT system performance, and identifies future CLT research directions based on the needs of the North American market. The majority of the research summarized herein is from a recent CLT research workshop in Madison, Wisconsin, USA, organized by the USDA Forest Products Laboratory. The opportunity and need for coordination in CLT research and development among the global timber engineering community are also highlighted in the conclusions of this paper.
Article
Glued-laminated timber (Glulam) – allowing for large span and curved elements to be produced – had revolutionized the way timber could be used in structural applications. As the importance of assessing large timber structures is growing, so is the interest of the professional community in assessment methods for existing timber structures. The performance of Glulam elements depends on the quality of the individual laminations, the quality of the finger joints, the quality of the glue-lines and the integrity of the cross-section. This paper presents and discusses feasible methods to: i) create a general overview of the structural integrity of Glulam elements; ii) assess the environmental conditions in which these are placed; iii) determine their moisture content; iv) map cracks; and v) assess the integrity of glue-lines. There are multiple methods available; each method, however, only allows assessing a certain type of property or damage. Therefore the application of just one method might not be suitable to enable confident decisions, making it necessary to combine different methods to derive a full picture about the integrity of Glulam elements. As a consequence, expert's reports treating the safety of a structure featuring Glulam oftentimes are set up from a standpoint which can be summarized as ''safe on the best knowledge we have''.