LATERAL 3

[haynewp]

I am reviewing existing structural dwgs of retail shops where knee brace frames were used in every bay along the front and there are masonry shear walls on other 3 sides. We usually rack the lateral force through the roof diaphragm into the rear walls in such situations. Does anyone know why the lateral force couldn't have been racked in a single story bldg that is 20'high X100'X100' in a non seismic area with 80 mph wind with a metal roof diaphragm? Besides less deflection, what else would be gained by using these frames?

 

[JAE]

haynewp:
Some engineers get concerned about the lateral drift that racking the diaphragm produces on the open end. Even with just wind load, the size of the building could be producing lateral drifts in excess of L/400 (or whatever limit is desired). Especially if there is a masonry veneer, a concern for cracking, and a metal deck diaphragm with low rigidity (shear stiffness).

Other than that, I can't think of anything other than the engineer just doesn't like to use the twist of the building to transfer the shear.

 

[phuduhudu]

I think I would be nervous too of having only point of lateral stability so far from the center of action of the force, probably for the reasons mentioned by JAE. 100' is quite a long wall. Carl Bauer

http://www.bauerconsultbotswana.com

 

[Ron]

Sounds like a "when in doubt, make it stout" approach. With only an 80 mph basic wind speed and no seismic, the only reason I can see for knee braces (which are rare in our area, which has much higher wind speeds)is the use of spindly columns and an assumption of low diaphragm stiffness. Even conservatively, the lateral load at the deck level is fairly low.

 

[dik]

With a retail shop... could be that the knee braced wall is filled with ductile glazing... just a thought <G>.

 

[LPPE]

I agrree with JAE. Probably the engineer didn't like the torsional effect.

 

[haynewp]

Thanks for advice, there are about 15 of these type shops w/glass fronts that will be going up. I checked one shop (100'deep by 120'sidewall length) which gave 1.03&quot; deflection with a 36/4/4 roof pattern, that is H/230 (H=wall height at front). Any thoughts?

 

[haynewp]

Also, there isn't any veneer along the fronts so I am mainly worried about the glass and it's enclosures. Exactly how &quot;ductile&quot; could these be? I don't have any add'l information on glass and attachments.

 

[JAE]

haynewp,
L/230....seems large to me. I checked a table that I use which came from AISC Engineering Journal (First Quarter, 1993) and for an exterior cladding of window wall, they recommend L/400. The footnote indicates that higher deflections could be allowed, but the cladding should be tested with a mock-up for water-tightness. The concern here is for the distress in the sealant and the ability of the connections of the windows to accept that kind of distortion.

1.03&quot; is the deck movement, the top of the window may be proportionately less, but still in the 3/4&quot; range. This would seem to be excessive for a typical 3/8&quot; or 1/2&quot; joint sealant.

 

[dik]

Usually use l/360 as an absolute minimum, and often l/600 for glazing. The only method of accommodating large deflections with glazing is to make the glazing frame independent of the opening and secure it against lateral movement by connection details and you may have to use large caulked joints and possible joint covers.

Good joints are expensive...

 

[haynewp]

Yes, I have decided to go with a more stringent, probably L/400, it may be actually less expensive in some of the shops that are (longer than they are deep) to go ahead and put in braced frames at the front instead of a rigid enough diaphragm to meet L/400 or better. Thanks for help.

 

[dik]

haynewp:
It's often less expensive to detail for movement than to increase the stiffness...

Might also want to consider using a trussed header over the window area.

If an architect is involved, then you should advise him of deflections anticipated.