Concrete beam loaded at the bottom? 2

[Althalus]

My team was given a cryptic message about our design from the design manager. And we won't be able to discuss with him for another day.

Design:
[ul]
[li]Concrete beam: 4ft x 3ft(H x W) with a 70ft clear span.[/li]
[li]30kip load hanging down from an embed plate at the bottom of the beam.[/li]
[li]Embedment uses Nelson D2L anchors.[/li]
[/ul]

He said that there is a problem with using a tension anchor at the bottom of a beam because they (D2L bars) didn't have heads (like the Nelson studs have). It had to do with the breakout cone on the tension side of the beam.

What further confused us was that he didn't say it had anything to do with the tension capacity of the anchor. But it was about the shear and bending reinforcement in the beam itself. Then he left. We were trying to figure out what he meant by all that.

Anyone have a clue what he was talking about? I want to be prepared for that meeting tomorrow.

 

[andrews2]

He could be referring to how concrete has a lower strength when tension is applied in the transverse direction. If you apply tension to concrete (Like the bottom fibers of a simple span beam) then pulling an anchor out of the bottom of the beam may be easier. His concern may be that if a flexural crack were to form at an anchor with no head, then the strength of the anchor may be compromised.

 

[TLHS]

Personally if I've got a hanger of any non-nominal size on a concrete beam I'd be using something that transfers load to the top of the beam whenever possible. Embedded plate with rebar welded to it that terminates in the compression zone at the top, for instance. The load path gets wonky with a tension load at the bottom, and your concrete is cracked. I'd much rather have the load moved to the top of the beam where it can form struts down into the shear reinforcement.

 

[slickdeals]

If your D2L anchor is developed to resist the tension in the member, then I don't see an issue. This would be no different than rebar being developed in a tension member (net uplift) anchored to a footing. I agree with TLHS that it is best to be conservative with the development length and use a long enough anchor (say 3.5' long). The other thing I have done in instances like this is to have closely spaced shear reinforcement that are centered on the load and developed fully around the headed stud or rebar used with the embed plate. The shear reinforcement is what will prevent a tension breakout.

 

[Althalus]

Thank you for your posts.

 

[Trippelsewe]

Here in Australia you would be required to take the tension force to the top of the beam.

 

[rapt]

It has nothing to do with being conservative. Concrete flexural shear design is based on the load being applied at the top of the member.

If the load is attached at the bottom, it must be transferred to the top at the point of application.

Steel member design has the same requirement.

 

[andrews2]

rapt, I agree that loads on concrete beams are intended to be applied to the top of the member, but where have you read that this is the case for steel beams? Loading steel beams from the bottom is actually better (Although not usually practical) because it helps to stabilize against lateral torsional buckling, so I'm curious what source is saying you are required to transfer loads to be applied to the top.