Unbraced Length of a Beam

[mkrei]

I have a steel beam supporting (4) elevator hoist beams (two cars). I am not sure if the contractor welded the elevator hoist beams to the supporting steel beam to provide a brace point. One end of the hoist beam is pocketed into a concrete wall with bearing plate the other end is bearing on my beam. (4) beams bear on the beam which is approximately 17'-0" long. Went back and ran the loads for unbraced for the 17' length and the beam works fine for the static load case. (This beam supports (2) elevator cars therefore the (4) beams.) When I run one pair of beam with the dynamic load and one pair of beams with the static loads I have a 25% overstress. The dynamic loads are approximately twice the static loads. The beam supporting the hoist beams frames into a concrete shear wall with a very large weld plate and a full height welded clip angle connection. My question is "does the welded connection to the concrete shear wall provide enough torsional restraint to reduce the unbraced length to something less than 17'-0"" If I run the beam as 15' unbraced length it works. I have a call into the contractor so see if they remember welding the machine beams down but if not someone would have to go in and perform the welding inside the elevator shaft or somehow brace the beam.

Please let me know your thoughts.

 

[civilperson]

Depending on the detail of the connections, the braced length of the support beam may be less than full length. If the elevator beams frame into the top third of the support beam or are connected to the top flange, then the elevator beams will provide braces and the unbraced distance is the spacing between the elevator beams.

 

[mkrei]

Thanks for the reply. I have spoken to the elevator installation people and they have assured me that the machine beams are indeed welded to the tops of the support beams therefore my assumption of a lateral brace is verified and the beams work fine at the unbraced length equal to the spacing of the machine beams. I have not had good luck at guessing the loading of the elevators. Seems like we always are out early with steel but do not get the elevator information until the steel is on site. I guess that is the way the world works these days.

Thanks again

 

[msquared48]

mkrei:

Not to be the devil's advocate here, but to be I guess.

GET IT IN WRITING FROM THEM AND CYA!



Mike McCann
MMC Engineering

 

[mkrei]

I dug into the final shop drawings that were submitted for the elevator based on the actual elevator and the beam will work unbraced for the 17'. I was using loads that matched up to elevators I had used in the past and they were slightly larger. You would think the elevator people would indicate on the shop drawings how the machine beams are anchored to the primary structure.