Are Drag Struts Always Required?

[JIMEY]

When doing wood frame design, is it always necessary to have drag struts going the full depth of the diaphragm? The standard practise in my office is to check if the diaphragm has enough strength not to fail assuming that it is only as deep as the shearwall. If this is true then no drag struts are required. Let me give an example. Let's say we have a simple box building. The diaphragm is 10m deep. It has shearwalls at each end that are only 2m long. There is 3 kN/m of shear load in the diaphragm. If I pretend the diaphragm is only 2m deep, my shear load gets bumped up to: 3 / 2 x 10 = 15kN/m. Now if my diaphragm could take this load, then I don't need to worry about drag struts extending out beyond the shearwalls.

However, I've been reading in many books and websites that you need to have drag struts extending to the end of the diaphragm regardless of how much capacity your diaphragm has. I proposed this to some of my co-workers who all told me I was crazy. I was just wondering how the rest of you guys design your diaphragms.

 

[msquared48]

Two things:

1. Don't forget the aqspect ratio limitations of your diaphragm(s).

2. Your deflections of the diaphragm(s) will be greater, and may tend to act independently (different frequencies) in a seismic event, ripping the roofing and causing leaks.

Personally, I would not do it.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

A third thing to consider is that due to the decreased diaphragm depth, your chord forces to resist will be higher - in your case by a factor of .5X (10^2/2^2) or 12.5 times as great.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[OldPaperMaker]

JIMEY,
The short answer to your question about drag struts (or collectors) is "yes".All diaphragms have boundary members; the chords are perpendicular to the direction of the load and the drag struts or collectors are parallel to the direction of the load.

The purpose of the drag struts is to drag the load from the diaphragm to the shearwalls below. Normally, the chords and drag struts are designed by using the rim board or double top plate at the level of the diaphragm.

 

[msquared48]

Actually, when you think about it, along each and every side of the roof diaphragm is a chord tie that functions as a tension/compreswsion member for the diaphragm when it acts as a deep beam in each major direction of loading. These members are also the drag struts you refer to.

So tho think that you can break these drag struts up, only having the drag collectors at the shear walls, is really just kidding yourself.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dcarr82775]

I don't think you always need drag struts. Lets say I have an apartment building where there are all sorts of partial length walls staggered all about the floor plate that I am using as shearwalls. I would not call for drag struts across the full width of the diaphragm at every wall. Where the diaphragm nailing along the length of the wall isn't sufficient I would add a drag strut.

Now if I have a building with few shearwalls, say just one at each end, then I probably would use a drag strut but the basic construction of such a building almost gives you a drag strut anyway.

 

[JIMEY]

A lot of different ideas. I suppose I will just have to use my judgement. Thanks to all who responded.

 

[OldPaperMaker]

JIMEY,
If you are in the US and you want to know more about the design of diaphragms, chords, collectors (drag struts) and shearwalls I suggest that you purchase the book, "Design of Wood Structures, ASD/LRFD" by Donald Breyer, Kenneth Fridley, Kelly Cobeen, David Pollock. It cost about $65 at Amazon. It will guide you through these subjects.

 

[MiketheEngineer]

BTW - very GOOD book