Loading exist concrete beam laterally below N.A.

[EngDM]

We are working on a reno where a 10kN load is being applied in the weak out of plane direction of a concrete beam. The equipment is intended to be hung from the beam with angles bolted into the concrete. I'm trying to determine whether or not the beam can resist the torsion and resolve it into the monolithic slab, or if I have to add some sort of diagonal angle bracing going up to the structural slab and check the slab for punching. Ideally I can just bolt my bracket on and move on.

Beam is 14"x26" deep, poured monolithic with a 5" slab.

I've never really dealt with concrete in torsion; I avoid it completely.

 

[jerseyshore]

Can you hang it a little closer to top of the beam/ slab? It's not a tremendous load, but no sense in trying to check an existing beam for torsion (unless you know the bar & stirrup configuration) if you can move up the attachment point.

 

[EngDM]

Can you hang it a little closer to top of the beam/ slab? It's not a tremendous load, but no sense in trying to check an existing beam for torsion (unless you know the bar & stirrup configuration) if you can move up the attachment point.

It's a pipe that has to clear underside of beam, so I'm keeping them 6" below underside of beam as a max.

I do know bar arrangement and stirrup configuration, although the building is from 1958 so the way they did the reinforcing is unlike I'm used to.

 

[KootK]

This doesn't sound like a terribly big deal. I'd be inclined to check the bottom of the beam as a fictitious girt. The goal being to demonstrate that the fictitious girt has gobs of capacity for this action such that torsion / interaction stuff doesn't warrant further investigation. If this doesn't pan out, then maybe look at other stuff.

 

[jayrod12]

This doesn't sound like a terribly big deal. I'd be inclined to check the bottom of the beam as a fictitious girt. The goal being to demonstrate that the fictitious girt has gobs of capacity for this action such that torsion / interaction stuff doesn't warrant further investigation. If this doesn't pan out, then maybe look at other stuff.

View attachment 4402

I'd go this route because it's highly unlikely that the stirrups are detailed for torsion, i.e. 135 degree bends.

 

[EngDM]

This doesn't sound like a terribly big deal. I'd be inclined to check the bottom of the beam as a fictitious girt. The goal being to demonstrate that the fictitious girt has gobs of capacity for this action such that torsion / interaction stuff doesn't warrant further investigation. If this doesn't pan out, then maybe look at other stuff.

View attachment 4402

Since I am providing an angle to midheight of beam, how much height of concrete would you use for the "fictitious girt"? I'd imagine the concrete bearing near the bottom ould take most of it for one direction, but the angle on the otherside would be held by tension of the anchors which would bring the resultant higher than the bottom say, 2-3". For this girt, would you just rely on the outermost bar (I have 3 bottom bars) and ignore the central and compression zone (when looking at lateral loads) bar?

 

[EngDM]

Also, the 10kN load is applied 3 times by 3 different fastening systems so the ficticious girt solution may actually be an issue, assuming that you take the beam span as the girt span, even though it is supported at the top by the slab.

 

[KootK]

Since I am providing an angle to midheight of beam, how much height of concrete would you use for the "fictitious girt"?

Designer's discretion. Maybe treat it as an L-beam using a compression block half the depth of the beam and the outer flexural rebar?

I'd imagine the concrete bearing near the bottom ould take most of it for one direction, but the angle on the otherside would be held by tension of the anchors which would bring the resultant higher than the bottom say, 2-3".

I don't think the connection location or direction really factor into my approach. You'll design the connection as anchorage using unreinforced concrete principles. After that, the load will have no trouble making it's way down to the bottom of the beam.

Also, the 10kN load is applied 3 times by 3 different fastening systems so the ficticious girt solution may actually be an issue, assuming that you take the beam span as the girt span, even though it is supported at the top by the slab.

I don't know what to tell you. The fictitious girt thing is just another way to check torsion. If fictitious girt doesn't work, then torsion might not work.

Even with the slab connection at the top, the bottom of the beam is still going to see lateral deformation as implied by the fictitious girt to some degree.

A 5" slab is not super convincing as torsional restraint for a beam of that size in my mind. And, depending on the detailing, it may only be suitable of loads directed inwards.

What's your beam span here?

it's highly unlikely that the stirrups are detailed for torsion, i.e. 135 degree bends.

I agree in a strictly theoretical sense. At the same time, there are many situations where I would be willing to utilize the slab top steel at the beam to complete the the square.

 

[ANE91]

It’s been a while since I did a concrete-beam-in-torsion calc, but isn’t there some threshold torsion in ACI 318 (Chapter 22) below which you simply neglect it?

 

[EngDM]

It’s been a while since I did a concrete-beam-in-torsion calc, but isn’t there some threshold torsion in ACI 318 (Chapter 22) below which you simply neglect it?

Not familiar with ACI but I could check A23.3 for a torsion mention.

The girt method I end up with quite a large moment, so it's looking like I need to brace to underside of slab.