Low-rise buildings constructed using wood are vulnerable to extreme wind storms and earthquakes. While several experimental measurements of the environmental loads (mostly wind) on the building envelope have been made at full scale, none of these studies directly linked these external loads with the internal forces and displacements of the structure.
This report presents the experimental and analytical work on two light-frame wooden structures, with one that already existed (Forintek shed in Québec City) and the other (UNB house) that was built specifically for the research project on the University of New Brunswick campus in Fredericton. The research goal was to devise and demonstrate methods of identifying load paths in light-frame wood buildings subject to environmental loads. This goal was achieved by carrying out experiments at the element level (studs, sheathings), subsystem level (shear walls) and on the whole-building level (finished and “realistic” light-frame timber buildings). The responses of these buildings to controlled static tests as well as natural environmental loads were observed and compared with a wind tunnel study and with detailed finite element models with good agreement.
Shear walls were tested in isolation and as a part of the whole structure. The tests indicated that neither the strength nor the stiffness decreased by the same magnitude as the wall effective length is reduced. For the Forintek shed, the structural monitoring was based on measurements of deformations within a representative segment of the wall and roof surfaces and a matching grid of wall and roof wind pressure taps supplemented with a wind tunnel study at Concordia University. In general, it was shown that the building surroundings had a great effect on the pressure distribution of the surface on the structure and that these effects cannot always be determined intuitively. Both mean and peak pressure coefficient were measured and they compared well with corresponding values obtained in the wind tunnel tests.
Results from controlled static loads on the UNB house indicated that the load was distributed to all walls, and significant load sharing was observed. The stiffness of the roof was sufficient to distribute load to walls farthest away from the load application point. It was also found that the internal forces are concentrated near the corners of the building. Under vertical loading on the roof, the load at the roof-to-wall interface was concentrated in a small region of the building plan around the application point. The test results also showed that the load was transferred to the transverse walls, even though there were only nominal connection between the wall and the roof trusses.
Analytical modeling results showed good agreement with the full-scale test results for shear walls as well as for the whole building. The 3-D model was able to simulate the sharing of racking forces between shear walls, based on experiments reported in the literature. In general, the errors in the numerical prediction were small. The model was able to predict the interaction between the roof system and the walls and the interactions amongst walls.
The objectives of the site visit were to document the damage to wood-frame and other wood buildings from the May 12, 2008 Wenchuan (Sichuan) earthquake and to compare the performance of wood-frame buildings with non-wood buildings of similar size.
Because of the limited number of wood-frame buildings in the affected region, all the available wood construction close to the seismically affected area was investigated as follows:
2 wood-frame houses in Dujiangyan
3 solid timber cabins in Dujiangyan
2 houses of wood-frame construction in Chengdu
6 houses of post-and-beam construction with wood-panel infill in Songpan.
The houses in Chengdu and in Dujiangyan are located, respectively, in low intensity and moderate-to-high intensity regions of shaking during that earthquake. From the inspection of the four houses and other concrete buildings nearby, it can be stated that even under light and moderate-to-high levels of seismic shaking the wood-frame houses examined suffered significantly less damage than nearby reinforced concrete houses of comparable size.
Based on a design-oriented analysis of seismic capacity it is shown that the wood-frame Houses A and B can withstand a pseudo-spectral acceleration of at least 0.89 and 1.01 g, respectively. This is judged to be a conservative estimate since the positive contribution of the exterior stucco and the second interior gypsum wall board (GWB) has not been included in the analysis.
The three timber cabins examined in Dujiangyan also performed very well, showing no signs of seismic-induced distress. The six post-and-beam wood buildings with wood-frame infill in Songpan also showed no signs of seismic damage, although for the latter the intensity of shaking was quite low.
From some examples of damaged concrete buildings, it was observed that numerous infill walls were damaged or had collapsed and thus subjected the inhabitants to mortal danger. Lightweight wood-frame infill walls for concrete frames could provide a safer alternative to the heavy and relatively brittle brick infill walls. Furthermore, the resulting reduction in building weight would further enhance seismic safety of the entire building.
It is recommended that for the Chinese code the GWB contribution be considered for normal seismic loading. However, the GWB should not be included in the design check for rare earthquakes because of the limited ductility of shear walls sheathed with GWB at the high levels of shaking associated with the rare seismic events.
Movement in structures due to environmental condition changes and loads must be considered in design. Temperature changes will cause movement in concrete, steel and masonry structures. For wood materials, movement is primarily related to shrinkage or swelling caused by moisture loss or gain when the moisture content is below 28% (wood fiber saturation point). Other movement in wood structures may also include: settlement (bedding-in movement) due to closing of gaps between members and deformation due to compression loads, including instantaneous elastic deformation and creep. Differential movement can occur where wood frame is connected to rigid components such as masonry cladding, concrete elevator shafts, mechanical services and plumbing, and where mixed wood products such as lumber, timbers, and engineered wood products are used.
Evidence from long-term wood frame construction practices shows that for typical light frame construction up to three storeys high, differential movement can be relatively easily accommodated such as through specifying “S-Dry” lumber. However, differential movement over the height of wood-frame buildings becomes a very important consideration for taller buildings due to its cumulative effect. The APEGBC Technical and Practice Bulletin provides general design guidance and recommends the use of engineered wood products and dimension lumber with 12% moisture content for floor joists to reduce and accommodate differential movement in 5 and 6-storey wood frame buildings. Examples of differential movement concerns and solutions in wood-frame buildings can also be found in the Best Practice Guide published by the Canadian Mortgage and Housing Corporation and the Building Enclosure Design Guide –Wood Frame Multi-Unit Residential Buildings published by the Homeowner Protection Office of BC Housing.
This document illustrates the causes and other basic information related to vertical movement in wood platform frame buildings and recommendations on material handling and construction sequencing to protect wood from rain and reduce the vertical movement.
Most buildings are designed to accommodate a certain range of movement. In design, it is important for designers to identify locations where potential differential movement could affect structural integrity and serviceability, predict the amount of differential movement and develop proper detailing to accommodate it. To allow non-structural materials to be appropriately constructed, an estimate of anticipated differential movement should be provided in the design drawings.
Simply specifying wood materials with lower MC at time of delivery does not guarantee that the wood will not get wet on construction sites and will deliver lower shrinkage amounts as anticipated. It is therefore important to ensure that wood does not experience unexpected wetting during storage, transportation and construction. Good construction sequencing also plays an important role in reducing wetting, the consequent wood shrinkage and other moisture-related issues.
Existing documents such as the APEGBC Technical and Practice Bulletin on 5- and 6-Storey Wood Frame Residential Building Projects, the Best Practice Guide published by the Canadian Mortgage and Housing Corporation (CMHC), the Building Enclosure Design Guide –Wood Frame Multi-Unit Residential Buildings published by the BC Housing- Homeowner Protection Office (HPO) provide general design guidance on how to reduce and accommodate differential movement in platform frame construction.
It is not possible or practical to precisely predict the vertical movement of wood structures due to the many factors involved in construction. It is, however, possible to obtain a good estimate of the vertical movement to avoid structural, serviceability, and building envelope problems over the life of the structure.
Typically “S-Dry” and “S-Grn” lumber will continue to lose moisture during storage, transportation and construction as the wood is kept away from liquid water sources and adapts to different atmospheric conditions. For the purpose of shrinkage prediction, it is usually customary to assume an initial moisture content (MC) of 28% for “S-Green” lumber and 19% for “S-Dry” lumber. “KD” lumber is assumed to have an initial MC of 15% in this series of fact sheets.
Different from solid sawn wood products, Engineered Wood Products (EWP) are usually manufactured with MC levels close to or even lower than the equilibrium moisture content (EMC) in service. Plywood, Oriented Strand Board (OSB), Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL), and Parallel Strand Lumber (PSL) are usually manufactured at MC levels ranging from 6% to 12%. Engineered wood I-joists are made using kiln dried lumber (usually with moisture content below 15%) or structural composite lumber (such as LVL) flanges and plywood or OSB webs, therefore they are usually drier and have lower shrinkage than typical “S-Dry” lumber floor joists. Glued-laminated timbers (Glulam) are manufactured at MC levels from 11% to 15%, so are the recently-developed Cross-laminated Timbers (CLT). For all these products, low shrinkage can be achieved and sometimes small amounts of swelling can be expected in service if their MC at manufacturing is lower than the service EMC. In order to fully benefit from using these dried products including “S-Dry” lumber and EWP products, care must be taken to prevent them from wetting such as by rain during shipment, storage and construction. EWPs may also have lower shrinkage coefficients than solid wood due to the adhesives used during manufacturing and the more mixed grain orientations in the products, including the use of cross-lamination of veneers (plywood) or lumber (CLT). The APEGBC Technical and Practice Bulletin emphasizes the use of EWP and dimension lumber with 12% moisture content for the critical horizontal members to reduce differential movement in 5 and 6-storey wood frame buildings.