Unbraced length of a (2) span beam

[Berniedog]

See attached. We have a 2 span beam. The bottom flange is not braced at all except at the column we are assuming it is braced. The results come back that it is inadequate. If we change the bottom unbraced length from 11’-0” to 5’-6” it is adequate.

The bottom flange is in compression for only about ¼ of the span. It makes sense that the unbraced length should be just the length of compression flange.

What is your opinion?

 

[fancypants]

I would not consider the inflection point a brace point. There is a fair amount of literature out there on this. If the bottom flange will not work at 11 feet with the uplift, then either get a bigger beam that will work or brace the bottom flange. I am assuming that your top flange is continuously braced. I did not look to close at your computer output.

 

[BadgerPE]

Berniedog,

AISC specifically states that "...the inflection point can not be considered a brace point because twist occurs at that point." See 16.1-425 Section 6.3 on beams. It sounds like you will need to use a larger section that can handle your loading or add bracing at the interior bearing point which connects the support to both flanges of the beam.

 

[frv]

Why can't you add a bottom brace at mid-span?

 

[Berniedog]

Thanks. I found a section in hte steel manual that states that you take the entire span as unbraced.

 

[ishvaaag]

This Quattro Pro sheet that I made around the mid nineties' addresses the case (the sheet is in spanish)

Arriostramiento lateral de vigas en zona de negativos por su alma arrancando empotrada de un forjado

i.e., the

LTB bracing of beams in negative moments zone by its web protruding from some structural floor with stiffness enough

It essentially determines the continuous bracing requirement in the compressed flange in the zone of the negative moment in terms of strength and stiffness and then determines if, assumed fixity in the stiff floor embedding the upper flange, it delivers such strength (in von Mises terms) and elastic stiffness as a lateral cantilever to sustain a force at the tip (per unit length).

Given the statement of the problem, it was thought for cases where the top flange is embedded in doubly reinforced slabs, quite likely then over 20 cm (8 in) thick or more. It could be extended to cases where the slab gets supported on the top flange with significant mutual restraint. It is unlikely that concreted decks 4.5 in and under in depth may be considered as stiff as to be able to deliver the kind of fixity thought of when conceiving this sheet, particularly for deep steel beams.

The kind of research implied by this concept maybe already done and is obviously dismissed by the current AISC regulations. It could be structurally sound, anyway, for a subset of the bracing on the negative moments' zone problems.

The sheet also determines the required strength and stiffness for discrete bracing for 1 to 10 points bracing the bottom flange.

The sheet obviously was not made to comply an inexistent AISC 360-10 at the era, but based on bracing recommendations on technical texts at the time.

The worksheet is made in Quattro Pro that amazingly continues to open its sheets just dumped even on windows 7 without any installation, something more user friendly than the complex installations we of necessity have been accustomed to (it seems was made portable more than one decade before most of us knew what a portable program is).

I think to have imported the Quattro Pro sheets to Excel with success in the past (I have not, however, presently, a Excel port of this sheet).

 

[Lion06]

If you have a composite beam, I've provided a full depth stiffer and counted that as a brace point. It really only works for composite beams, though. The studs into the concrete deck at in a similar fashion to bolts through the bottom flange into a support with full depth stiffeners at the ends. It's upside down, but the same thought applies.

 

[ToadJones]

I've used stiffeners as Lion says but with a "kicker" from the bottom flange of one beam to the top flange of a parallel beam in the deck to act as a torsional brace. simple and cheap.