Step three: Distribute loads to shear walls
The Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) put together a best practices guide to assist engineers in the design of these types of buildings.(More information can be found in the guide, Structural, Fire Protection and Building Envelope Professional Engineering Services for 5 and 6 Storey Wood Frame Residential Building Projects (Mid-Rise Buildings), published by APEGBC) Section 3.5.2 (j) explicitly states both flexible and rigid analysis be performed to determine the maximum loads on each shear wall. If the rigid diaphragm forces increase more than 15 per cent when compared to the flexible case, design for the envelope of forces is required.
The flexible diaphragm distribution is straightforward and only requires one iteration. The flexible diaphragm depends on the tributary area on each shear wall. Once the shear wall layout is provided, this tributary area never changes (unless shear walls are modified). The rigid diaphragm, on the other hand, becomes very onerous. In the rigid diaphragm case, the floor is assumed to move as one entity. Thus, the deflections of all shear walls should be the same, regardless of material properties, force, and length of each wall. The problem becomes challenging because the deflections of the shear walls are dependent on the load and stiffness in the wall. Each time a shear wall is modified (i.e. stiffness or length changes), this process must repeat itself until the deflections converge (Figure 3).
In addition to the load distribution analysis, the forces dissipated in a flexible versus rigid diaphragm load case are different (Figure 4). For a simple two-span floor with three shear walls, the forces for each case could be either over- or underestimated. For the exterior wall, the flexible analysis would underestimate the wall force while for the interior shear wall the flexible load case would govern. It became clear through research and analysis that both the rigid and flexible load distributions were mandatory in the design of six-storey LWWF buildings.
Step four: Evaluate for deflection and strength
Once the loads are properly distributed to each shear wall, the next step is to check the strength and deflection elements to determine whether the walls perform as intended.
Strength
Shear loads are cumulative through the shear wall. In simple terms, the shear load at the sixth floor is the ‘least,’ while the load at the bottom floor is the ‘largest.’ The shear properties of a wood panel shear wall vary depending on the following properties:
- sheathing thickness, material (i.e. oriented strandboard [OSB] or plywood), wood panels on one side of wall or on both sides of wall;
- nail spacing (i.e. 152-mm [6-in.] spacing versus 102-mm [4-in.] spacing, etc.); and
- nail size (i.e. length) or shank diameter.
The moment component is somewhat similar in nature. The moment is ‘least’ at the sixth floor and gathers at the bottom floor where it becomes the largest. The moment resistance of a wood panel shear wall is resisted by the tension/compression couple at the end of the shear walls, where the tension and compression loads are equal to the moment. The tension/compression load—T6/C6—would be much less than the tension/compression load of T1/C1, as noted in Figure 5.
Tension and compression loads are designed in various ways. For six-storey buildings, the design and research team found a continuous threaded rod tie-down system helps control building drift and deflection much better than other traditional tie-down systems used for smaller four-storey buildings. Additionally, the tension loads found in six-storey buildings are much higher than that of four-storey buildings, requiring the use of this tie-down system for tension. The compression value is resisted by wood posts on either side of the tension tie (Figure 6).
Image courtesy Simpson Strong Tie
