Optical Fiber Sensor for Timber Structure Monitoring

Abstract

The paper deals with the experimental analysis focused on the determination of strain in wooden building structures using optical fibers with FBG sensors. Firstly, optical fibers were used to measure displacements of layers and surface strain of the lower surface of the ceiling panels made from mechanically jointed cross laminated timber (CLT). The measured displacement values of the optical fibers were compared with the results obtained from the inductive displacement sensors. Secondly, optical fibers were used for long-term monitoring of the roof structure of the sports hall. The fibers were mounted on chords of timber trusses and their correct function was verified by a load test.

Full text

Optical Fiber Sensor for Timber Structure Monitoring
VČELÁK Jan
1,a*
,
KUKLÍK Petr
1,b
, ZELENÝ Richard
1,c
, VELEBIL Lukáš
1,d
,
DVOŘÁK
2,e
and DEMUTH Jaroslav
2,f
1Czech Technical University in Prague - University Centre of Energy Efficient Buildings
Třinecká 1024, 273 43 Buštěhrad, Czech Republic
2Safibra, s.r.o., Černokostelecká 1621, 251 01 Říčany, Czech Republic
ajan.vcelak@uceeb.cz, bpetr.kuklik@uceeb.cz, crichard.zeleny@uceeb.cvut.cz,
dlukas.velebil@uceeb.cz, emilan.dvorak@safibra.cz, fjaroslav.demuth@safibra.cz
Keywords: Fiber Optic Sensor; Strain Measurement; Cross Laminated Timber; Long-term
Monitoring; Timber Truss.
Abstract. The paper deals with the experimental analysis focused on the determination of strain in
wooden building structures using optical fibers with FBG sensors. Firstly, optical fibers were used
to measure displacements of layers and surface strain of the lower surface of the ceiling panels made
from mechanically jointed cross laminated timber (CLT). The measured displacement values of the
optical fibers were compared with the results obtained from the inductive displacement sensors.
Secondly, optical fibers were used for long-term monitoring of the roof structure of the sports hall.
The fibers were mounted on chords of timber trusses and their correct function was verified by a
load test.
Introduction
In recent years an interest in the long term monitoring of structures increases. One of the reasons is
due to the repeated collapses of roofs that brought disastrous consequences. Especially flat roofs and
roofs of halls may have a problem with snow loads, which can lead to exceeding the permissible
load. Monitoring the static stress of the roof structure helps to minimize security risks due to the
possibility of early warning.
Therefore the reconstructed sports hall was selected to verify the proposed monitoring system.
After uncovering of suspended ceilings during reconstruction of the sports hall damage of several
timber chords in the truss structure was detected. In some cases, the lower chords were completely
fractured, which could lead to fatal consequences. Monitoring system of critical points in the truss
structure has been designed simultaneously with the reconstruction and replacement of defective
chords. Continuous monitoring system has been designed in such a way that the measurement is
meaningful and provides early warning of construction overload due to the strong winds or
excessive snowfall. Conventional resistance strain gauges are not suitable for this application, due
to shorter durability. So the monitoring system has been created using the innovative technology of
fiber-optic sensors based on FBG principle, which are alternative to strain gauges. Application of
FBG sensors for monitoring of timber structures complements current utilization for long-term
monitoring of concrete [1] and steel [2] building structures.
Small Scale Experiments
Ceiling Panel. Two experimental specimens (I., II.) were made from 5-layer ceiling panels of CLT.
The layers of spruce wood planks with thickness 27 mm were stacked together and screwed by
threaded galvanized screws. Two edge layers were in longitudinal orientation (parallel to span) and
the middle one was in orthogonal orientation. Four screws per each lamella’s crossing were used for
the layer connection. Dimensions of each tested floor panel were 2600 x 1000 x 135 mm.
Experimental Set-up. Four point bending tests were used to compare results with inductive
displacement sensors. In the experiments, horizontal displacements of the upper two layers relative
Applied Mechanics and Materials Submitted: 2015-06-16
ISSN: 1662-7482, Vol. 827, pp 344-347 Revised: 2015-10-09
doi:10.4028/www.scientific.net/AMM.827.344 Accepted: 2015-11-10
© 2016 Trans Tech Publications Ltd, All Rights Reserved Online: 2016-02-22
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