load distribution -- wood sws in same line

[mpete]

A peer reviewer is stating that loads to wood shear walls in a single line must be distributed per wall stiffness instead of per wall length. He cites AF&PA '08 sect 4.3.3.4. I cannot find design examples using this method, nor much discussion on the topic.

Any direction pls?

 

[jdgengineer]

The plan reviewer is correct, typically the shear is distributed to wood shearwalls on the same line based on their stiffness. Essentially, to calculate stiffness you need to apply a unit load to the shearwalls to determine how much they deflect. The inverse of this is their stiffness. Add up all the stiffnesses on the line and then distribute the force to each shearwall based on the ratio of their stiffness / total wall line stiffness.

Section 4.3.2. of the NDS SDPWS provide a method to calculate deflection of wood shearwalls. Alternatively, you can use the deflection shown in section 2305.3 of IBC.

 

[JAE]

In many (most?) cases, length of a shearwall is proportional to its stiffness.

 

[JAE]

...assuming they are constructed the same.

 

[mpete]

...I understand the concept of calculating stiffness. However, in my limited experience, this method of distribution in a single line has typically been applied to rigid (ie: conc, masonry) shear walls.

It seems to me that applying this method to wood shear walls would result in a long iterative process, given the substantial design changes due to redistributing load to wall panels. I, personally, do not know any engineers using this method.

 

[jdgengineer]

We always use this method. Although, it is not done by hand it is done through MathCAD...

As JAE noted, you can probably get 80-90% of the stiffness by looking at the big ticket items. Most of the deflection will be governed by the length of the wall and the boundary elements. From what I remember (and I could be wrong) the nail slip and holdown deflection are usually small in comparison. I would guess that depending on your proportions, 70% of deflection is governed by length of wall, 20% by boundary elements, 5-10% by holdown & nail slip. Keep in mind I made up those percentages. Either way, plan reviewer would probably be ok with neglecting the smaller contributions if you are doing this by hand to avoid iterations.

 

[RFreund]

I see as 2 reasonable ways of going about this.

You also could use the Ga term and multiply by length in the NDS 2005 tables and in the 2008 Wind and Seismic I believe. The Ga would account for nail pattern (nail slip) and sheathing used. (There may be a reduction for large aspect ratios). Or better yet use the 3 term deflection equation and given in this document found here:

http://www.awc.org/pdf/2008WindSeismic.pdf

You could possibly forgo hold down slip and then use the same hold down for all walls.

EIT

 

[abusementpark]

A peer reviewer is stating that loads to wood shear walls in a single line must be distributed per wall stiffness instead of per wall length. He cites AF&PA '08 sect 4.3.3.4. I cannot find design examples using this method, nor much discussion on the topic.

You have a peer-reviewer on a wood-framing building? Never heard of that. How many stories is the building?

For wood shear walls, everyone I know distributes the load based on length of the wall. And I seriously doubt that use of that method has ever been the reason for failure.

 

[EngineeringAdam]

I agree that distributing load based on length of wall is acceptable, but I too use the full deflection equation in the IBC.

If the plan reviewer wont budge I would use excel and enter the equation for wood shear wall deflections,

This way, you can quickly complete your analysis for each variation of wall.

 

[EngineeringAdam]

I actually should clarify my comment above before I give someone the wrong idea.

Distributing the load based on the length of walls should only be considered acceptable if the walls are fairly similar. Specifically the boundary elements.

If you have a two shear walls that are identical in length and nailing but their boundary elements are vastly different, e.g. 4x4 post versus a 4x12 post the actual deflections can vary by a factor of 2 or so. So, IMO, if you find that you need large boundary elements to stiffen some walls and not others I consider it mandatory to determine deflection based on the IBC formula.

 

[msquared48]

Interesting variation to the increased forces due to Rho in the older IBC's where the value varied from 1.0 to 1.6, and the largest rho value was applied to the whole story.

In effect, the weakest shearwall line controlled the design of the whole story for that direction.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com