DRAG STRUT? 4

[nrguades]

north
A B C

xxxxxxx xxxxxxxxxxx xxxxxxx xxxxxx 1
y y y
y y y
y y y
xxxxxxx xxxxxxxxxxxx xxxxxxxxxxx 2
CORRIDOR
xxxxxxx xxxxxxxxxxxx xxxxxxxxxxx 3
y y
y y
y y
xxxxxxx xxxxxxxxxxx xxxxxxx xxxxxx 4

south


Wooden floor trusses are to be installed parallel with the partition wall (y's) making this walls, non-load bearing but acting as shearwall against N - S wind direction. Along shearwall line B, on the southern side, no shearwall can be provided due to architectural space requirement. Is it necessary to provide drag strut along line B on the southern side? thanks...

 

[JAE]

Yes, I would think so - otherwise your diaphragm has an abrupt change in load at the end of the B wall - possible stress concentration and tearing.

 

[jtstruct]

I agree with JAE. You will need some sort of mechanism for developing the shear loads into your diaphragm. Based on your sketch, you appear to have two relatively close shear walls on either side, so the shear load may not be too significant. You may be able to get away with using a joist or beam in line with your shear wall and using the appropriate straps and nailing pattern to get the load into the diaphragm. Just a matter of checking the numbers to see what you need.

 

[haynewp]

Assuming all wall lengths are equal and of equal strength per foot, and assuming a flexible diaphragm:

Would the diaphragm work if there is no shear wall along B at all? If it would, then I don't think that having a wall at B on only the northern side makes a worse case does it?

If the diaphragm relies on the shear wall at B on the northern side to work, then yes I would put in a drag strut.

 

[nrguades]

Thanks guys for confirming that in this particular case it is safer to put a drag strut inorder to ensure deformation compatibility and prevent localized tearing of diaphragm. But supposing we are not goin to put any collector elements and no chord to resist the bending moments, how will we check the bending moment capacity of the diaphragm since the IBC will just give us only the shear capacity of the diaphram.

 

[JAE]

Well, we usually use the perimeter collectors as chords that resist the lateral bending in the diaphragm - not depending on the "web" of the diaphragm at all.

 

[nrguades]

thanks JAE, perhaps it is really necessary to neglect the contribution of the plywood sheathing to the flexural capacity of the diaphragm acting as a deep beam and just let the boundary members acting as chord such as double top plate of the wood frame to resist the total applied moment. I just wonder if somebody have tried designing without the aid of this elements and how did they do it.

 

[pmkPE]

why not ignore "B" alltogether and allow "A" and "C" on the north and south sides to take the loads. Drag struts will be required over the corridor. If you can evaluate the stresses in the diaphram and they check out then the question is moot. Otherwise add drag strut(s).

 

[JAE]

pmkPE - Yes I can generally agree with your statement - but let me suggest this also, that the mere presence of wall B, connected to the roof, will have some effect - it has some stiffness whether you design it or not. I would think that you need to be a little careful about ignoring stiffness in your structure as it does affect the behavior -

A drag strut might possibly be used along B anyway to keep the B interference "soft" and not create any tears at the end of B.

 

[ChipB]

Personally, I fully agree with pmkPE. I'd minimumly attach it to the roof and call out on the plan for the plywood not to end on it (nail spacing reasons). Gyp both sides as required and send it on its (flexible) merry way.

The other walls should have boundary nailing called out on the plan in order for the shear to go to them and not fully to the outside walls.

A fully sheathed shearwall is significantly stiffer than a partition wall, therefore, that's where the load is going to go.

If you put a drag strut in and treat B as a shearwall, you'll have (approx) double the shear in it as the other two, thereby affecting your nail spacing and anchorage spacing.