Braced Steel Commercial Building
Braced Steel Commercial Building
(OP)
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?
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?






RE: Braced Steel Commercial Building
I would never skip one of the 50' sides as this would be a really bad aspect ratio.
If you have glass on the unbraced wall then deflection of the front face would be an issue.
I would recommend you avoid if at all possible as it is more trouble than it is worth.
RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
With twist, be aware that the opposite sides of the buildng will deflect in the same direction due to overall drift plus different directions from the twist. Thus, some sides of the building will experience greater story drift than other sides and window-walls, etc. should be carefully detailed to account for this.
RE: Braced Steel Commercial Building
The more I learn about structural engineering, the more I learn I don't know.
RE: Braced Steel Commercial Building
We all have more to learn, If you ever meet a structural engineer that claims to know everything then steer clear of him, he is an arrogant fool.
The most important thing is to realise when you are out of your depth, and to seek help.
A good engineer is confident in what they know and is aware of what they dont know.
Anyway, tell us a bit more about your building. Is it portal frame in one direction and braced in the other or just braced in both directions, is it metal clad or does it have concrete panels?
Here is a good place to start:
ht
RE: Braced Steel Commercial Building
You asked for a bit more info on my building. Well its 50ftx150ft as I said. The one 50' side has a portal frame. The other 50' side has a diagnol HSS for brace. One of the 150' sides also has a diagnol HSS for brace. The other 150' side has no braces as it is mostly windows and there is no room for one. The building is ~22ft high, conventional steel construction, wide flange ridge beams with HSS columns and steed stud framing. Not sure what panel material is yet. I believe metal or glassboard.
Roof is OWSJ with metal deck.
RE: Braced Steel Commercial Building
Thanks for the complement, but I'm here at Eng-Tips precisely because of what csd72 states...to learn. I certainly don't know everything and I'm grateful for the vast flow of info that comes into this site...again, it's why I'm here.
RE: Braced Steel Commercial Building
Seems we start out knowing everything and end up knowing nothing.
I never use the three sided scenario, even though it is allowed in special circunstances under the codes. The numbers may work out, but in my guts, it just does not "feel" right, and I have laearned to trust my gut instincts, especially in the "PRACTICE" of Engineering, if you know what I mean.
Mike McCann
McCann Engineering
RE: Braced Steel Commercial Building
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RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
Other than providing some form of a moment frame, which can be expensive, the alternative is the concept above, where you use three sides of the box and a relatively rigid diaphragm to engage all three in resisting lateral forces.
RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
The load (be it wind or seismic) is brought into the building near it's center and the parallel brace is eccentric to that force...thus there is a moment induced by the force on the two perpendicular braces which resist that moment.
For obviously rigid diaphragms this creates direct shear forces in the braces. For flexible diaphragms you have to include the deck distortion in the calculations for deflection to see if it is too flexible (i.e. allows too much story drift on the one side).
Sometimes this just doesn't work as the diaphragm deflects both in a flexural mode and in a shear deflection mode and allows the "loose" end to sway too much.
RE: Braced Steel Commercial Building
As for the decking. Will the joists need to be considered in the flexibility of the diaphram, or just the deck?
RE: Braced Steel Commercial Building
Here is a diaphragm design example power point:
http:
But it doesn't deal with a three-sided brace situation.
RE: Braced Steel Commercial Building
It seems like there is no information on three-sided braced situations. I should start a post just to find some good material.
RE: Braced Steel Commercial Building
Watch the sign convention.
RE: Braced Steel Commercial Building
Couple more questions about your notes if you don't mind.
1.Can your example take into account stiff or flexible diaphrams with some assumptions?
2. R, i am assuming is relative stiffness of the lateral supporting member. Any suggestions or rules of thumb on calculating this?
3. V, at the center of the building, would you typically use a sum of all the wind or seismic forces acting in that direction? or on that face (Face 4 in this case and perhaps some suctino on face 3)
Sorry for all the questions, but this is very helpful!
RE: Braced Steel Commercial Building
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.
RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
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
RE: Braced Steel Commercial Building
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.
______
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|______|
RE: Braced Steel Commercial Building
brace _/___
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no | |
brace | |
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| |\brace
|______|
/ brace
RE: Braced Steel Commercial Building
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.
RE: Braced Steel Commercial Building
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.
RE: Braced Steel Commercial Building
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?
RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
RE: Braced Steel Commercial Building
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.
RE: Braced Steel Commercial Building
What moment do you plan on designing the chords for on the 3 sided diaphragm?