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Choosing which bays to be braced
3

Choosing which bays to be braced

Choosing which bays to be braced

(OP)
Hello all,

How does one usually know what is the optimum location of the braces? What considerations can one take into account when deciding on which bay is to be braced. I suppose trial and error on an analysis model would do but I was curious if there are rules of thumbs out there about this (one being for structures supporting cranes, bracings are best to be placed at the end bays.)

Also, if at one wall I add a second braced bay, does that help at all with the drift or does it simply allow me to use lighter members (since we have braced two bays instead of one)

Thanks!

RE: Choosing which bays to be braced

Whenever I've designed these, it's typically architectural considerations that dictate where they are put. But I'd be interested to know what others think, as I would assume that so long as you have relatively symmetric braces, it really shouldn't matter where you put them.

On another note, is drift really an issue for you with braced frames?

RE: Choosing which bays to be braced

(OP)
With this particular project that I am working with, it could very well be, I have a 150 ton crane, with the horizontal and impact forces drift could very well be an issue or would I be wrong in assuming so?

RE: Choosing which bays to be braced

In commercial buildings, you are usually limited by the architecture where to locate braces: generally in the cores where there are stairwells, elevators, toilets, etc.

In industrial buildings, you have a little more freedom as to locations. The restrictions are usually limited by louvers, doors, equipment, future considerations, etc.

I try to avoid end bays since corner columns usually have little dead weight to resist uplift. Another consideration is that the structure should be free to grow and so I like braces to be near the center so as not to restrain growth. I also prefer to locate braces where I have the most dead weight to conteract any uplift.

 

RE: Choosing which bays to be braced

(OP)
Thanks Jike.

You brought up interesting points. Temperature is an issue with my building, so placing the bays in the center would allow to structure to grow, but at the same time, the crane stop loads will have to travel all the way from the edge of the building to the center where the bracings can take them to the foundation. It is a trade-off.   

RE: Choosing which bays to be braced

Clansman,

With a 150 ton crane, I would have separate crane columns and place the bracing for the crane in the plane of the crane rails.  The braces between the crane columns would be in the end bays to take the crane stop loads.  The building braces for wind load would be between the main building columns and would be located in bays to coincide with the roof bracing.

RE: Choosing which bays to be braced

IMHO for portal framed buildings the end bays are always the ones that should be braced.  If internal bays are braced for other than structural reasons then members must be introduced to transfer the longitudinal loads (e.g. wind loads) to these internal bracing bays and because the load path is longer it would be more expensive.

So I always try to brace the end bays.

RE: Choosing which bays to be braced

Tips:

1. Your proposed  brace layout should always be in symmetry. (e.g if you brace on the left sode of the structure, be sure there's a brace on the right, top or down, on same leves, etc.) The point is to avoid overall torsion in the structure, which leads to another point.

2. The braces should be in symmtery with respect to the center of mass of the structure.

Well that's the ideal. Unfortunately architectural necessities might differ. In any case, be sure to do a 3D model analysis, to capture all those twist forces on your members. ;)

RE: Choosing which bays to be braced

I agree with jike--putting a braced frame at each end of a building is not a good idea--the building will try to grow thermally, and put tremendous lateral force into the two braced frames.

DaveAtkins

RE: Choosing which bays to be braced

civeng80-

Unless you design your diaphragm to transfer the forces to your X-brace only for the span of the X-Brace, you'd have to drag the force in anyway. In our neck of the woods a typical X-Brace plan dimension is 15-30 ft. This means you would have to transfer your entire diaphragm force over this distance.

If you allow three or four bays to transfer your force, your diaphragm forces are greatly reduced. For a flexible diaphragm, you may have no option but to do so.

RE: Choosing which bays to be braced

frv,

With a 150 ton crane in the building, you are not likely to be using diaphragm action to distribute the forces.  These industrial buildings require discrete bracing.

Some of the advice here is generic about bracing, but Clansman's building was identified in his second post as a specific case.

Jike and Dave,

Your points are valid, but I think the crane forces must take precedence in any decisions.

RE: Choosing which bays to be braced

Fry

From Australian design we tend not to rely on Diaphragm action here (although I wish we did at times!).  Certainly for the larger buildings and the ones with crane loads I would brace the end bays if at all possible.   

RE: Choosing which bays to be braced

I don't know if it is a "rule-of-thumb", but bracing every third bay seems to have a certain logic to it (if there are no other constraints). That way one end of each horizontal beam can be fixed to a braced bay.

www.SlideRuleEra.net idea

www.VacuumTubeEra.net r2d2

RE: Choosing which bays to be braced

Makes sense.

I guess I hear "bracing" and automatically jump to wind and seismic.

RE: Choosing which bays to be braced

Get a copy of the AISC Design Guide #7 Industrial Buildings.  It will have lots of good info on the design of industrial buildings with overhead cranes.

RE: Choosing which bays to be braced

Also, AISC Design Guide #3 Seviceability considerations for Low-Rise BUildings and Design Guide #5 Low and Mid-Rise Steel Buildings are both also helpful in design steel buildings.

RE: Choosing which bays to be braced

As a general note, if you need larger footings to resist the bracing uplift then 2 adjacent bays rather than 2 separate bays will reduce the number of larger footings from 4 to 2.

 

RE: Choosing which bays to be braced

     My rule of thumb for a braced building is to match the number of bays with braces.  Thus, a three bay by six bay building would have nine sets of diagonal braces, one each brace crossing each line of bays.  I prefer interior columns for the ends of the braces.  If moment frames are required for architectural reasons, I double the number of frames with moment connections at both top and base of columns.

RE: Choosing which bays to be braced

civilperson,

What kind of building are you talking about?  With all those braces, Clansman's overhead crane wouldn't work very well.

RE: Choosing which bays to be braced

Yes civilperson

We sometimes have narrow views of buildings.  I think most of us here are thinking of portal framed buildings and the diagonal bracings of the portals.  I guess if there are many bays e.g. 10 to 20 bays then some internal bays must also be braced rather than just the exterior bays.  So how does one decide which internal bays if any?  

It would be interesting if you could describe the sort of buildings you have in mind civilperson !

 

RE: Choosing which bays to be braced

Clansman,
I'm guessing this is a crane operating in a single bay steel portal frame building.  For a 150 ton crane (overhead travelling type), I'd go with Hokie66 and use separate columns to support the gantry beams and brace between them.  You don't say how long the building is but conservatively provide movement joints every 40m (135ft ?)  These should go between pairs of columns and through the gantry beam and rail.  The crane braces are designed for the stop force and any thermal forces.  The gantry carries a compression force...

For a building housing a large crane, there is likely to be a big door.  If is often convenient to locate a bracing bay adjacent to large doors as it helps in stabilising the area.  It then makes sense to position bracing at the other end as well.  It is possible that this reduces the sizes of logitudinal members as the gable end wind forces go directly to the beaces.  My experience would indicate that this is rarely of significance.

There really isn't a single right answer for the question.  Whatever solution you adopt pay attention to load paths and make sure all your design loads can be taken down to the foundations.

RE: Choosing which bays to be braced

Reminds me of my ol' dawg.  Wanders in, innocently drops a big one, then disappears.

RE: Choosing which bays to be braced

hokie66

I know 150 ton crane is quite a large one but why would you have  separate columns for this ?  Or do you mean a stepped column?  I hope we are on the same wavelength here and looking at portal frames spaced evenly with a gantry.

Also what's a dawg (LOL) ?

RE: Choosing which bays to be braced

civeng80,

We are on the same wavelength.  And stepped columns can certainly be used.  I prefer using a separate UC column, rotated 90 degrees to the building column and braced to the building column with flexible or vertically sliding braces so that the shortening of the crane column does not affect the building column.  This arrangement avoids the need for some of the fabrication involved in the stepped column, and sometimes the end clearance on the crane requires a big step.

The object of my barb knows what a dawg is, but as he is a hit and run contributor, may not read about it.

RE: Choosing which bays to be braced

Thanks hokie66!

Always a pleasure reading your comments.

Cheers!

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