Compression Flange Bracing 4

[rjw57]

Hi,

I have the AISC Manual of Steel Construction 8th Ed. (something I use very infrequently since I do not often get involved in steel design). I have been reading thru on the design of beams for a current project and am confused by terminology which I think has the same meaning. Is there a difference between "lateral bracing of the compression flange" and "laterally supported steel beams"? Can anyone please describe the difference (if any) in the context of beam design.

I have personally interpreted these phrases to have the same meaning and that some sort of stiffener is placed at specified distances to prevent lateral buckling of the compression flange (after twisting?). Where is the design of this bracing alluded to in the Manual? Your help will be greatly appreciated...

Thanks,
Bob

 

[JAE]

Stiffener plates attached to the beams will not brace the compression flange. The bracing must be external to the beam such as another intersecting beam, joist or slab, or specific braces tied into another relatively rigid element of the structure.

The idea is to prevent the beam from moving laterally and buckling. The compression flange behaves similar to a column under compression and will buckle laterally to avoid the axial stress.

The terms you describe are the same.

 

[ishvaaag]

Both phrases refer to bracing in prevention of LTB or Lateral Torsional Buckling. If bracing is not provided at proper distances this mode of (buckling) failure controls the design. Then 1) one needs care about using proper design equations for the beams accounting the separation of effective bracing of the compression flanges and 2) one needs to care about providing a bracing that is effective enough to sustain any likely forces generated by any likely LTB.

This is made using the proper equations in the LRFD code. Chapter F deals with beam design and Chapter C paragraph C3 deals with bracing in the LRFD 99 code.

 

[rjw57]

Hi Folks,

Sorry I have been so long in replying. Your answers all helped and I even found some reference to this phenomena in an old college text. I have it under control now.

Thanks Again,
Bob

 

[UcfSE]

Lateral bracing of the compression flange is bracing designed to resist forces from buckling of the compression flange and provide stiffness to prevent translation and rotation.

Laterally supported steel beams are those which have continuous bracing, such as from a slab, so that lateral instability of the compression flange is not a design issue.

 

[JAE]

"Laterally supported steel beams are those which have continuous bracing".....

UcfSE - a laterally supported steel beam does not have to have continuous bracing...just bracing close enough to adequately bring the maximum moment capacity above that required. You could have a beam with braces intermittent (say at 24" o.c.) and still develop the full plastic moment of the section.

 

[rjw57]

JAE

Thanks for your reply. I agree with your assessment based on my understanding of the referenced manual. It does not necessarily need to be continuous, although the best scenario for bracing is, as UcfSE mentions, a beam in a slab. Again, thanks everyone for your answers. I just haven't had anyone yet answer one of my original questions: Where is the design of this bracing alluded to in the Manual (AISC)? Ishvaaag mentioned the LRFD code, but not the AISC manual.

 

[UcfSE]

Continuous bracing and bracing close enough to develop the plastic moment are not different from the point of view of preventing LTB. Continuous bracing is essentially a design concept that isn't practical unless the compression flange is embedded in a concrete slab or similar. The point is that lateral instability is not a design issue. I should have put the continuous in quotes.

I'm not sure that the ASD code addresses bracing design though you may find it in older versions of texts such as a steel design text by Salmon and Johnson, 2nd Ed. I would recommend checking out the LRFD code since it is more up to date.

 

[SlideRuleEra]

rjw57 - You are not going to find the design of this bracing in the AISC Manual (ASD). I have taken a good look back through my older reference books: AISC 5th Ed. & 6th Ed., Bethlehem Steel Manual (1934), Carnegie Manual (1923), and Hool & Kinne (1943). Most refer to design details in Railway Specifications (AREA) from 1920 or earlier. Appears that the the steel industry just more or less accepted a standardize mathematical way of addressing the problem in 1937. The design details seem to have been "left behind".

A decent, concise desciption of the bracing concept can be found in a rather odd place:

http://www.awc.org/Publications/TR/

Take a look at paragraphs 1.2.6 (and subparagraphs) in Technical Report 14 "Designing For Lateral-Torsional Stability In Wood Members" (free .pdf download). A beam is beam, and the math is the same, whether wood or steel.

 

[rjw57]

SlideRuleEra

Very helpful post! Especially after I looked at the price for the LRFD code manual. They have some great free info at this site. I'll be creating a file of this info for sure.

Thanks,
rjw57

PS - Had a slide rule when I entered my first year of college (but shifted to a TI calculator the following). Great tools, weren't they? My father taught me how to use one as well as his log tables book from Engineering school. Who needs calculators?

 

[SlideRuleEra]

rjw57 - You are welcome. I came along at a unique time - in 1974 took (and passed) the PE exam using both a four-function calculator to it's limits and my "top of the line" Pickett slide rule for everything else (trig, exponentials, logs, etc). Seems like a strange combination now, but was the "best available technology" then - unless you had several hundred dollars for for an "advanced" HP calculator.

 

[DRW75]

I do not entirely agree with what has been said with regards to stiffening a beam to improve it performance in LTB. Although I do agree with the fact that stiffeners placed perpendicular to the longitudinal axis of a beam will have only a nominal contribution to the overall stability of a beam, and that to increase its stability with this method would require a tremendous (and uneconomical) number of stiffeners. Consider instead providing 'stiffeners' along the longitudinal axis on the compression flange. For example, say you have a beam that is limited by LTB, and you would like to get more capacity from it. Then by increasing the beam's stiffness in its weak axis, you effectively can increase a beams resistance to LTB. Say you've stitched on some angles to the top flange of a beam, you will get more capacity simply as a result of its new resistance to sway.

Thus I would think that 'bracing' does not necessarily have to be external to the beam itself. I do agree to the fact that external bracing is a preferred method of developing the full plastic moment, or a higher elastic moment, I think it should be noted that it is not absolutely required.

I am not very familiar with the AISC, as I am in Canada and follow the CISC & CSA codes. The AISC may state that the 'bracing' must be external. If this is the case please inform me.

I hope that I haven't nit picked the issue too much.

 

[JAE]

DRW75,

AISC does not state explicitly that "bracing" must be external, they don't have to because bracing, by its own definition IS external to the beam.

Adding angles or plates to a compression flange doesn't "brace" the beam, rather, it simply increases the beams internal properties and thus, its capacity....but we're talking semantics here, I know.

I agree with you that adding plates and angles would increase the elastic moment capacity of the beam - but the original question of this thread was:

Is there a difference between "lateral bracing of the compression flange" and "laterally supported steel beams"?

Both phrases are dealing with lateral bracing. Adding substance to a compression flange decreases the tendancy of the beam to laterally buckle, but doesn't brace the beam.

I also agree with you that external bracing is preferred...for the simple reason that a few, strategically placed 2x2 angles tied into an adjacent diaphragm or adjoining beam are much more effective in adding capacity than the labor-intensive welding of additional sections longitudinally.

 

[UcfSE]

I would say that bracing reduces the buckling length of the compression flange. That implies that it is external to the beam section, as you say. It provides a point at which lateral deflection does not occur and is the location of a node in the buckled shape. This reduces "KL" in the Fcr=pi^2(E)/(KL/r)^2 buckling equation. I know this equation isn't directly used in beam LTB design (for inelastic buckling though it is in some form for elastic buckling) but the concept is the same as far as how the different variables affect buckling strength.

Adding angles or longitudinal stiffeners does not change the buckling length, or unbraced portion but rather increases the "r", radius of gyration of the compression flange. In the above critical buckling stress equation increasing "r" increases the critical buckling stress.

So you have two different approaches to increase beam strength against LTB, with or without bracing. You get a similar affect as far as strength but adding bracing is different from providing a stronger geometry for the cross section. Does that sound reasonable?

 

[DRW75]

JAE,

I very much agree, hence the reason why I put the word 'brace' in quotations, because really what was proposed was reinforcement of the beam. I have certainly used (or abused) the word 'brace' to its limit of appropriateness.

UcfSE,

Absolutely agree, the reinforcement does not change the effective length of the member (unless of course you have also tampered with the end connections). It is more like cheating LTB failure with a different means.