Architectural Institute of Japan Structural System
Timber elements, which are different from other structural elements, have a characteristic problem in that the load bearing capacity decreases due to self-burning in the case of a fire, and this self-burning may continue after other fuel in the room has been exhausted. Therefore, the structural fire performance of timber elements should be clarified during not only the heating phase, but also the cooling phase. However, in examining the load bearing capacity of timber elements in a fire, few studies have considered the cooling phase. In the present paper, the fire performance of glued, laminated timber beams is discussed based on load-bearing fire tests that take the cooling phase into consideration.
At the Institute of Structural Engineering at the ETH Zurich numerous of investigations are
conducted to analyse the load bearing capacity of glued laminated timber beams. The investigations are part of the research project ’Influence of varying material properties on the load bearing capacity of glued laminated timber (glulam)’.
The investigations are taking place on 24 glulam beams with well-known material properties.
The glulam beams are fabricated out of 400 timber boards. From those boards the material
properties are investigated non-destructively within a former research project. During the glulam
fabrication it is particularly focused to keep the information of the timber boards; i.e. after the
glulam fabrication the position of each particular timber board within the glulam beam and
thus the position of each particular knot is still known.
The glulam beams are investigated during a 4-point bending test. On the glulam members
the load bearing capacity, the bending stiffness and the density is measured. Furthermore
local strains within the glulam beams are investigated using an optical coordinate-measurement
device. Following the test the failure is investigated in detail. Hereby the type of failure (knot
cluster, finger joint, clear wood) and the amount of failure (number of damaged lamellas) is
documented. Afterwards the failed glulam beams are loaded again to analyse the remaining
bending strength and the corresponding remaining bending stiffness.
The major aim of the experimental analysis is the investigation of the load bearing capacity
of glulam beams with well-known local material properties. The gained results can be used for
an investigation of the influence of local weak zones, such as knot clusters or finger joints, on the
load bearing capacity of glulam. In addition a data basis is produced to develop a new model
(or to evaluate existing models) for the estimation of the load bearing capacity of glulam.
The force-displacement behaviour of structural timber members subjected to axial compression or combined axial compression and bending is distinctively non-linear. This behaviour is caused by the non-linear increase of the deformation due to the increasing eccentricity of the axial load as well as by the non-linear material behaviour of timber when subjected to compression. The present report describes experimental investigations on glued laminated timber members subjected to eccentric compression. The aim of these experimental investigations was to create a data base, which can be used to validate theoretical calculation models and to assess the accurateness of the design approaches given in the design codes for timber structures.
The specimens for the main bunch of experiments were produced using lamellas made of Norway spruce grown in Switzerland. For this purpose, a total of 336 lamellas were available. In the first step, non-destructive tests on the lamellas were performed. These tests aimed at the collection of data in order to characterise the raw material.
In the second step, the lamellas were strength graded. The aim of the grading process was to select two classes of lamellas for the production of the test specimens. The lamellas were selected so that they were suitable to produce glued laminated timber of strength classes GL24h and GL32h. Within the grading process, visual grading criteria as well as machine grading criteria were used.
In the third step, the graded lamellas were used to produce glued laminated timber members. Five tests series were produced. Each of the test series consisted of ten specimens. Three series were made of glued laminated timber GL24h and two series were made of glued laminated timber GL32h. The length of the timber members was varied between the different test series. The lengths were L = 1’400 mm, L = 2’300 mm and L = 3’200 mm respectively. During the production, the setup of the test specimens was recorded. Hence, the position and the orientation of every lamella within the test specimen were documented. Additionally, some non-destructive tests were performed using the test specimens.
In the last step, the glued laminated timber members were subjected to buckling tests. The test specimens were loaded with an eccentric compression force up to failure. During the tests, different measurements were carried out in order to document the experimental investigations as accurate as possible. Amongst others, the applied loads as well as horizontal and vertical deformations were recorded. For a subsample of 20 test specimens, additional local deformation measurements were performed using an optical measurement device.
Dowel-laminated timber (DLT) elements consist of lamellae arranged side-by-side that are connected with beech dowels. Due to the glue-free DLT element layup, joints and shear walls potentially suffer from considerable reduction of stiffness and load carrying capacity as metal fasteners inserted perpendicular to the element plane may be placed in gaps between the single lamellae. Tests on typical joints showed that, depending on the fastener diameter, the remaining load carrying capacity of joints in DLT in comparison to joints in solid wood may be only 25%. Tests on DLT shear walls with different sheeting proved that the use of DLT structures as shear walls is only possible if at least one-sided sheeting is used. Cyclic tests on DLT shear walls demonstrated that the DLT construction typology has energy dissipation properties similar to traditional timber frame construction. Analogously, preliminary behaviour factors for DLT buildings evaluated with numerical models were also similar to those for timber frame buildings.
Inspection, Testing, and Monitoring of Buildings and Bridges
Depending on the severity, fire damage can compromise the structural integrity of wood structures such as buildings or residences. Fire damage of wood structures can incorporate several models that address (1) the type, cause, and spread of the fire, (2) the thermal gradients and fire-resistance ratings, and (3) the residual load capacity.
The investigator should employ engineering judgment to identify those in-service members that are to be replaced, repaired, or can remain in-service as they are. Suchjudgment will likely be based on the visual inspection of damaged members, connections, and any protective membranes.
In the presented paper, results of theoretical and experimental investigation of timber-concrete composite members with adhesive connection are described. For the timber part of composite beams Cross Laminated Timber and for concrete part lightweight concrete was used. For the composite connection special adhesive to bounding wet concrete and timber was applied. For experimental investigation two types of composite beams with different dimensions was used. Due to the shrinkage of lightweight concrete small precamber of timber beams during concrete hardening was applied. CLT panels combined with concrete slab dispose of higher load-carrying capacity, lower deformation and vibration. In case of theoretical analysis, simplified analytical -method was used to consider shear flexibility of the CLT cross layer. Results of presented experimental and theoretical analysis provide wider scope for further research and application of adhesively bonded CLT-concrete composite members.
Within this paper a comparison of different reinforcement concepts for timber beams with round holes is carried out. Therefore currently applied standardized methods and two recently developed approaches are considered. By means of numerical and analytical investigations it becomes apparent that the analysed reinforcement methods divergent to those given in current standards have great potential: shear stresses as well as tensile stresses perpendicular to the grain in the critical areas around the beam opening can be reduced significantly. Hence, the maximum load carrying capacities of the new reinforcement concepts supposedly exceed the standardized ones considerably. For verification of the results experimental investigations on beams with different reinforcement methods are planned.
International Journal of Advanced Structural Engineering
This paper investigates the mechanical performance of longitudinally cracked glulam columns under eccentric compression loads. Experimental investigation was conducted to explore the influence of initial cracks on the failure modes and load bearing capacity of glulam columns. Two different crack patterns named DC and IC, and two column lengths (i.e. 600 and 1100 mm) were considered in the experiments. It was indicated that these two crack patterns reduced the capacity of slender glulam columns and the difference of failure modes was observed between glulam columns with and without initial cracks. Further, a numerical model was developed and validated by the test results. With the application of cohesive zone material model, the propagation of initial cracks could be considered in the numerical modeling. A parametric study was carried out by the verified model and the influence of crack lengths and crack locations was further investigated. From the numerical analysis, it was found that through cracks reduced the capacity of glulam columns significantly. Also, crack location impacts the capacity of glulam columns and the extent of impact relates to the slenderness ratio of the columns, while cracks with different lengths have similar influence on the capacity of columns.
Cross laminated timber (CLT) members are especially suited for in-plane loads due to their high shear strength and stiffness. However, available connection techniques show limited load-carrying capacities and stiffness values in comparison to the shear capacity of CLT. To use the potential of CLT under in-plane loading, new connection techniques, so called contact joints, with increased stiffness and load-carrying capacities were developed. 10 different types of these contact joints, varying geometry and connector material, were studied. The developed contact joints can substitute traditional connection techniques.
In cross-laminated timber (CLT) buildings, in order to reduce the disturbing transmission of sound over the flanking parts, special insulation layers are used between the CLT walls and slabs, together with insulated angle-bracket connections. However, the influence of such CLT connections and insulation layers on the seismic resistance of CLT structures has not yet been studied. In this paper, experimental investigation on CLT panels installed on insulation bedding and fastened to the CLT floor using an innovative, insulated, steel angle bracket, are presented. The novelty of the investigated angle-bracket connection is, in addition to the sound insulation, its resistance to both shear as well as uplift forces as it is intended to be used instead of traditional angle brackets and hold-down connections to simplify the construction. Therefore, monotonic and cyclic tests on the CLT wall-to-floor connections were performed in shear and tensile/compressive load direction. Specimens with and without insulation under the angle bracket and between the CLT panels were studied and compared. Tests of insulated specimens have proved that the insulation has a marginal influence on the load-bearing capacity; however, it significantly influences the stiffness characteristics. In general, the experiments have shown that the connection could also be used for seismic resistant CLT structures, although some minor improvements should be made.