Wood shearwall deflection questions 1

[GA_Engineer]

I have a few questions on what others do in calculating the deflection of a wood shearwall. In my specific case I am looking at the deflection due to wind.

1. In the 3-term deflection equation in SDPWS, it uses Ga (apparent stiffness). For seismic conditions this value is given in the nominal shearwall capacity table. However for Wind it is not given. When checking deflection for wind do people generally use the 4-term equation per the SDPWS commentary or just use the stiffness given for seismic for wind as well?

2. Within the equation for shearwall deflection, we must take into account the deflection/elongation of the hold down/anchorage at the end of the wall. For the hold downs at the foundation, this is given to me by Simpson for the hold downs required. However , I have a multi-story condition with strapping between floors at the end of the walls. Where can I find the values for straps as I’m not seeing it in the literature?

 

[Yao1989]

Can't help with 1, as in Canada we just use same Bv for both wind and seismic.

for 2, you should be able to estimate axial deformation using length & load, although there may be some nail slippage that's difficult to account for, so your best bet is to contact simpson.

With that said, straps doesn't do well with platform framing due to shrinkage; they are usually more suitable for engineered floor system which has less shrinkage. I hope you've already taken in consideration. I have used straps single-family homes with dimensional lumber before and have not gotten in trouble yet (probably not a great idea in hindsight). I typically ask the contractor to make sure they install the strap as late as possible for single family homes, but multi-story is a different story (unintentional pun).

 

[GA_Engineer]

@Yao1989 We are indeed using an engineered floor system with engineered wood trusses and LSL rim joists, which should limit shrinkage considerably.

 

[Celt83]

engineered wood trusses ...... which should limit shrinkage considerably.

If this is platform framed (Trusses bearing directly on the walls) then the shrinkage is actually increased due to the addition of 2 or more extra 2x plates making up the truss top and bottom chords.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[GA_Engineer]

If this is platform framed (Trusses bearing directly on the walls) then the shrinkage is actually increased due to the addition of 2 or more extra 2x plates making up the truss top and bottom chords.

The pre-engineered trusses that bear on the wall should have an end vertical member which would limit the wood shrinkage. Even if there isn't a vertical member at the end of the wood floor truss, the addition of 2 or 3 more top chords (at 1.5" thick) would still have less shrinkage than a floor using typical solid sawn floor joists (8"-12" deep nominally)

 

[JoshPlumSE]

1. In the 3-term deflection equation in SDPWS, it uses Ga (apparent stiffness). For seismic conditions this value is given in the nominal shearwall capacity table. However for Wind it is not given. When checking deflection for wind do people generally use the 4-term equation per the SDPWS commentary or just use the stiffness given for seismic for wind as well?

**The SDPWS commentary gives an example for how to calculate the apparent shear stiffness Ga.... See Example C4.3.2-1. It just uses the nail slip values from the old 4 term equation to calibrate the shear stiffness term to the level of nail slip (which is more non-linear than the other terms) to the expected load levels. **

2. Within the equation for shearwall deflection, we must take into account the deflection/elongation of the hold down/anchorage at the end of the wall. For the hold downs at the foundation, this is given to me by Simpson for the hold downs required. However , I have a multi-story condition with strapping between floors at the end of the walls. Where can I find the values for straps as I’m not seeing it in the literature?

**I don't think you'll find it in the literature. It shouldn't be all that hard to develop a "ball park" number for your self based on the PL/AE of the strap material itself + some fudge factor based on nailing or screws or such. Personally, I think this is a weakness of the provisions.... Or, more accurately something that should be better addressed in design examples or such. **