Beam-Column Design 1

[ChipB]

I'm analyzing an existing roof of a warehouse. I have a beam that is supporting two bar joists. There is NO kicker from the bar joist bracing the bottom flange against twist, however, it appears that they considered the bar joist as bracing in the weak direction. Is this standard? The axial load on the original design drawing is 147k. The beam used is a W21x44. Its span is 20'. Fy=36ksi. The bar joist spacing is 6'-8". What are you guys feelings on this? There is no way the tensile bending moment will zero out the compression force in the bottom flange.

 

[LPPE]

If the bar joists are seated (and connected properly) to the top flange of your W21x44, I would assume the beam is braced at your bar joist locations. Is this a spandrel beam? Are you concerned about weak axis bending of the W21x44 due to lateral loads (wind?)? Perhaps the wall system frames directly to the top flange of the W21x44 which is then welded to the roof deck. If this is so, the lateral force should transfer directly to the roof diaphram without affecting your W21.

 

[ChipB]

The roof diaphram loads are transfering to this beam, hence the 147k axial force. If I use the full length, 20', Fa=4.18ksi, while fa=11.31ksi which is 2.7 X the allowable. This is before taking the bending stresses into account. The wall system is braced by this beam. I really don't think it would be good engineering judgement to consider the beam being braced by the wall.

 

[LPPE]

The roof diaphram forces should be transfered to some sort of lateral bracing system. Do you mean the W21 is part of a moment frame? Or some part of a lateral frame?

The W21 is a spandrel beam, 20' long, supporting 2 roof joists at 6'-8" o.c.. Is the exterior wall a shear wall?
I dont follow how you'd be getting axial force in your beam unless the beam is part of a frame.

 

[Taro]

Is it feasible to add kickers at the bottom flange to cut down the unbraced length? If not, the beam must be checked for torsional buckling. Most texts that cover stability will address columns that are braced laterally on one flange only (common situation with wind girts). This is analogous to your situation.

 

[ChipB]

Sorry, I should have clarified.
Yes, the beam is part of the wind bracing. The bay is X-braced. The beam is also supporting the roof.

Can you name a text that covers columns braced one one flange only? I have never considered girts as bracing a column. However, the question of girts does bring up something else that I have often wondered. Do you consider the siding bracing the girt or is the unbraced length column line to column line? I go column line to column line since the siding is only on one side and would not help in suction.

 

[Taro]

Timoshenko & Gere, 1961, develop the equation for torsional buckling of a column with lateral restraint at one flange. It is recommended to design for 90% of this buckling load to account for actual conditions which deviate from the idealized.

 

[JAE]

At 127 kips, the lateral bracing required for the beam would typically be 1% to 4% of the axial load. Yura has also demonstrated that, in addition to strength, the bracing needs a certain minimum level of stiffness as well. I would be concerned that a couple of bar joists would have enough stiffness to resist 5000 lbs. of lateral force. At the least, I would add the bottom chord stiffeners and move on with life.