Through long-term measurements of climate data (temperature, relative humidity) and timber moisture content on large-span timber structures in buildings of typical construction type and use, data sets were generated which deliver information on the sequence and magnitude of seasonal variations. The measurement of moisture in different depths of the cross-section is of particular interest to draw conclusions on the size and speed of adjustment of the moisture distribution to changing climatic conditions. The moisture gradient has direct influence on the size of the internal stresses and possible damage potential. Similarly, the results provide a review and extension of the previous classification of buildings into use classes. They allow for a more precise indication of range of resulting equilibrium moisture content for the specific use, enabling the installation of timber elements with adjusted moisture content. The results of the research project also support the development of appropriate monitoring systems, which could be used in the form of early warning systems based on climate measurements.
This thesis deals with the shear design of Cross Laminated Timber (CLT) elements stressed by concentrated loads which are locally reinforced by means of self-tapping screws with continuous threads. A simplified model is presented using an effective width for the calculation of the shear stresses in the vicinity of point supports or concentrated loads. Laboratory tests supply material-mechanical principles to determine the interaction of rolling shear stresses and compression perpendicular to the grain. In addition to experimental tests theoretical models are developed to examine the load bearing behaviour of CLT-elements reinforced by self-tapping screws. Preliminary tests with plate elements provide initial experience with these reinforcements under biaxial load transfer. Finally a design concept validated by means of the test results is proposed.
Concentrated loads on Cross Laminated Timber elements (CLT) in areas of point supports or load applications cause high local shear stresses. Inclined self-tapping screws with continuous threads have turned out to be an effective reinforcement. As neither the German design standard DIN 1052 nor technical approvals cover this construction method a research project funded by the AiF was conducted to gather basic information for its application. These basics include the determination of shear stresses next to concentrated loads, the interaction of compression perpendicular to the grain and rolling shear stresses as well as theoretical and experimental examinations of the load bearing behaviour of reinforced CLT-elements. This paper presents the main research results. A design concept validated by means of the test results is proposed.
International Conference on Structural Health Assessment of Timber Structures
Numerous large-span, load-bearing structures, e.g. for public venues, sports halls or industrial facilities, utilize glued-laminated timber (glulam) due to its versatility and aesthetic appearance. Since glulam is an organic and composite material consisting of wooden lamellas joined with glue, mistakes during planning, fabrication and use of the structure can lead to deficiencies or even damages. Following a visual inspection suitable to detect surface deterioration, further holistic investigation of the glulam material is necessary to appraise structural safety or estimate the need for restoration. Besides the integrity of the wooden lamellas, the glue bond of the lamellas is vital for the operational reliability of glulam. A currently practiced, semi-destructive assessment method to revalue the condition of the glue lines consults a shear test on drill core samples with included glue line. The presented paper links this method to medium-scale shear tests and large-scale 3-Point bending-shear tests representing a practical loading situation. To provoke shear failure during bending, reinforcements of the specimens at critical tension and compression zones included glued-on beech veneer lamellas and self-tapping screws, respectively. Executed on the same sample material, shear resistance was determined for all three testing formats. The sampling included aged and new glulam. Published values derived from comparable test programs augmented the database. Based on the evaluated test results of the drill cores under shear loading, suggestions regarding the drill core extraction and the implementation of the valuation method of EN 14080 are outlined. After incorporating a size effect to account for varying dimensions of the bendingshear specimen, their shear resistance values correlated well with the obtained values from the drill core tests. This adumbrates the possibility to derive the shear resistance of structural members from shear values of drill core samples taken from an existing glulam structure.
In this study a timber-based integrated solution is presented to solve at once common issues affecting typical reinforced concrete (RC) existing buildings, such as seismic and energy performances, providing an eco-friendly alternative to steel external bracing systems. Cross-laminated timber (CLT) walls are provided perpendicularly to the external façades as strengthening elements while interposed CLT slabs are foreseen at each floor level to host new architectural units together with a new envelope. While the connections to the foundations and to the existing RC frames are provided respectively with steel brackets and axial-connectors distributed along the height of the building, a post-tensioned connection, between CLT panels (PT-CLT connection), is implemented in the system to guarantee resistance to horizontal actions acting parallel to existing façades with consequent structural independence and architectural freedom. On this regards a first look at the findings of an experimental campaign carried on the Technical University of Munich are presented. A numerical model is developed with finite element software characterizing each type of connector for linear and non-linear analyses. Modal analyses with response spectrum are performed to verify structural elements and connectors, while pushover analyses with target displacement checks are performed to assess the obtained seismic improvement. Finally, the preassembled architectural components that allow to renovate the envelope and the provided assembly procedure are revealed.
This study investigates five fire stop variants used to limit the spread of fire on wooden façades. For this purpose, five fire tests using various types of wooden façade claddings and different fire stops were conducted as full-scale tests and compared to the existing findings. The influences and interactions between the material qualities of the external wall behind the façade cladding, the construction type of the wooden façade cladding, the design of the substructure, the depth of the ventilation gap, and the design of the fire stops were investigated. In evaluating the fire stops, the design of the interior corners, the joint design, and the influence of thermal expansion were examined. Finally, design proposals for the design of fire stops at wooden façades in order to limit the spread of fire were derived based on this evaluation. The outlook presents further needs that need to be investigated in the future in order to clarify undiscussed aspects or points that were ultimately not evaluated within the scope of this study.
Following the Bad Reichenhall ice-arena collapse, numerous expertises on the structural safety of wide-span timber structures were carried out at the Chair of Timber Structures and Building Construction. It became evident that inadequate structural design and detailing as well as inadequate manufacturing principles were the main reasons for observed failures. The design and manufacture of connections in wide-span timber structures are still amongst the most challenging tasks for both the structural engineer as well as the executing company. This paper will, on the basis of two exemplary expertises, discuss specific issues in the structural reliability of connections in wide-span timber trusses and give recommendations towards a state-of-the art design of such connections.
Beech laminated veneer lumber (LVL) circular hollow sections for use as nails in temporary geotechnical soil nailing systems were developed within this research. For the soil wall stability, beside a sufficient load carrying capacity of the nail cross section, a load carrying connection between the cement annulus and the nail is necessary. As cement grout ensures the bond between the nails and soil for the mobilisation of a soil reinforcement effect, a reliable connection between cement and wood surface is required. Occurring shear forces must be transferred from the nail into the soil through the wood-cement interface similar to reinforced concrete. Pull-off tests on clear LVL specimens with different cement and wood surface modifications as well as the results of large-scale push-out tests are presented. The results reveal a sufficient short-term bonding strength between the cement annulus and the wooden member to transfer the geotechnical loads, but also highlight the necessity of further research on long-term behaviour.