Repair options for buckled bottom flange of steel beam
Repair options for buckled bottom flange of steel beam
(OP)
We are doing an investigation of existing building (main structure built in 1980). The pre-engineered steel rigid frames have span of 117' and spaced about 25' OC. One column has a height of 13 ft and the other 39 ft. Both the column and beams are taped. The deepest sections for both column and beam are at column- beam connections. The frames are supporting W8 roof beam @ 5'-0" oc. We also found the structure drawings but since the rigid frame were designed by others; there is not much information about the frame.
The building engineer noticed big deflections at the center of rigid frame and distorted bottom flange of beams.
Some engineers of our company were called for a site visit. They found that there 8" deflection at the center of rigid frame (117x12/8=176, under dead load only). The bottom flange of beams close to the beam-column connections (at the low column side) are distorted (we think it is compression flange local buckling). There is no braces for the beam bottom flange. No other unusual were oberserved. Some measurements has been done for beam close to low column side.
We modeled the rigid frame as a portal frame. The two supports are modeled pin connection. The beam-column connections are fixed connections. Under DL+SL, the moment in beam is about -660 kip-ft at the low column, -1982 kip-ft at the high column, and +1165 kip-ft at the center of the beam. The negative moment length along the beam is about 10' on low column side and 23' on the other side. We also checked other load combinations with WL and E. DL+SL resulted the max moments.
Then we tried to calculate the capacity of the beam, especially for the beam close to the low column . We checked the 3 failure modes per F5 section of the specification. The lowest is Ma=610 kip-ft (F.3 compression flange local buckling). Since the beam is taped, we do not know how to check the lateral-torsional buckling. The unbraced length Lb is about 117', if we use equation F5-4, Cb=1.92, and the Fcr=1 ksi. The result seems unreasonable. If we use Lb=10', Fcr=15.2 ksi, Ma=434 kip-ft.
Since there is compression flange local buckling and no lateral-torsion buckling of the beam, it seems we can assume that the lowest Ma is due to local buckling, then Ma=610 kip-ft. (there is 1.67 safety factor, so the 660kip-ft load should not cause the local buckling)
We are thinking to just strengthen the bottom flanges with some plate at negative moment area. We are also thinking about adding braces to the bottom flange of beams. We are doing more investigations.
Any comments and suggestions are welcome.
The building engineer noticed big deflections at the center of rigid frame and distorted bottom flange of beams.
Some engineers of our company were called for a site visit. They found that there 8" deflection at the center of rigid frame (117x12/8=176, under dead load only). The bottom flange of beams close to the beam-column connections (at the low column side) are distorted (we think it is compression flange local buckling). There is no braces for the beam bottom flange. No other unusual were oberserved. Some measurements has been done for beam close to low column side.
We modeled the rigid frame as a portal frame. The two supports are modeled pin connection. The beam-column connections are fixed connections. Under DL+SL, the moment in beam is about -660 kip-ft at the low column, -1982 kip-ft at the high column, and +1165 kip-ft at the center of the beam. The negative moment length along the beam is about 10' on low column side and 23' on the other side. We also checked other load combinations with WL and E. DL+SL resulted the max moments.
Then we tried to calculate the capacity of the beam, especially for the beam close to the low column . We checked the 3 failure modes per F5 section of the specification. The lowest is Ma=610 kip-ft (F.3 compression flange local buckling). Since the beam is taped, we do not know how to check the lateral-torsional buckling. The unbraced length Lb is about 117', if we use equation F5-4, Cb=1.92, and the Fcr=1 ksi. The result seems unreasonable. If we use Lb=10', Fcr=15.2 ksi, Ma=434 kip-ft.
Since there is compression flange local buckling and no lateral-torsion buckling of the beam, it seems we can assume that the lowest Ma is due to local buckling, then Ma=610 kip-ft. (there is 1.67 safety factor, so the 660kip-ft load should not cause the local buckling)
We are thinking to just strengthen the bottom flanges with some plate at negative moment area. We are also thinking about adding braces to the bottom flange of beams. We are doing more investigations.
Any comments and suggestions are welcome.






RE: Repair options for buckled bottom flange of steel beam
They provided a draft paper on the subject - "Steel Design Guide: Frame Design Using Web Tapered Members" dated January `07.
I`m not sure how to post files here, but there must be copies of this kicking around somewhere.
RE: Repair options for buckled bottom flange of steel beam
My point is that it's possible that you'll get very low allowable compressive stresses if the bracing is missing. The PEMB designers are very clever in optimizing their designs. If it's cheaper to brace a flange than upsize it, that's what they'll do.
RE: Repair options for buckled bottom flange of steel beam
RE: Repair options for buckled bottom flange of steel beam
RE: Repair options for buckled bottom flange of steel beam
Finally I went to the swimming pool to do the field investigation yesterday. We found those steel gusset plates along the bottom flanges of the beams. It's very clear to me they are for the braces. They should send me there earlier. Please see the picture. You also can see the distorted bottom flange.
I also found the lateral resistance system along the column grid lines are diagonal tension rods (between columns). They put diagonal tension rods in one of the five bays. But the diagonal tensions rods between the 13 ft high columns are missed (they add a room between those two colummns during the renovation). The holes on those two columns for the tension rods are still there. The diagonal tension rods at the roof (at this bay) are still there.
Any suggestions for repairs?
I may add diagonal braces back to the steel beam. And weld a steel plate at the distorted bottom flanges.
RE: Repair options for buckled bottom flange of steel beam
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RE: Repair options for buckled bottom flange of steel beam
However, and as you know, these frames are designed very close to the line, and there may be other problems which have been caused by the 1992 modifications. If possible, I would want them completely analyzed by one of the PEMB manufacturers, the original supplier if possible.
RE: Repair options for buckled bottom flange of steel beam
You might be able to deal with that by bashing it back into place and using a stiffener. Personally, I'd be tempted to put in a new load path to completely avoid the damaged bottom flange. You could run a plate along the beam, either above or below the plate, to take the flange forces and then tie it into the existing flange once you hit an undamaged area. Placing it on the outside is obviously preferable, but you may have to do some creative detailing if the flange is buckled outward in some places.
Also, if this area is as highly stressed as it seems to be, I'd be really concerned about the weakening that will occur when you heat up the steel to weld it. You may need to shore the beams somehow while you're making modifications.
RE: Repair options for buckled bottom flange of steel beam
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Repair options for buckled bottom flange of steel beam
I have a feeling that even the lateral braces of steel beam bottom flange were not revmoved. The bottom flange may also buckle.
Because the braces are about 10'-0" OC. The thickness of the bottom flange is only about 5/16". The flange width is 8" (I need to double check that next Monday when I am back in the office). The KL/r will be really high, which will lead to a low Ma due to local buckling. I remember some explanations in a concrete text book for the buckling of a plate with three sides simply supported and one side free (similar to the flange). Any suggestions for study materials for this issue are welcome.
The beams may be overloaded due to the big snow storm in 2010 winter. (it broke the record in some locations of our area)
The web of the beam is about 3/16" thick. And the depth of beam at the column-beam connection is about 50".
The PEMB engineer really wanted to save materials.
I did not see any problem in the columns. The thickness of column flanges vary (along the height of the column). The flange is thicker than those of beams (due to column flanges is unbraced?). I will also need to check that. The depth of column also vary.
I also found a note in the structural drawings saying grade 36 steel shall be used in column and grade 50 steel shall be used in beam. But I am not sure if the PEMB engineer follow that. But the Ma due to local buckling of beams won't change more than 10%.
Oh, I should enjoy my weekend.
RE: Repair options for buckled bottom flange of steel beam
RE: Repair options for buckled bottom flange of steel beam
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Repair options for buckled bottom flange of steel beam