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?

 

[csd72]

It is possible, but it may be restricted by your building code. The roof bracing would need to be designed to take this load and the perpendicular bracing needs to be ckecked for the push/pull from the eccentricity.

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.

 

[JrStructuralEng]

also, does anyone know where I can find good information on this topic, and also roof diaphrams. (Preferably Canadian information)

 

[whyun]

three sided structure is possible provided that you have a rigid diaphragm (not flexible) and torsional irregularity has been mitigated to acceptable limits in the building code.

 

[JAE]

And keep in mind that with the torsional response necessary to engage the three brace lines together that the building will twist. The story shear limits, in the IBC anyway, dictate that the drift be measured at the geometric center of the story.

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.

 

[JrStructuralEng]

Dang! Jae, your knowledgeable on all these topics. Where do you work, the company I work for needs to hire you!

The more I learn about structural engineering, the more I learn I don't know.

 

[csd72]

JrStructuralEng,

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:

http://www.cisc-icca.ca/content/publications/publications.aspx

 

[JrStructuralEng]

csd72 thanks for the reply: If I see someone who knows everything I will let you know. I have the handbook for steel construction. It didn't seem to have much on this topic. Would you recommomend any of those other publications at that site for this topic ?

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.

 

[JAE]

JrStructuralEng -
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.

 

[msquared48]

Ditto...

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

 

[prsconsultant]

Follow the link below for examples.

http://www.ce.memphis.edu/WTNSEA/Diaphragm Design.pdf

 

[abusementpark]

I don't have much experience with this, but if you want to only provide one line of bracing for the 150' direction of building wouldn't it best to try and locate it close to the center of the building to alleviate the aforementioned torsional problems??

 

[JAE]

Well, abusementpark, there are many times when a building can have three sides with available locations for braces and a fourth side where a brace can't be placed, due to full window-walls, or a building expansion joint.

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.

 

[abusementpark]

JAE, when you say all three engage in resisting lateral forces, does that mean the other two lines of bracing contribute to resisting the lateral forces when the wind is perpendicular to them? Also, how does one ensure a rigid enough diaphragm?

 

[JAE]

Yes, all three resist the force. The lateral force is resisted by a combination of direct shear into the parallel brace and a force couple from the two perpendicular braces.

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.

 

[JrStructuralEng]

JAE, do you know where we can find information on roof diaphrams as you describe?

As for the decking. Will the joists need to be considered in the flexibility of the diaphram, or just the deck?

 

[JAE]

The deck has a shear stiffness (G') that is primary to the deck deflection. The deck also would utilize perimeter chord members (your collectors) that add to the overall stiffness a bit (Ad²). So a combination of shear deflection in the deck plus flexural deflection of the collector/chords.

Here is a diaphragm design example power point:

http://www.ce.memphis.edu/WTNSEA/Diaphragm Design.pdf

But it doesn't deal with a three-sided brace situation.

 

[JrStructuralEng]

Thanks,
It seems like there is no information on three-sided braced situations. I should start a post just to find some good material.

 

[JAE]

See the attached..this is a rough (very rough) layout of how the forces in a building are analyzed. This layout shows four "shearwalls/bracelines" but it works with three.

Watch the sign convention.

 

[JrStructuralEng]

Very helpful Jae, when is your next training seminar? Sign me up. Perhaps Las Vegas this year.

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!