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Wood Material Science & Engineering
ISSN: 1748-0272 (Print) 1748-0280 (Online) Journal homepage: http://www.tandfonline.com/loi/swoo20
Construction management for tall CLT buildings:
From partial to total prefabrication of façade
elements
Eugenia Gasparri, Angelo Lucchini, Gabriele Mantegazza & Enrico Sergio
Mazzucchelli
To cite this article: Eugenia Gasparri, Angelo Lucchini, Gabriele Mantegazza & Enrico Sergio
Mazzucchelli (2015) Construction management for tall CLT buildings: From partial to total
prefabrication of façade elements, Wood Material Science & Engineering, 10:3, 256-275, DOI:
10.1080/17480272.2015.1075589
To link to this article: http://dx.doi.org/10.1080/17480272.2015.1075589
Published online: 11 Sep 2015.
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ORIGINAL ARTICLE
Construction management for tall CLT buildings: From partial to total
prefabrication of façade elements
EUGENIA GASPARRI, ANGELO LUCCHINI, GABRIELE MANTEGAZZA, &
ENRICO SERGIO MAZZUCCHELLI
Department of Architecture, Built environment and Construction engineering (ABC), Politecnico di Milano, Via Ponzio 31,
20133 Milan, Italy
Abstract
Cross-Laminated Timber is one of the most widely used engineered wood products, thanks to its numerous advantages,
among which construction speed is the most appreciated, both by clients and by designers. However, construction
scheduling compression refers exclusively to CLT structures, while the rest of the construction process still requires a
longer phase to complete vertical enclosures. The aim of the research work presented in this paper is to outline advantages
brought about when the degree of envelope prefabrication of tall timber buildings is increased. Results are presented in
two sections. The first includes the definition of a case study together with an overview of possible technical details for
entirely prefabricated façade solutions, ready to be installed without the need to work via scaffolds. The second deals with
construction site management analysis for the case study building, where the determination of specific factors having an
influence on time and costs is achieved by varying the prefabrication degree of the various façade configurations and
repeating the analysis process. The main findings of this research work demonstrate that comprehensive façade
prefabrication allows not only consistent compression of construction scheduling to be achieved, but also for immediate
protection of wooden elements from weather agents.
Keywords: CLT timber construction, façade prefabrication, time/cost optimization, construction site management, technical details,
case study building.
Introduction
In the last decade, timber construction has achieved
an increasingly larger consensus within the construc-
tion market panorama, striving competitively with
other building materials, such as concrete and steel,
even in the case of large-scale projects. Achievements
and more recent developments in the field of engin-
eered timber products have raised new insights and
possibilities for the design of tall buildings using
wood as the main construction material (Timmer
2011, Abrahamsen and Malo 2014). Comprehensive
studies on this issue illustrate how building at height
with wood is nowadays not only possible, thanks to its
excellent mechanical properties and new products’
structural performances, but also safe, efficient, econ-
omically and environmentally sustainable (Bryan
2012). In addition, the opportunity to speed up con-
struction time represents the main reason why clients
and designers are more and more often directing their
choices towards timber building systems (Gardino
2010). This, for instance, has been the case in two
very well-known buildings in the field of timber con-
struction: the Stadthaus in London (2009) and The
Forté building in Melbourne (2012). For both pro-
jects, Cross-Laminated Timber technology was pre-
ferred to on-site cast concrete according to a
preliminary evaluation of project development that
highlighted advantages of the CLT system in terms
of environmental aspects (CO
2
-equivalent emission
reduction) and construction time saving (see Anon-
ymous 2009,2014).
Contraction of scheduling is also one of the main
reasons why, in the timber construction field,
advanced prefabricating systems are consistently
gaining market share (Lehmann 2013,Mikkola
2014). In fact, wood is characterized by certain
(Received 15 March 2015; revised 19 July 2015; accepted 20 July 2015)
© 2015 Taylor & Francis
Correspondence: Eugenia Gasparri, Department of Architecture, Built environment and Construction engineering (ABC), Politecnico di Milano, Via Pon zio 31,
20133 Milan, Italy. Tel: +39 333 3816295, +1 778 984 0833. E-mail: eugenia.gasparri@polimi.it
Wood Material Science & Engineering, 2015
Vol. 10, No. 3, 256–275, http://dx.doi.org/10.1080/17480272.2015.1075589
intrinsic features, such as low weight and simple
manufacturing, which make it suitable for the
realization of prefabricated components for the
construction industry (Smith 2014). Moreover,
computer-aided design and production applied to
new timber technologies allow even higher quality
standards to be reached and enhance possibilities
for product customization, according to the
specific needs of individual buildings (see Figure
1(a) and (b)) (Staib et al.2008, see Research
Report 2009).
Prefabrication of envelope components for timber
buildings is a very debated and constantly evolving
field of research, which attracts interest from mani-
fold sectors of expertise (Lehmann 2013, Mikkola
2014, see Research Report. 2011). However, pub-
lished studies in this field concern mainly the use of
timber frame technologies, light weight and inexpen-
sive, usually applied to low or mid-rise constructions
(Kapfinger and Kaufmann). Other timber technol-
ogies have not been the object of detailed research
on this issue so far.
CLT, for instance, has gradually been spreading
across international markets across the world,
thanks to the potential it offers from very different
points of view, as an example structural performance,
seismic behaviour, sustainability rate and so on
(Zumbrunnen and Fovargue 2012, Laguarda Mallo
and Espinoza 2014). Construction speed is one of
the principal “sources of pride”of this technology,
nevertheless, state of the art in the CLT construction
system presents rather unbalanced work phases. The
completion of façades through outer layers installa-
tion from scaffolds requires much longer time than
structure construction. In most cases, timber struc-
ture is exposed to weathering agents during this
period. Thus, the extremely systematized
construction of the building’s structural skeleton is
followed by a much more traditional construction
phase for façade completion from the outside,
causing consistent disadvantages in terms of site
management. Moreover, on the basis of past con-
struction site experience, it is possible to affirm that
working teams dedicated to scaffolds installation
often do not cope well with the consistently more effi-
cient CLT structural panels installation, causing time
slippage or logistic interferences (Presutti and Evan-
gelista 2014), as shown in Figure 2 (a) and (b).
The research work presented in this paper aims to
investigate advantages brought about by the increase
of envelope prefabrication degree for tall timber
buildings, using Cross-Laminated Timber as struc-
tural system (Falk 2013). It is meant to be a prepara-
tory study for a wider research project focused on the
development of fully prefabricated façade systems
based on CLT technology.
As a general rule, it can be stated that the design
effort needed from the start of product, component
or building conception is higher when the prefabrica-
tion level increases (Sarja 1998,Lessing2006,
Smith 2015). So, it is fundamental to examine the
convenience, in terms of both time and costs, at the
earliest possible phase of the design process and deter-
mine the most appropriate approach for an efficient
product or project development. A construction
management analysis is carried out through a case
study building, to give evidence of the hypothesized
benefits and provide thorough results from both a
qualitative and a quantitative point of view. Despite
output from calculation is related to the analysed
case, conclusive results offer interesting starting
points for further research development and provide
the order of magnitude of a system convenience over
the other.
Figure 1. (a) Manufacturing phases of prefabricated timber frame housing elements at the production site. (b) Storage of prefabricated timber
frame walls ready for the transportation phase.
Construction management for tall CLT buildings 257
Preliminary considerations
The first step carried out in order to carefully guide
work development has been a qualitative analysis of
the main advantages that the prefabrication of façade
elements brings about within a CLT
building construction process. The shifting of a large
amount of manufacturing operations from the con-
struction to the production site not only guarantees a
higher matching between design and product, but
contributes to enhance construction process quality
and control (Johnsson and Sardén; Velamati 2012).
Off-site prefabrication of large parts of a building,
including all necessary layers needed to ensure
timber protection during service life, certainly rep-
resents a consistent advantage for the risk of weather
agents exposure during the construction phase as
well. This is certainly another important issue that
needs to be carefully addressed within the design
phase, in order to prevent wooden products from
being afflicted by future mould problems, due to
incorrect humidity levels (AITC 111-2005). Changes
in the moisture content also cause wood swelling
and shrinkage, which represent a particularly relevant
concern for tall wood buildings (FP Innovations
2013). Furthermore, most of the weather protection
systems commonly used in the construction field are
insufficient to guarantee adequate shelter and protec-
tion to building components. Should temporary
roofing structures be applied, their higher efficiency
is counterbalanced by high realization times and
costs (Serrano 2009, Mahlum 2014). One example
of such structure is shown in Figure 3.
Figure 2. (a) Aerial view of a timber structure with scaffolds (Eng. Presutti). (b) CLT panel positioning through crane (Eng. Presutti).
Figure 3. Temporary protective roofing at the Limnologen Vaxjio construction site, Sweden.
258 E. Gasparri et al.
Table I includes a comprehensive list of the main
advantages related to façade prefabrication and the
absence of scaffolds, according to four different
topics: quality, time, cost and risk.
Case study definition
A case study building has been developed in order to
verify the feasibility of the research project based on a
dimensional model. The design phase has been
characterized by the research of the maximum level
of simplicity and reproducibility, in order to obtain
results that could be generalized or extended to
other project having similar characteristics.
The layout of the building has been determined
through the “universal floor plan”(see Figure 4)
method by using a design formula (Timmer 2011)
based on the definition of the following data:
.the shape of the floor plan (w×w) of the case
study building, considering that rectangular
and square shapes are common shapes for
tower buildings;
.the core dimension (c), designed according to
Italian fire regulation (see DECRETO MINIS-
TERIALE n. 246/97) for residential usebuilding;
.the floor span (d), predominately defined
according to daylighting requirements;
.the gross-net floor ratio (R
f
).
In particular, considering a floor span fixed equal
to 7.10 m, the main plan dimension of the case
study building can be easily calculated. So, the
square plan results in a side length of 23.6 m and ver-
tical connections (stairs and elevators) with a square
footprint of 9.50 m each side. To facilitate the
evaluation processes, every floor is characterized by
the same layout (see Figure 5(a) and (b)).
Italian regulations and standards have been used to
define architectural requirements, such as room sizes
and collocation, windows sizes and number, storey
height and so on. The latter is 3.40 m, resulting
from a height of 3.20 m between two consecutive
floor slabs plus 20 cm of the floor slab thickness.
Panels’main dimensions have been determined in
order to minimize material waste, according to
Stora Enso/Austria production line. The building is
nine storeys high: the ground floor is 4 m high and
is the only storey that differs to the others, as it is
Table I. Comparison of the main advantages related to façade prefabrication and construction without use of scaffolds.
Façade prefabrication Construction without scaffolds
Quality •Less storage areas (a reduced footprint could be a
relevant issue in urban areas)
•Less work teams and construction site fluxes
(materials, machinery and workers) to be coordinated
•Waste minimization (e.g. scraps reduction)
•No storage space needed neither work teams to be coordinated
Time •Higher installation efficiency within the production site
•Time saved for storage procedures (e.g. insulation,
cladding, etc.)
•Time saved within the design phase, as scaffold design and safety
measures are often required for tall or complex buildings
•Time saved for storage, installation and dismantling procedures
Cost •Reduced cost of risks
•Very short return of investment cost (m
2
available for
sale in record time)
•Cost saving for design effort
•Cost saving for labour and rent (transportation, installation and
dismantling)
Risk •Major safety for workers due to manufacturing in the
production workshop
•Minimization of weather variability related risks
•Reduction of time slippage risk due to materials supply
problems
•Lower probability of mistakes during work execution
•Less contractors to cope with
•Major safety for workers due to the consistent reduction of work on
height
•Reduction of time slippage risk due to lack of coordination among
scaffold and CLT installer
Figure 4. Universal Floor Plan scheme.
Construction management for tall CLT buildings 259
assumed to be made of reinforced concrete. The
building height is equal to 34.80 m.
Since structural analysis is not within the scope of
the present work, a simplified approach has been
adopted to define horizontal and vertical structural
component sizes. However, structural design has
been necessary to evaluate volume and weight of
each considered panel, in order to be able to make
practical considerations about site logistics and trans-
portation systems, as shown in the following. Table II
summarizes thickness values for both horizontal and
vertical structural elements of the case study.
Façade options definition
This paragraph includes an overview of the façade
options to be analysed, as far as the definition of the
executive size and the number of panels the façade
is made of. This classification has been carried out
to define specific boundary conditions and provide
all data needed to perform the quantitative analysis
of the construction process.
It is important to note how such an elaboration is a
common characteristic for every project involving
CLT technology, as the production process is totally
mechanized. Size definition criteria and final outcome
are illustrated in this work to allow easier understanding
of the adopted evaluation method, but might differ
from case to case. In fact, they have an influence on
thefinaloutcomeonlyunderan“absolute”point of
view, while relative results are supposed to have the
same percentage relevance for both options A and B.
As a design assumption, the platform framing
approach has been used for the case study develop-
ment, thus external walls are intended to be posi-
tioned in between floors for the analytical process.
Option A –nonprefabricated wall elements. This is
intended to consistently represent the current state
of the art with respect to CLT constructions. More
specifically, CLT panels are transferred to their
final position directly from the truck, through the
help of a crane. All other layers of the cross-section
are installed on-site, working from scaffolds. Two
different cases will be detailed for this first option.
Large-size Panelling (A-LP): it helps to push con-
struction speed and save on costs for vertical connec-
tions between consequent panels. It consists of six
different panels, as shown in Figure 6. Just one
front of the case study building, which serves as a
sample application of the proposed method, is
reported here.
Small-size Panelling (A-SP): it has never been
comprehensively investigated, according to the
reviewed literature. Clearly, a number of small
Figure 5. (a) Typical floor layout. (b) Typical floor structural layout.
Table II. Collection of horizontal and vertical structural elements
thicknesses for the case study building.
Storey Floor slab elements Wall elements
0–ground floor Concrete Concrete
1 200 mm 180 mm
2, 3, 4 200 mm 160 mm
4, 6, 7 200 mm 140 mm
8, 9 200 mm 120 mm
260 E. Gasparri et al.
panels are sometimes necessary even in the previous
case. For this option, anyway, small panels are
designed according to the decomposition of the
façade in modular elements, as it happens for
instance in the case of glazed unitized façades. Parti-
tioning large panels in smaller sub-elements can
provide the designer and the system with a higher
architectural freedom. Moreover, this option can be
favourable in specific contexts, due to transportation
and handling advantages provided.
The Small-size Panelling option consists of 12
different wall elements, as shown in Figure 7. Every
panel, except for M and N, have the same size, that
is, to say, 2.95×3.20 m. Most of them are different
from the standard element because of window size
and position, but this aspect does not imply any
waste of resources because of the production line
characteristics.
Option B –entirely prefabricated wall elements. This
option refers to the solution proposed in this work,
where external wall elements are completely preas-
sembled off-site, as shown in Figure 8(a) and (b).
In this case, they arrive at the construction site
loaded on trucks and ready to be installed on their
final position by workers operating from the desig-
nated floor. Façade elements are provided with insu-
lation and finishing layers, doors, windows and all of
the necessary connection predispositions. This
implies that no further works on the exterior wall
surface are needed to complete façades through scaf-
folds. Anyway, according to the specific case, it is
Figure 6. Option A/B –Large size Panelling (LP) –north/south front.
Construction management for tall CLT buildings 261
possible to install on site some vulnerable edge parts
before handling façade elements to their designated
floor.
As for the previous option, two further detail cases
have been analysed:
.Large-size Panelling (B-LP)
.Small-size Panelling (B-SP)
For both solution, the same panel size as Option A
has been taken into account, in order to achieve com-
parable results.
Table III shows the main work phases characterizing
the defined case study, in order to outline relevant
macro-differences between the two options, A and B.
Façade design
The definition of all outer wall layers needed to satisfy
performance requirements has been carried forward
before going further into the study of joint technical
solutions between façade panels. This design phase
has been developed following two main steps: func-
tional analysis of the solution and energy perform-
ance validation.
Chosen materials and layer positioning have been
established both in accordance with prescriptive
requirements identified for vertical enclosure design
and builders’experience. Hygrothermal and acoustic
behaviour analysis have been performed, according
to boundary conditions referred to the city of
Milan, Italy. The wall cross section (see Figure 9(a)
Figure 7. Option A/B –Small size Panelling (LP) –north/south front.
262 E. Gasparri et al.
and (b)), from the interior to the exterior, consists
mainly of:
.CLT structural panel
.breathable vapour barrier
.double layer of wood fibre insulation layer,
thickness equal to 6+4 cm
.breathable waterproofing membrane
.wooden vented façade
The choice of the finishing should not be considered
fixed, as many possible solutions could be further
developed. The presence of a vented façade represents
one of the best solutions in order to keep the wall dry,
guarantee the best thermal performance, allow wide
architectural flexibility and confer a pleasant aestheti-
cal result according to the studied prefabricated
system (FP Innovations 2013, Lucchini 2013).
Technical detailing. A preliminary study of the techni-
cal solution has been considered an interesting step to
go through, in order to outline the main issues to be
tackled within the design phase and, on the other
hand, what perspectives an extensive study of this
subject would be able to offer.
It is fundamental to specify that technical detailing
solution has been developed after the analysis of the
proposed system benefits, which will be illustrated
in the next paragraph. This choice was made to
avoid establishing fixed borders for the numerical
analysis. In fact, assumptions made for the compara-
tive time/cost analysis allowed to perform a lighter
and more repeatable calculation process, which can
be easily and efficiently applied to other projects,
despite a non-significant loss of precision for the
obtained values.
For the purpose of this work, 2-D horizontal and
vertical sections of a planar joint between two contig-
uous panels have been represented. In this context,
façade elements such as balconies, eaves and so on
have not been considered. The first step has been
the construction details design as far as the traditional
way to build CLT construction is concerned, that is,
to say, Option A (see Figure 10(a) and (b)). Layers
needed to complete the technical solution from the
inside have not been taken into account in time and
cost analysis, as they do not have any relevance
within the scope of this work.
The study of the newly proposed preassembled
system has followed two parallel design paths.
Table III. Work phases for the two proposed design options.
Option A Option B
a Excavation for foundations Excavation for foundations
b Reinforced concrete
foundations construction
Reinforced concrete
foundations construction
c Reinforced concrete load-
bearing walls construction
Reinforced concrete load-
bearing walls construction
d Concrete slab construction Concrete slab construction
e First and second level of
scaffolding installation
−
f Vertical load-bearing CLT
panels installation
Vertical load-bearing CLT
panels installation
g Horizontal CLT panels Horizontal CLT panels
h Repetition of e, f, g phases n
−1 times, where nis the
building storey number
Repetition of f, g phases n−1
times, where nis the
building storey number
i Thermal insulation
installation
−
l Waterproof canvas
installation
−
m External finishing installation −
N Scaffolds dismantling −
Figure 8. (a) External view of totally prefabricated façade (binderholz X-LAM BBS_1 © www.binderholz.com). (b) Internal view of totally
prefabricated façade (binderholz X-LAM BBS_1 © www.binderholz.com).
Construction management for tall CLT buildings 263
Figure 10. (a) Option A, vertical detail. (b) Option A, horizontal detail.
Figure 9. (a) Vertical cross section. (b) Horizontal cross section.
264 E. Gasparri et al.
The first one aims to mimic window functioning,
so interrupting the movement of fluids from indoor
to outdoor and vice versa. This is why this has been
defined as a “geometrical method”, as it uses
shaped wooden elements in order to prevent air/
water exchange between the indoor and outdoor
environments, as shown in Figure 11(a)–(c). This
method has been used for the evaluation of benefits
in the case of both B-SP and B-LP.
In a platform framing approach, the use of an OSB
panel positioned at the edge of the floor slab to guar-
antee a continuous contact at the wall–floor interface
has been considered a good way to stiffen, protect and
contain the thermal insulation layer. As far as the
installation sequence is concerned, it proceeds
storey after storey, as in Option A.
The second approach has been developed after
results of the process analysis had been processed. In
fact, as demonstrated in the next paragraph, the weak-
ness of the entirely prefabricated solution using small
size panels is the high number of vertical joints. This
fact not only implies an increase in construction
costs due to the higher number of fasteners, but also
to a higher air leakage risk through panel joints.
To address this issue, an implemented version of
the previously proposed solution has been studied.
The ambition is to introduce an innovative technical
system, imitating the functioning of unitized glass
façades (Rigone 2014), which guarantees air and
water tightness through the use of gaskets (see
Figure 12(a)–(c)). This allows, at the same time,
full compatibility of opaque elements with an all
height glass modular element, providing designer
with a higher freedom level as far as façade texture
design is concerned.
In this case, panel connection in the vertical section
is realized at the floor slab extrados. This design
choice makes panels less vulnerable during the load/
unload onto and from the truck handling phase.
Results and discussion
The technical solution introduced in Figure 9 has
not to be considered in any way as executive. Many
other factors are worth investigating thoroughly
before thinking about launching the proposed
system on the market, from both a performance and
an architectural point of view.
In the case of Option B, for instance, air and water
tightness through joints are fundamental aspects to be
tackled, preferably by means of a thorough laboratory
testing program. Moreover, it is necessary to face
structural issues like interaction of different com-
ponents depending on both thermal expansion and
shrinkage, or floor deformation due to wood floors
compression stress perpendicular to grain. Fire
safety is also an important issue to be fully addressed.
Concerning architectural flexibility within the
design phase, it would be extremely interesting to
investigate more in depth different possibilities
offered by the system. This would imply studying the
analysis of the briefly summarized following issues:
.building types and different uses;
.system compatibility with different kinds of
structures;
Figure 11. (a) “Geometrical method”. Broken-down vertical detail. (b) “GM”, vertical detail. (c) “GM”, horizontal detail.
Construction management for tall CLT buildings 265
.system layers diversification both for the external
wall wood technology and for the cladding
system, in order to give designers and clients
the chance to customize their projects. As an
example, should the load-bearing function no
longer exist, it could be more convenient to take
into account cheaper timber-based solutions,
such as timber frame walls or LVL panels.
The work presented by the authors makes a step
further in the timber prefabrication field, introdu-
cing a load-bearing massive system which allows
for the construction of high rise buildings. In
addition to this, it also has the aim to solve panel
joints through the sole element positioning with no
need to work from the external side of the building.
The common practice in such systems is to complete
external finishing and panel joints on site through
the use of fixed or mobile scaffolding (Kobler
2011, see Anonymous 2012). The use of finished
façade elements would avoid the presence of open
storeys on site, offering immediate protection to
CLT horizontal and vertical elements as the con-
struction rises.
Construction management analysis
The determination of all system boundaries through
the definition of the case study, together with possible
façade options, represented a propaedeutic phase
necessary to perform the construction management
analysis. Within the scope of this work, the compu-
tation process has taken into account external wall
components, discounting all other construction
elements such as floor slabs, internal walls, building
services and systems. For these reasons, final results
are not to be considered representative for the entire
construction process of the defined case study. They
intend to only provide relative results by quantifying
the convenience, in terms of time and costs, brought
about by the proposed façade solutions.
Work Breakdown Structure
As a first step, the Work Breakdown Structure (WBS)
has been developed in order to correctly organize the
evaluation process and, at the same time, to identify
every component to take into account within both the
bill of quantities and the length of works calculation.
The splitting of the external wall element into its
sub-components has been carried out according to
the Italian voluntary standard UNI 8290-1, which
provides the following classification rank:
.classes of technological units (e.g. structure)
.technological units (e.g. elevation structure)
.classes of technical elements (e.g. vertical
elevation structure)
.technical elements
Categorization for CLT technology follows a
slightly different approach when compared with
other construction systems. In fact, CLT panels
have a load-bearing function and, at the same time,
they are also part of the building envelope (together
with other components for each functional layer),
Figure 12. (a) “Technological method”. Broken-down vertical detail. (b) “TM”, vertical detail. (c) “TM”, horizontal detail.
266 E. Gasparri et al.
thus belonging to two different classes of technologi-
cal units for both options A and B (see Table IV).
Differences between the two options become
evident from classes of technical elements breakdown
level because, in the totally prefabricated cases of
Option B –LP/SP, windows have not been separately
classified but included within the external wall
category.
This fact needs to be acknowledged from the very
beginning, as it represents the starting point to go
through the analysis process. Hereafter, if in the
case of the non-prefabricated solution every different
material is taken into account individually in order to
evaluate its installation cost directly from quantities,
the same approach could not be followed for the pre-
fabricated systems as far as the totally preassembled
solution is concerned, given that the external wall
arrives at the construction site ready to be installed
in “one-shot”. This aspect will be further illustrated
within the following paragraphs.
Cost analysis
The realization of a detailed WBS served as a basis for
the bill of quantities elaboration.
Data involved in the calculation have been mainly
deduced from Italian price lists, which indicate the
cost of a particular work according to a work-
related unit price, including a specification on the
percentage cost to be assigned to manpower.
When a work was not available in the Italian price
list or the description was judged to be too far from
the actual design needs, a detailed price analysis
through work splitting has been performed. There-
fore, starting from unitary costs of material and the
amount of manpower needed to complete the exam-
ined work, the prices were calculated €according to
Equations (1) and (2) (Gottfried and Di Giuda 2011):
Pu=Cu+(Cu·GE)+ [C+(Cu·GE)] · PR(€),(1)
Cu=CM+Cm,(2)
where P
u
is the unitary price, C
u
is the marginal unit
cost, C
M
is the manpower cost, C
m
is the material
cost, GE is the general expenses (15%), P
R
is the
profit (10%).
Where needed, cost of machinery will be evaluated
separately.
However, when considering the hypothesis of
totally prefabricated elements, a new parameter has
been introduced: increased productivity. This
accounts for the fact that wall assembling work
takes place in a controlled and fully organized
environment, so unforeseen events and downtime
are extremely reduced. Moreover, workers are sup-
posed to enjoy the best possible working conditions,
as far as both comfort and safety are concerned, so
their productivity will be definitely higher.
Option A –LP/SP
For the cost evaluation of every technical element
reported within the WBS, the following layers have
been analysed:
.CLT panel (for each different thickness value);
.αir-tightness canvas;
.thermal insulation;
.water-tightness canvas;
.vented façade.
The total price for the installation of each layer in
its final position is derived from the product
between the unitary price, including both cost of
material and work, and the computed quantity.
Each listed element reports a unique identification
code directly related to the reference price list.
As evident from the sample codes reported in
Table V, the only two cost voices that have been com-
puted by performing a price analysis are vented
façade installation (which required a project custo-
mized evaluation) and CLT panel installation. In
this latter case, the detailed cost analysis has been
useful to highlight differences between big and
small panels, showing that manpower cost incidence
on total cost is in a ratio of one to three (Mantegazza
2014).
The iteration of the analytical process demon-
strated in Table V for all works related to each
defined technical façade element reveals the final
price for the case study vertical enclosure. In order
to better outline differences among the analysed
cases, some partial results have been extracted out
of the total price estimation. This is the case of verti-
cal joints and scaffolds.
Table IV. WBS example for Option A –LP.
Classes of
technological
units
Technological
units
Classes of
technical
elements
Technical
elements
1 Structure
2 Enclosure
1.1 Elevation
structure
2.1 Vertical
enclosure
1.1.1 Vertical
elevation
structure
2.1.1 Vertical
external
wall
Panel A
Panel B
Panel C
Panel D
Panel D
Panel E
2.1.2 Vertical
door or
window
Window
Construction management for tall CLT buildings 267
In Tables VI and VII results from Option A cost
analysis have been reported, respectively, for Large-
size and Small-size Panelling solutions.
As far as the cost analysis for joints is concerned,
assumptions related to their quantities estimation
can be briefly summarized as follows:
.specific screw prices have been determined,
thanks to the information provided by certain
manufacturers, as they were not available on
Italian price lists;
.vertical connections between consecutive
panels requires a total of 10 screws along the
entire joint length (45° inclination);
.horizontal connections consist of angle brackets
fixed to the floor through screws and to the wall
through nails. Two connection points per meter
have been taken into account.
Finally, the cost estimation for scaffolds has been
developed considering the use of a pipe-joint-type
scaffold for the entire on-site work period. At a first
step, the outcome value has not been included
within the final cost as the price reported has to be
considered per month. This issue will be addressed
in the Result section, after timing considerations
have been presented.
Option B –LP/SP
A different approach has been followed in evaluat-
ing the final price of installation as far as the totally
prefabricated option is concerned. In fact, applying
the same methodology to this case would have
meant performing an overestimation of costs as,
despite material cost being the same for both
options,
1
manpower incidence needs to be calibrated
taking into account the higher productivity a worker
can achieve in a factory.
This consideration has led to the development of a
simplified price analysis by applying the inverse
formula of Equation (1) for each designed preas-
sembled panel, in order to calculate the actual mar-
ginal unit cost, excluding general expenses and
profit. Once C
u
is available, it is possible to determine
the cost of manpower only, according to the percen-
tage incidence suggested in national price lists or
derived from price analysis. Finally, this cost (C
M
)
has been decreased by a reduction factor equal to
0.5,
2
applied to the specific works which will benefit
from the shifting of working location from the con-
struction site to the factory, that is, to say, all wall
outer layers which would normally be installed
through the use of scaffolds. On the other hand,
Table VI. Cost analysis outcome for Option A –LP.
Ex. walls (€) Windows (€) V. Junctions (€) Total (€) Scaffolds (€/month)
575,476.44 138,875.31 5775.45 720,127.20 8030.03
Table V. Panel A (CLT 18 cm) –- cost analysis example for Option A –LP.
External wall Code P
u
(€/m
2
) Quantity (m
2
) Price (€)
CLT panel A.P.02 172.30 42.18
3
7,266.90
Air-tightness canvas A95022 11.05 34.60 382.33
Thermal insulation 02.12.01.15 16.76 34.60 579.89
Water-tightness canvas A95045 5.95 34.60 205.87
Vented façade A.P.01 36.23 34.60 1253.68
9688.66
Window Code P(€/each) Quantity (n) Price (€)
120×140 cm C25027c 546.29 2 1092.58
160×140 cm C25033b 728.39 2 1456.77
2549.35
Total price 12,238.00
Table VII. Cost analysis outcome for Option A –SP.
Ex. walls (€) Windows (€) V. Junctions (€) Total (€) Scaffolds (€/month)
669,828.55 138,875.31 20,214.06 828,917.92 8030.03
268 E. Gasparri et al.
manpower contribution related to CLT panel instal-
lation and their connections would not experience
any substantial variation between the two analysed
options, A and B. As in the previous case, Table
VIII summarizes calculation results.
It is important to emphasize that an extremely accu-
rate and detailed evaluation of costs related to each
specific option would be necessary to develop a price
analysis for each single work of the WBS, based on
close collaboration with manufacturing and construc-
tion companies to obtain the exact data from previous
project experiences. This would allow for the defi-
nition as to which parameter has a consistent influence
on cost and it would be easier to keep it under strict
control according to the specific project case.
This very detailed works factorization would have
required a significant analytical effort and a very time-
consuming data search phase, which could not justify
the scope of this work, whereby the aim is to produce
an approximate estimation to evaluate the order of
magnitude of a system convenience over the other.
As well as for Option A, some partial results have
been extracted out of the total cost and reported in
Tables IX and X, respectively, for Large-size and
Small-size Panelling solutions. In this case,
however, the diversification of the external wall
element and window would have not made sense.
As far as the cost analysis of joints is concerned,
assumptions made for Option A are also valid in the
case of the totally prefabricated off-site option too.
This choice allows comparable results between all
different cases. On the other hand, scaffolding will
not be considered anymore.
Results and discussion
Results from the cost analysis confirmed that prefab-
ricating façade elements off-site allows for a signifi-
cant reduction in costs (see Figure 13(a) and (b)).
It is noteworthy that this is also true without consid-
ering the expenses derived from scaffolds.
Moreover, outcome have shown how differences
in terms of cost among Large- and Small-size
Panelling solutions, in the case of both A and B
options, are equally significant (see Tables VI,
VII,IX and X) as it makes no sense to take into
account the second one without rethinking pro-
duction and installation process according to a
Table VIII. Panel A (CLT 18 cm) –Cost analysis example for Option B –LP.
External wall Code C
m
(€/m
2
)C
M
(€/m
2
)C
u
(€/m
2
) Quantity (m
2
) Cost (€)
CLT panel A.P.02 122.40 13.80 136.20 42.18 5744.92
Air-tightness canvas A.P.15 5.68 1.53 7.21 34.60 249.47
Thermal insulation A.P.14 9.41 1.92 11.33 34.60 392.02
Water-tightness canvas A.P.16 1.98 1.36 3.34 34.60 115.56
Vented façade A.P.01 7.60 10.52 18.12 34.60 626.95
7128.92
Window Code C
m
(€/ea) C
M
(€/ea) C
u
(€/ea) Quantity (n) Cost (€)
120×140 cm A.P.12 384.35 23.75 408.10 2 816.20
160×140 cm A.P.11 512.46 31.67 544.13 2 1088.26
1904.46
Tot Cost 9033.38
GE (15%) 1355.01
P
R
(10%) 1038.84
Tot Price 11,427.22
Table IX. Cost analysis outcome for Option B –LP.
V. Enclosure (€) V. Junctions (€) Total (€) Scaffolds (€/month)
641,553.71 5775.45 647,329.15 –
Table X. Cost analysis outcome for Option B –SP.
V. Enclosure (€) V. Junctions (€) Total (€) Scaffolds (€/month)
759,493.85 20,214.06 779,707.91 –
Construction management for tall CLT buildings 269
more sustainable strategy. The most relevant differ-
ences are determined by manpower incidence
related to panels load on truck, on-site handling
through crane and installation (see Table VIII).
These phases clearly need to be repeated for the
high number of designed panels, causing a consist-
ent time and cost increase. In this case, a more
coherent solution would imply studying new wall
element dimensions in order to allow for superim-
posed pallet transportation, as it already happens
for unitized glass façades, which are commonly
used for the major part of high-rise building
façades nowadays. The adoption of such a system
would also foster a quick and simple truck unload
and on-site storage phase (or at the foot of the
building) and agile handling/installation phase
through the use of a reduced radius operating
crane (Friblick et al., Mantegazza 2014).
Another factor which has an influence prices raising
is the higher number of fasteners needed to structu-
rally connect two consecutive walls (see Table XI).
Anyway, this issue can be easily addressed by using a
heavy-duty connection system preinstalled on panel
edges during the factory assembling phase.
Scaffolding prices have been computed according
to work durations obtained from the project
Gantt diagram, for each of the considered options.
Savings generated from the preassembly of the off-
site façade can easily be reported as a unit price divid-
ing the specific saving value by the analysed façade
area, that is, to say, 824 m
2
into four fronts. For
instance, the comparison between Options A and B,
as far as the B-LP case is concerned, shows how the
unit price saving may be substantiallyreduced, in
this case equal to 31.83 €/m
2
, but it can reach consist-
ent amounts if the involved surfaces are large enough
(see Table XII).
Finally, it is fundamental to outline how this
analysis deals with direct costs only, that is, to say,
from resource savings in terms of material, man-
power and tools. However, including all indirect
costs mentioned within the advantages analysis illus-
trated at the beginning of the present paper (see
Table I) would make cost reduction even more sig-
nificant. Starting from this main finding, it would
be useful and extremely interesting to perform a
life-cycle cost analysis as further development of
the present work.
Time analysis
The analysis of the four different defined solutions as
far as time effort is concerned has been carried
out through two main steps: the duration determi-
nation for each work included in WBS and schedul-
ing through different on-site phases in order to
obtain a realistic work estimation of hypothesized
options.
The approach used to perform the first goal is
based on statistical/probabilistic prevision (Gottfried
and Di Giuda 2011) and makes use of empirical
data from operators’experience. The evaluation
method is strictly related to the closed bill of quan-
tities estimation phase, as the manpower percentage
incidence in terms of costs is the starting point to
determine every work median duration. Calculations
have been developed based on Equation (3) and (4),
which are briefly explained below.
CM=Cu·%M,(3)
CMday =CMh ·nh,(4)
where C
M
is the manpower cost, C
u
is the marginal
unit cost (for each work), %
M
is the percentage inci-
dence of the manpower, C
Mday
is the manpower cost
per day, C
Mh
is the manpower cost per hour, n
h
is the
working hours per day.
Figure 13. (a) Partial price graphic comparison for the four pro-
posed design options. (b) Total price graphic comparison for the
four proposed design options.
270 E. Gasparri et al.
Once having easily fixed with these data, it is possible
to go further through the determination of a fictitious
indicator, representing the amount of workers required
in order to be able to complete a work of any sort in only
one day (see Equation 5). This parameter, together
with the definition of the standard operating team for
the examined work, allows to evaluate the duration of
the work itself through Equation (6).
Pday =CM/CMday,(5)
Dn=Wday/ST,(6)
where P
day
is the person-days (see COUNCIL DIREC-
TIVE 92/57/EEC), ST is the number of workers
making the standard operative team, D
n
is the work
normal duration.
Finally, the outcome duration value has been con-
sidered by defining both optimistic and pessimistic
scenarios, in terms of percentage of occurrence, in
order to take into account various unforeseen
events that might affect the ordinary progress of the
construction process (see Equation (7)–(9)):
DO=%O·Dn(7)
DP=%P·Dn(8)
Dme =DO+DP+4·Dn
6(9)
where D
O
is the optimistic duration, %
O
is the opti-
mistic percentage, D
P
is the pessimistic duration,
%
P
is the pessimistic percentage, Option A –LP/SP.
Following the process described above, each work
duration for both Option A –Large- and Small-size
Panelling –solutions has been evaluated. As in the
case of cost estimation, this analysis has also been
performed for each work included in the WBS.
Table XIII reports evaluation outcomes as far as the
necessary time for work completion on a certain
level is concerned.
It is important to underline that, on a real construc-
tion site, with the increase in the level height, some
works will naturally require a longer time to be per-
formed. As an example, it is possible to consider the
CLT panel transportation path on the crane from
the ground floor to a constantly increasing level of
the building. In addition, many works require stricter
safety procedures, which are likely to make work
proceed at a slower pace to guarantee workers’
safety. However, this aspect produces differences
that are not so significant and may be not taken into
account within the scope of this work (Mantegazza
2014). Thus, durations in Table XIII, related to the
first floor, have simply been multiplied by the
number of storeys of the case study building.
Figure 14 (a) and (b), which show the percentage
duration divided according to the installation of
various components, clearly highlights that the
impact in terms of time related to façade installation
(CLT panels) is significantly more in the case of
small panels than in the case of large panels.
Option B –LP/SP
The same approach has been followed to carry out
the construction site works length calculation insofar
as the entirely prefabricated façade solution is con-
cerned. In this case, the only activity needed to be
taken into account concerns the installation of preas-
sembled external walls, thus the starting value for
durations estimation has been manpower incidence
Table XI. Numeric results comparison for all different options.
Option A –SP Option B –SP Option A –LP Option B –LP
Ex. walls 669,828.55 –575,476.44 –
Windows 138,875.31 –138,875.31 –
V. enclosure (€) 808,703.86 759,493.85 714,351.75 641,553.71
V. Junctions 20,214.06 20,214.06 5775.45 5775.45
Price w/o scaffold (€) 828,917.92 779,707.91 720,127.20 647,329.15
Months number
4
5–4–
Scaffolds 40,150.15 –32,120.12 –
Total price (€) 869,068.07 779,707.91 752,247.32 647,329.15
Table XII. Cost savings related to each option compared to the other alternatives.
Savings
5
(€) Option A –SP Option B –SP Option A –LP Option B –LP
Option A –SP 0.00 89,360.16 116,820.75 221,738.92
Option B –SP 0.00 27,460.59 132,378.76
Option A –LP 0.00 104,918.17
Option B –LP 0.00
Construction management for tall CLT buildings 271
related just to CLT panel installation. Results are
summarized in Table XIV.
When performing the calculation, a 15% increased
value has been considered in order to consider the
desirable longer time needed for the preparation,
handling and installation phase due to both higher
vulnerability of the system itself and precision
required to ensure façade surface planarity.
Manpower incidence cost for the façade outer
layers has been useful in order to obtain the total
costs for the solution proposed, but they have not
been taken into account within on-site scheduling
determination, as their installation durations do not
belong to the analysed construction phase.
If the actual works duration, as far as the only
façade completion on the four building fronts is con-
cerned, would come from the algebraic sum of single
works contributions, the prefabricated and non-
prefabricated options would show a huge difference.
As an example, in the traditional large panels
option, façade completion through scaffolding
would require around 7.5 months.
For this reason, a general work program for the
considered solutions has been developed, allowing
for the performance of an effective and coherent com-
parison among the various solutions, considering the
contemporary works in each of the cases.
Gantt chart. The final construction time analysis for
the case study building has been developed through
a bar diagram, currently used in the field of project
management to arrange various project activities
and guarantee a proper and well-balanced use of
resources. Once more, it is useful to underline that
work duration is only related to the four fronts
façade completion. Therefore, final duration out-
comes have to be intended as relative values, with
the aim of allowing a reasonable comparison
between the various options of the case study.
The diagram construction is performed according
to the typical project development for these kinds of
buildings, one storey after the other. Scheduling
analysis was based on the following assumptions:
.works on the façade, if needed for the specific
case, are supposed to begin when the structure
installation has already reached the third level.
The presence of an intermediate storey
without any type of work has been guaranteed
in order to avoid any eventual interference;
.scaffolding installation grows up together with
structural CLT panels in order to provide pro-
tection from “fall from height”to workers oper-
ating on various storeys;
Table XIII. Sum of durations in days for option A –LP/SP.
Option A
Large size Panelling Small size Panelling
1
Storey
All
storeys
1
Storey
All
storeys
CLT panel 2.50 22.50 7.25 65.25
Air-tightness canvas 1.50 13.50 1.50 13.50
Thermal insulation 1.25 11.25 1.25 11.25
Water-tightness
canvas
1.00 9.00 1.00 9.00
Vented façade 8.00 72.00 8.00 72.00
Windows 2.00 18.00 2.00 18.00
Scaffolds 1.50 13.50 1.50 13.50
V. Junctions 0.25 2.25 0.50 4.50
Total (days) 18.00 162.00 23.00 207.00
Figure 14. (a) Large Panelling, work duration distribution accord-
ing to components installation. (b) Small Panelling, work duration
distribution according to components installation.
272 E. Gasparri et al.
.links of precedence between different activities
have been determined according to construc-
tion time logic and experience.
Analysis results for each solution in terms of
working days are reported in Table XV.
Results and discussion. Results from on-site scheduling
elaboration show how component prefabrication can
achieve significant time savings. Unfortunately, con-
struction schedule and cost reports related to best
practice case studies are not easily made available
by design firms or general contractors companies,
making analytic comparison and research results vali-
dation hard to perform. In fact, most of the studies
concern already built project or patented construc-
tion systems, whose specific data sheets are very
seldom made public or published. Thus, only quali-
tative information on these case studies is available
(Waterfront Auckland, Miloni et al.2011).
Table XVI collects outputs from both analytical
steps followed within scheduling estimation. It is
interesting to note how, in the case of Option B,
there are no differences in terms of works duration
compared to the mere sum of each activity duration.
This is quite a predictable result considering the fact
that façade panels arrive at the construction site ready
to be installed in their final position and no additional
work is needed. As such, only the following activities
have been considered for each panel:
.truck preparation, lifting to the floor, vertical
positioning and horizontal connection to floor
slab (all included under V. Enclosure, see
Table XIV);
.vertical connection to the adjacent facade
panel.
Despite both prefabricated solutions perform more
strongly compared to non-prefabricated ones (see
Figure 15), time saving guaranteed from Option B
–SP, within the defined boundary conditions, is
not large enough to justify this choice compared to
the traditional construction way.
Thus, as demonstrated for the cost estimation
phase, outcomes from time analysis also prove how
prefabricating façade elements off-site could bring
about lots of advantages to the construction process
of a tower building. This issue is even more relevant
when dealing with timber-based structure, where
minimizing exposure to weather agents could
become a crucial aspect to take into account during
on-site works. Moreover, it is necessary to underline
how construction speed concurs widely in saving
money as well and it is often the key factor which
drives clients to choose timber as construction
material for their investments.
Conclusions
This paper presented the results of a study of the
influence of envelope degree of prefabrication for
tall CLT buildings, in terms of cost and time
savings but also concerning on-site work quality
and safety issues. The analysis was carried out
through the use of a case study building, demon-
strating that comprehensive façade prefabrication
not only allows consistent compression of construc-
tion scheduling to be achieved, but also for the
immediate protection to wooden elements from
weather agents. In addition, it also guarantees
more efficient construction site organization. The
latter is a main concern when dealing with timber
construction.
Results prove that off-site prefabrication of façade
elements allows a significant reduction in costs. It is
particularly interesting that this also applies without
considering scaffolding costs. Moreover, the
outcome showed how differences in terms of cost
among Large-size and Small-size Panelling solutions,
in the case of both Options A and B, are equally sig-
nificant. However, it would certainly be possible to
optimize production and installation processes in
Table XV. Gantt chart output.
Real
duration
Option A –
SP
Option B –
SP
Option A –
LP
Option B –
LP
Days 103.25 81.00 85.75 29.25
Months 5 4 4 1.5
Table XIV. Sum of durations in days for option B –LP/SP.
Option B
Large size Panelling Small size Panelling
1 Storey All storeys 1 Storey All storeys
V. Enclosure 3.00 27.00 8.50 76.50
Scaffolds ––––
V. Junctions 0.25 2.25 0.50 4.50
Total (days) 3.,25 29.25 9.00 81.00
Table XVI. Comparison between the sum of partial durations and
the Gantt chart output for each option.
Days
Option A
–SP
Option B
–SP
Option A
–LP
Option B
–LP
Sum of
durations
207.00 81.00 162.00 29.25
Actual
duration
103.25 81.00 85.75 29.25
Construction management for tall CLT buildings 273
favour of a more sustainable strategy for the latter
option. In fact, the most relevant differences were
encountered in manpower incidence related to panels
load on truck, on-site handling through crane and
installation. Changing the system to allow for a more
efficient transportation and handling phase could be a
very promising research line for the next years.
Furthermore, within the frame of this work, cost
analysis focuses only on direct costs, that is, to say,
those derived from resource savings in terms of
material, manpower and tools. In addition to this,
indirect savings derived from better construction site
organization and other related aspects would certainly
further reduce total costs. Analysis carried out out-
lined how a prefabricated envelope strategy applied
to tall timber constructions brings about numerous
advantages. Moreover, it is easily inferable how
benefits become even more consistent by increasing
building height. On the other hand, the use of the pro-
posed system for smaller buildings needs to be inves-
tigated further to better verify its cost effectiveness.
In this case, a partial prefabrication of the envelope is
likely to be more convenient, as it is certainly possible
to solve joints between wall elements from the outside
through the use of aerial platforms.
Finally, results from detailed construction site
scheduling outline how components’prefabrication
permits consistent savings in terms of time and
costs and, more generally, improves construction
process quality. In particular, the issue of time
reduction is even more relevant when dealing with
timber-based structure, where minimizing exposure
to weather agents is certainly among the main con-
cerns. In addition to this, construction speed
concurs widely in saving money and is often the key
factor driving clients towards the choice of wood as
construction material for their investments.
Acknowledgements
The development of this work has been supported by our
research partner the institute for Construction Manage-
ment and Economics of the Technical University in Graz/
Austria, where parts of this research was specifically
carried out. Special thanks to Prof. Dr Detlef Heck and
Dipl.-Ing. Joerg Koppelhuber of the department for their
kind collaboration and useful suggestions. Further
acknowledgments go to Stora Enso Wood Products
GmbH for the technical information provided. The corre-
sponding author wants to thank Annalisa Andaloro for pro-
viding comments that greatly improved the manuscript and
for her friendly support.
Disclosure statement
No potential conflict of interest was reported by the authors.
Notes
1. In the case of prefabricated solution, additional pieces (e.g. OSB
frame) have been computed but are not reported to facilitate an
effective comparison. However, graphs summarizing results
consider also their contribution.
2. The exact value of this factor is currently under study at the
Construction Management Department of TU Graz, Austria.
3. CLT panel surface is larger as manufacturers compute the
whole surfaces, including windows cutting.
4. Average on the working days number obtained from the Gantt
analysis (Microsoft project).
5. Calculated for total prices of each option.
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