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.
