Steel Beam Bracing Basic 1

[palk7 EIT]

Hi,

When there are primary beams 30 foot long on 5 or 6 rows and there are secondary beams connected to the primary beams at 5'-0" O.C via full-depth stiffeners and both beams are flush ( at the same level). Assuming there is no decking or flooring diaphragm spanning and connected to the secondary beams. The depth of both primary and secondary beams are the same.

Does the 5'-0" O.C center secondary beam coonx. provide brace points for the 30-foot-long beams and reduce their unbraced length to 5'?

Thank you

 

[XR250]

Only if the secondary beams have some other means of resistance longitudinally.

 

[Lomarandil]

As XR250 states, the most common way for your statement to be true (especially for building structures) is for the secondary beams to be connected to another element that restrains lateral movement parallel to the secondary beams. The concern is that without that "hard point", each of the primary beams could roll over in the same direction, and the bracing just goes with it.

Also, in two less common cases:

1) the secondary beams (and connections!) have enough bending stiffness to restrain the primary beams from rolling over. With both beams being the same depth, you have a chance at making this work.

Or

2) the applied load is not uniform, such that a lightly loaded primary beam can help stabilize its heavily loaded neighbor ("lean-on bracing"). This is uncommon

 

[palk7 EIT]

Thank you!,
So if a floor decking in the form of metal deck is introduced to span between secondary beams, in this case a diaphragm action is achieved, So now would it be ok to consider the braces length as 5' on the main beams ?

 

[BridgeSmith]

In bridge superstructures, to be considered bracing members, the secondary 'beams' (AKA diaphragms) have to meet the slenderness requirements (r > L/140) so they don't buckle in the weak direction, and they have to be more than 1/2 the depth of the girder.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

both beams are flush ( at the same level).

I take that statement to indicate that the infill beams tie into the sides of the girders. Everything that follows is predicated upon that assumption.

Only if the secondary beams have some other means of resistance longitudinally.

I disagree. This system is generally conceived of as torsional or "roll beam" bracing which does not require lateral restraint. Twin girder diaphragm bracing in bridges is sort of an extreme version of this. Perhaps you and Lomarandil are envisioning beams that run over top of the girders?

The concern is that without that "hard point", each of the primary beams could roll over in the same direction, and the bracing just goes with it.

For the same reason that I mentioned above, I believe that this is also incorrect. The "same direction" thing is a phenomenon of translational bracing, not torsional bracing.

Does the 5'-0" O.C center secondary beam coonx. provide brace points for the 30-foot-long beams and reduce their unbraced length to 5'?

That's right, even in the absence of decking. Done by the book:

1) The girder to beam connection should be at least 0.6 x the depth of the girder and;

2) The girder to beam connection should be capable of a nominal amount of moment transfer. Shear tabs, end plates, and clip angles are commonly deemed appropriate.

3) There's an AISC section that dictates the strength and stiffness that the roll beams out to have in order to perform the bracing function.

 

[Lomarandil]

KootK, you're describing what I meant with my first option. Torsional bracing.

(Sorry if my wording there was overly pessimistic, I've been dealing with a rash of engineers telling me "my beam is connected to X, therefore it must be braced". Not necessarily!)

Palk, that depends on thetype of metal decking and connection to the beams below. A sheet metal deck with regularly spaced connections can provide very good bracing (much like a wood floor or shear wall). A metal grate decking panel bolted down at the corners, not so much.