Braced Steel Commercial Building 3

[JrStructuralEng]

Perhaps this is already posted somewhere (I looked)

But I heard that in a steel framed building it is only neccesary to brace 3 of the 4 sides (i.e. with diagnol steel bracing in 3 bays of a rectangle building) . The building in question is ~50ftx150ft. Can the roof diaphram act as a 'beam' and distribute the load.

Any thoughts?

 

[JAE]

1. The method I posted assumes a rigid diaphragm. I suppose you could have a somewhat less than perfectly rigid diaphragm and it would still work, but as the flexibility increases, the distribution of the forces to each brace becomes less accurate.


2. R is the stiffness of the member and the design method works if you correctly use "relative" rigidities in terms of getting a force in each brace. For deflection calculations you must use actual rigidities. Normally, a rigidity can be determined by placing a 1kip load at the top of the brace and determining its deflection. This is the flexibility in terms of kips/inch. Then take 1/[Δ] to get the stiffness, R (some use the term k) in terms of inches/kip.

3. V is all the forces acting on the diaphragm at that story, either wind or seismic.

 

[hokie66]

Without a rigid diaphragm, I would use diagonal roof bracing. Then just analyze the whole thing with your 3D frame program, which will give you the deflections. To control the deflections to an accecptable amount, you then adjust the member sizes.

 

[DaveAtkins]

I think we have all digressed quite a bit from the original post (not that there's anything wrong with that :-D

For a three sided diaphragm, relative stiffness doesn't come into play. And although most textbooks and college professors might say a steel deck is a flexible diaphragm, I have no problem with a 150' deep diaphragm being considered rigid.

For the problem originally posed, all of the lateral force on the 50' side of the building will be resisted by the braced 150' side. Since there is an eccentricity (of 25') between the load and the resisting force, a moment is created in the diaphragm, equal to the force times 25'. This force is resisted by a force couple (150' between the forces) at the two perpendicular walls.



DaveAtkins

 

[JrStructuralEng]

Dave, that was exactly what I was looking for! So relative stiffness only comes into effect with a flexible diaphram?

To further your discussion, would you size the single brace (the one on the 150' side) to take approximately 25' of loading? With other 25' being taken out by the moment couple

i.e.

______
| |
| |
| |
| |
| |
| |
| |
| |
| |
|______|

 

[JrStructuralEng]

whoops hit submit instead of preview. here is my drawing again:

brace _/___
| |
| |
| |
| |
no | |
brace | |
| |
| |
| |\brace
|______|
/ brace

 

[csd72]

JrStructuralEng,

No you have misunderstood the recent comments.

Relative stiffness is important for a rigid diaphragn not a flexible one, but as this is a statically determinate diaphragm, then it makes no difference to load distribution in this case.

100% of the wind load gets taken out by the wall perpendicular to the wind.

It is the moment from the 25' eccentricity (between center of load and center of reaction) that gets taken out by the two short walls in a moment couple.

 

[JAE]

So relative stiffness only comes into effect with a flexible diaphram?

No - relative stiffness in your braces is especially important with rigid diaphragms. With flexible diaphragms the load is usually apportioned to the braces based on tributary width and no brace stiffness..... this for buildings with braces on four sides.

For buildings with a 3 sided brace system (the point of your original post) you must have some degree of rigidity in your diaphragm to make it work. For theoretically "pure" flexible diaphragms, this system doesn't work....but as Dave states, large metal deck diaphragms have some level of rigidity to transmit the rotational diaprhagm moment to the orthogonal braces.

 

[JrStructuralEng]

Thanks,

What would be an example of a flexible diaphram? And I'm assuming relative stiffness is still my tool for figuring out deflections with a partially stiff diaphgram?

 

[JAE]

Most engineers assume a metal deck roof is flexible. Also plywood sheathed decks are assumed flexible. For large metal decks, you can calculate the stiffness (G') and proceed with the "partial" rigid deck assumption but most just assume flexible and go with that. If you have only the three sided brace condition, then you must utilize the deck stiffness as a partially rigid deck.

 

[TimmyH76]

Could you ( and would you ) consider a plywood sheathed deck ridgid if you used metal strapping across it?

 

[JAE]

I've never done so...guess I'd have to think about it for a while. The strapping tends to be limited in size and still stretches a bit.

I think what I would do is look at the guidelines in the IBC or ASCE 7 where there is a definition (I think) of a limiting deflection ratio for the diaphragms in terms of defining whether they are rigid or flexible.

 

[haynewp]

What moment do you plan on designing the chords for on the 3 sided diaphragm?