New Zealand Society for Earthquake Engineering Conference
Research Status
Complete
Notes
April 13-15, 2012, Christchurch, New Zealand
Summary
Driven by sustainability, locally available resources and expertise, and economy, the design of the Carterton Events Centre focused on timber for the majority of the main structural and non-structural components. Combined with a client desire for minimization of earthquake damage, dissipative post-tensioned rocking Laminated Veneer Lumber (LVL) shear walls (Pres-Lam) were considered for the lateral load resisting system. During design development various structural forms were explored and tested through costing to determine an economic design solution meeting the project drivers. Advanced numerical analyses carried out by the University of Canterbury validated the design process assuring confidence with the design of the technology.
Wood-frame is the most common construction type for residential buildings in North America. However, there is a limit to the height of the building using a traditional wood-frame structure. Cross-laminated timber (CLT) provides possible solutions to mid-rise and high-rise wood buildings. CLT offers many advantages such as improved dimensional stability, a quicker erection time and good performance in case of fire. In order to introduce the cross-laminated timber products to the North American market, it is important to gain a comprehensive understanding of its structural properties. This paper focuses on the seismic performance of CLT connections. Over the last few years FPInnovations of Canada has conducted a test program to determine the structural properties of CLT panels and its application in shear walls. The test program comprised of more than 100 connection tests which followed the loading procedures of CUREE and ISO test protocols as specified in ASTM Standards ASTM E 2126-09 (2009). These tests were performed parallel and perpendicular to the grain of the outer layer, respectively. The impact of different connections on the seismic performance of CLT walls was investigated in a second phase on full size shearwall. CLT panels are relatively stiff and thus energy dissipation must be accomplished through the ductile behaviour of connections between different shear wall elements and the connections to the story below. A literature review on previous research work related to damage prediction and assessment for wood frame structures was performed. Different approaches for damage indices were compared and discussed. This paper describes how the energy-based cumulative damage assessment model was calibrated to the CLT connection and shear wall test data in order to investigate the damage under monotonic and cyclic loading. Comparison of different wall setup provided a deeper insight into the damage estimation of CLT shear walls and determination of the key parameters in the damage formulation. This represents a first published attempt to apply the damage indices to estimate the seismic behaviour of CLT shear walls.
Källsner and Girhammar have presented a new plastic design method for wood-framed shear walls at ultimate limit state. This method allows the designer to calculate the load-carrying capacity of shear walls partially anchored, where the leading stud is not fully anchored against uplift. The anchorage system of shear walls is provided from anchor bolts and hold downs. Anchor bolts provide horizontal shear continuity between the bottom rail and the foundation. Hold downs are directly connected from the vertical end stud to the foundation. When hold downs are not provided, the bottom row of nails transmits the vertical forces in the sheathing to the bottom rail (instead of the vertical stud) where the anchor bolts will further transmit the forces into the foundation. Because of the eccentric load transfer, due to forces acting in the same vertical plane, transverse bending is created in the bottom rail and splitting often occurs. It is important to evaluate this cross-wise bending and to ensure that no brittle failure occur in the bottom rail.
The bottom rail is experimentally studied with respect to two primary failure modes, splitting along the bottom of the bottom rail due to cross-wise bending and splitting along the edge side of the bottom rail due forces perpendicular to the grain from the sheathing-to-framing connections. The parameters varied are the size of the washer and the orientation of the pith. The bottom rail was subjected to loading perpendicular to grain through two-sided sheathing. In this report the different set of series are presented. Five sets were conducted depending on the size of the washer and in each set the pith was placed upwards and downwards.
The tests showed three different failure modes. In addition to the failure modes that the testing program was aimed at, splitting along the bottom or side of the bottom rail, the final failure was also due to plastic bending and withdrawal of the sheathing-to-framing nails. The results show that the size of the washer has a significant influence on the maximum load and the failure modes. The results show also that the orientation of the pith have a significant influence on the maximum load.
Källsner and Girhammar have presented a new plastic design method for wood-framed shear walls at ultimate limit state. This method allows the designer to calculate the load-carrying capacity of partially anchored shear walls, where the leading stud is not anchored against uplift. The anchorage system of shear walls is provided by anchor bolts in the bottom rail and hold downs at the leading stud. Anchor bolts provide horizontal shear continuity between the bottom rail and the foundation. Hold downs are directly connecting the vertical leading stud to the foundation. Sometimes hold downs are not provided and only the bottom rail is anchored to the substrate. In this case the bottom row of nails transmits the vertical forces in the sheathing to the bottom rail (instead of the stud) where the anchor bolts will further transmit the forces into the foundation.
In this report hold downs have been experimentally studied with respect to the strength and stiffness of the connection. Four different types of hold downs have been tested. The specimen was subjected to tension load applied to the stud. Four tests series are presented. Each series was divided into different sets according to the type of fastener used with the hold down device.
The results show that the failure load is higher when hold downs with anchor bolts are used, up to ten times higher than the anchorage that uses only screws or nails. The failure mode vary with the type of hold down and the type of fasteners used. The tests showed three primary failure modes: failure of the stud when a bolt is used as the fastener between hold down device and stud, failure due to pull-out of the screws or nails from the rail and failure due to failure or pull-out of screws or nails from stud. Also, failure of the stud itself occurred in some tests caused by some defect of the timber.
New Zealand Society for Earthquake Engineering Conference
Research Status
Complete
Notes
April 13-15, 2012, Christchurch, New Zealand
Summary
The Nelson Marlborough Institute of Technology Arts and Media building was completed in 2011 and consists of three seismically separate complexes. This research focussed on the Arts building as it showcases the use of coupled post-tensioned timber shear walls. These are part of the innovative Expan system. Full-scale, in-situ dynamic testing of the novel building was combined with finite element modelling and updating to obtain an understanding of the structural dynamic performance within the linear range. Ambient testing was performed at three stages during construction and was combined with forced vibration testing for the final stage. This forms part of a larger instrumentation program developed to investigate the wind and seismic response and long term deformations of the building. A finite element model of the building was formulated and updated using experimental modal characteristics. It was shown that the addition of non-structural elements, such as cladding and the staircase, increased the natural frequency of the first mode and the second mode by 19% and 24%, respectively. The addition of the concrete floor topping as a structural diaphragm significantly increased the natural frequency of the first mode but not the second mode, with an increase of 123% and 18%, respectively. The elastic damping of the NMIT building at low-level vibrations was identified as being between 1.6% and 2.4%.
This study investigates the in-plane stiffness of CLT floor diaphragms and addresses the lateral load distribution within buildings containing CLT floors. In practice, it is common to assume the floor diaphragm as either flexible or rigid, and distribute the lateral load according to simple hand calculations methods. Here, the applicability of theses assumption to CLT floor diaphragms is investigated. There is limited number of studies on the subject of in-plane behaviour of CLT diaphragms in the literature. Many of these studies involve testing of the panels or the connections utilized in CLT diaphragms. This study employs numerical modeling as a tool to address the in-plane behaviour of CLT diaphragms. The approach taken to develop the numerical models in this thesis has not been applied so far to CLT floor diaphragms. Detailed 2D finite element models of selective CLT floor diaphragm configurations are generated and analysed in ANSYS. The models contain a smeared panel-to-panel connection model, which is calibrated with test data of a special type of CLT connection with self-tapping wood screws. The floor models are then extended to building models by adding shearwalls, and the lateral load distribution is studied for each building model. A design flowchart is also developed to aid engineers in finding the lateral load distribution for any type of building in a systematic approach. By a parametric study, the most influential parameters affecting the in-plane behaviour of CLT floor diaphragm and the lateral load distribution are identified. The main parameters include the response of the CLT panel-to-panel connections, the in-plane shear modulus of CLT panels, the stiffness of shearwalls, and the floor diaphragm configuration. It was found that the applicability of flexible or rigid diaphragm assumptions is primarily dependent on the relative stiffness of the CLT floor diaphragm and the shearwalls.
This building is a typical one-storey commercial building located in Vancouver, BC. The plan dimensions are 30.5 m x 12.2 m (100’ x 40’), with a building height of 5 m. The walls are wood-based shear walls, with a wood diaphragm roof and a steel moment frame at the storefront. The roof plan is shown in Figure 1. The site is Seismic Class ‘C’. Wind, snow and seismic figures specific to the project location are taken from the current version of the British Columbia Building Code (2012). Roof dead load is assumed to be 1.0 kPa and the wall weight is 0.5 kPa. The weight of non-structural items including mechanical equipment and the storefront façade has not been included in this example for simplicity.
The 2009 edition of CSA Standard O86, Engineering Design in Wood (CSA 2009), provides an equation for determining the deflection of shear walls. It is important to note that this equation only works for a single-storey shear wall with load applied at the top of the wall. While the equation captures the shear and flexural deformations of the shear wall, it does not account for moment at the top of the wall and the cumulative effect due to rotation at the bottom of the wall, which would be expected in a multi-storey structure.
In this fact sheet, a mechanics-based method for calculating deflection of a multi-storey wood-based shear wall is presented.
Figure 1 shows a floor plan and elevation along with the preliminary shear wall locations for a sixstorey wood-frame building. It is assumed some preliminary calculations have been provided to determine the approximate length of wall required to resist the lateral seismic loads.
If the preliminary design could not meet the drift limit requirement using the base shear obtained based on the actual period, the shear walls should be re-designed until the drift limit requirement is satisfied.
Utilizing Linear Dynamic Analysis (LDA) for designing steel and concrete structures has been common practice over the last 25 years. Once preliminary member sizes have been determined for either steel or concrete, building a model for LDA is generally easy as the member sizes and appropriate stiffness can be easily input into any analysis program. However, performing an LDA for a conventional wood-frame structure has been, until recently, essentially non-existent in practice. The biggest challenge is that the stiffness properties required to perform an LDA for a wood-based system are not as easily determined as they are for concrete or steel structures. This is mostly due to the complexities associated with determining the initial parameters required to perform the analysis.
With the height limit for combustible construction limited to four stories under the National Building Code of Canada, it was uncommon for designers to perform detailed analysis to determine the stiffness of shear walls, distribution of forces, deflections, and inter-storey drifts. It was only in rare situations where one may have opted to check building deflections. With the recent change in allowable building heights for combustible buildings from four to six storeys under an amendment to the 2006 BC Building Code, it has become even more important that designers consider more sophisticated methods for the analysis and design of wood-based shear walls. As height limits increase, engineers should also be more concerned with the assumptions made in determining the relative stiffness of walls, distribution of forces, deflections, and inter-storey drifts to ensure that a building is properly detailed to meet the minimum Code objectives.
Although the use of LDA has not been common practice, the more rigorous analysis, as demonstrated in the APEGBC bulletin on 5- and 6-storey wood-frame residential building projects (APEGBC 2011), could be considered the next step which allows one to perform an LDA. This fact sheet provides a method to assist designers who may want to consider an LDA for analyzing wood-frame structures. It is important to note that while LDA may provide useful information as well as streamline the design of wood-frame structures, it most often will not be necessary.
