Permanent deflection of steel beams.

[stevenspm]

Hello,
I am working on a steel and wood framed building built back in 1915. The contractor, while performing demolition noticed a deflection a steel beam forming the edge of an auditorium balcony. Using a laser level, he determined that the beam has a deflection of about 2-1/4". The beam is an old 12"-I beam spanning 52' and is suspended from a 26" Bethlehem Girder beam above by tie rods located 16' from each end. The beam, at the time of the measurement was surrounded by plaster and wood furring. The stresses I am getting, based on the renovation design, in the 12"-I beam is 8.4ksi and in the 26" girder beam is 13.1ksi. Both of these stresses are under the allowable 16ksi, which I found in an old Carnegie Pocket Companion from 1913. I have not been able to find any information about whether or not a permanent deflection would cause a stress/strength reduction in steel beams. The question I have is would the permanent deflection cause a reduction in the allowable stress / strength of the beam.

Thanks for the help,
Phil

 

[CANPRO]

I'm not sure what you mean by permanent deflections...there will be some self-weight and dead load deflection - how do your expected dead load deflections compare with the 2.25" measured deflection? You wouldn't know if it is permanently deformed unless you removed the beam and put it into a "no-load" condition...and even then, you don't know if the beam was initially perfectly straight or not.

 

[JAE]

And theoretically, if the beam stresses were always below the yield point, there wouldn't, shouldn't be any creep deflections developing while in the elastic range.

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[Ron247]

I also do not know what permanent deflection is. If the beam is still loaded, you are close to L/240 for total load based on 52' span. But if you are 2.25" based on 52'-(2x16') = 20' you have a real problem it seems unless they just ignored deflection in the original design. Is the beam spanning 52' or 20' between rod supports? What does your analysis say the beam should be sagging based on calculation?

Based on the old theory, I would assume permanent deflection in an unloaded state would have been permanent plastic deformation.

 

[SlideRuleEra]

stevenspm - Determine deflection at several points along the entire 52' length to get the beam's general shape. Just knowing that deflection is 2 1/4" (at the center?) is not enough to reach any conclusion. Assuming the beam is uniformly loaded (say, even if only by self-weight) and stress is in the elastic range, deflections at the ends of the two 16' cantilevers should be greater than 2 1/4" and equal to each other.

What to look at next depends on the deflections obtained.

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[BAretired]

The beam is an old 12"-I beam spanning 52' and is suspended from a 26" Bethlehem Girder beam above by tie rods located 16' from each end.

That can be interpreted two ways. A sketch might help.

BA