Unbraced Length

[EIT2]

Hollowcore Precast Concrete Plank Manufacturers will provide weld anchor plates in the underside of the plank to be welded to the top flange of the steel beam support.

Does anyone consider a 4" long at 4' or 8' on center, uninspected field weld, that has cracking potential, to be a credible source compression flange bracing?

Thank you!

 

[dbuzz]

Remember that for a beam to laterally-torsionally buckle it must undergo two movements; one lateral and one torsional. If you restrain either of these movements you have an effective restraint against lateral-torsional buckling.

Can the weld transfer 2.5% of the compressive force in the beam flange? If so it is likely to be an effective lateral restraint.

Are the intermittent fillet welds aligned on each side of the beam flange? If so they are likely to be an effective torsional restraint.

If you are concerned about the potential reduced quality of the overhead field weld, perhaps you can apply a different capacity factor. In Australian practice, we have General Purpose (GP) and Structural Purpose (SP) welds. Category GP is used for low-stressed and non-structural welds and category SP for relatively highly stressed welds. The capacity factor is 0.6 for GP fillet welds and 0.8 for SP fillet welds.

 

[AlohaBob]

YES

Even though the welding may not have a continuous program of testing and inspection, nonetheless, visually inspect this yourself to see that the welds are there and that they are serviceable. Then rightly use the added capacity due to decreased Lu.

 

[bendit]

Just asking - but, depending upon the size and thickness of the anchor plate (mass) does the heat of welding have any effect on the concrete?

 

[jheidt2543]

EIT2,

You've raised an interesting question. I seem to remember being told years ago, by a long time precast engineer that weld plates weren't needed in most (not all) cases. He maintained that the dead load friction from the precast provided enough load for LTB support of the beam. Now that your question jogged my memory, I'll have to revist this to satify my curiosity.

Thanks

 

[AlohaBob]

jheidt2543,

You suggest using friction as a connection. Not okay in seismic areas. Statics clearly verify the effect you say, but the connection is a nominal connection first. Conditions may require analysis of loads later.

 

[jheidt2543]

AlohaBob,

I should have clarified that I'm not in a seimic zone, thanks for noting that. We tend to comment based on our past experiance and just the thought of working in a seismic zone gives me the shakes!

I'm not sure what you mean by "nominal connection first", but I agree that other loading considerations can lead to the weld plate connection requirement.

 

[AlohaBob]

Nominal: The building code prescribes minimum detailing requirements for specific areas. The most stringent being seismic and hurricane prone. Analysis may suggest mechanical connections are not required to define a load path, nonetheless they are required as a starting point for tying a structure together.

I've seen buildings come unzipped. The process begins at points of poor detailing. Nominal loadpath ties were missing or poorly defined. Structural weak points inadvertently employed.

In any case, I make it a practice to have basic ties incorporated into detailing before I begin analysis. I remove nominal connections under justified insistence rather than justify their ommision as a practice.