Choosing which bays to be braced 3

[GalileoG]

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!

 

[frv]

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?

 

[GalileoG]

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?

 

[jike]

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.

 

[GalileoG]

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.

 

[hokie66]

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.

 

[civeng80]

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.

 

[mazingerz]

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.

 

[DaveAtkins]

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

 

[frv]

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.

 

[hokie66]

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.

 

[civeng80]

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.

 

[SlideRuleEra]

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.

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[frv]

Makes sense.

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

 

[structuresguy]

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.

 

[structuresguy]

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.

 

[csd72]

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.

 

[civilperson]

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.

 

[hokie66]

civilperson,

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

 

[civeng80]

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 !