ACI 10.4 - Lateral beam support? 1

[pelotoner]

ACI 10.4.1 states that "Spacing of lateral supports for a beam shall not exceed 50 times the least width 'b' of compression flange or face." However, it makes no mention of bracing the compression flange, only "lateral" bracing.

I have a 8"x72" upturned beam supporting a 9" slab at the bottom on one side and a roof diaphragm on the other. The span = 43'-4" = 65xwidth (exceeds 50). The beam is supporting a masonry wall and 2 stories of concrete above. Approximately 48" of the beam will be above the 9" slab.

Is the beam braced?

 

[frv]

Is the roof diaphragm at the top?

 

[pelotoner]

No, both the slab and roof diaphragm are about 24" from the bottom, so the beam is upturned 48" from both lateral supports.

 

[JAE]

Since concrete beams rarely have LTB issues ACI hasn't spent much time on research in this area compared with steel.

I would think that the 2% rule could be at least looked at for your condition. You have your upturned beam that is designed for a C and T. Take the C x 0.02 and that is the lateral force that you have to resist. The slab at the base then would have bending moment in it based upon say this moment arm:

72" - 9"/2 - x/2 where x is the depth of your conc. compressive stress block (Whitney).

There is also the concern that, while the slab may be strong enough, it may NOT be stiff enough to properly brace the top of the beam. What to do about that I'm not sure off the top of my head unless you could look into the AISC bracing rules (See Appendix 6 in the 13th Manual specification) and see if the steel beta stiffness would work. I'm not sure it is applicable to concrete beams but in theory I'd like to think it is.

 

[pelotoner]

Thanks JAE. I thought about that also. That wouldn't just be a 9" concrete slab, but a 9" concrete diaphragm...that won't have a problem with stiffness.

I was thinking of adding extra stirrups to resist the out-of-plane moment caused by compression block. However, the 2% rule is for nodal bracing. This would be continuous, therefore relative bracing, so 0.8%/ft would be the force.

 

[JAE]

If your concrete slab is on the bottom of the beam, and the top of the beam is in compression, then your LTB action in the beam would involve twist, or sectional rotation about its longitudinal axis, or about a longitudinal axis along the beam near the bottom of the beam.

This rotation in the beam would cause bending in the 9" slab. The LTB is not resisted by diaphragm action here. Yes the slab acts as a diaphragm in preventing the bottom of the beam from translating laterally, but the top of the beam can translate laterally by the beam rotation.