×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Braced Steel Commercial Building
3

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?

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

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

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

2
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.

RE: Braced Steel Commercial Building

(OP)
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.

RE: Braced Steel Commercial Building

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

RE: Braced Steel Commercial Building

(OP)
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.

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

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

RE: Braced Steel Commercial Building

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??

RE: Braced Steel Commercial Building

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.  

RE: Braced Steel Commercial Building

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?

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

(OP)
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?  

RE: Braced Steel Commercial Building

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%20Design.pdf

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

RE: Braced Steel Commercial Building

(OP)
Thanks,
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

(OP)
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!

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

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

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

(OP)
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.

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

RE: Braced Steel Commercial Building

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

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

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

Quote:

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.

RE: Braced Steel Commercial Building

(OP)
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?

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building

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

RE: Braced Steel Commercial Building

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.

RE: Braced Steel Commercial Building


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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources