Design of Embed Plates in concrete beams

[ec29]

I have the usual references for design of embed plates in concrete walls, concrete slabs but how do you design an embed with headed studs into a concrete beam. The embed is used to conect a steel beam with a very large reaction (concrete breakout won't work). The plate is as deep as the concrete beam.

 

[JedClampett]

If it won't work, you have a problem. You're not going to get any code sanctioned extra capacity because it's going into a beam. You could assume that there's auxiliary reinforcing, but that's only going to get you so far. I would consider a beam ledge (L shaped beam) if the loads are high.

 

[ec29]

Thanks Jed. If concrete breakout doesn't work Appendix D allows you to take the force with rebar. This is easy if you are at the edge of a wall or a slab. But can you do this in a beam? Could you use shear friction of the longitudinal reinforcement? any other way? If this works somehow I don't want to change the detail. We are in construction administration now.

 

[JedClampett]

Instead of headed studs can you use deformed bar anchors, like Nelson D2Ls? This is a cheap change, and might allow you to use lapped bars to carry the force.
How big is you reaction?

 

[hokie66]

Any chance of a detail? If you can't make a connection work, maybe the concrete beam is not big enough.

 

[ec29]

Thanks. I am attaching a quick sketch of the detail

 

[hokie66]

I won't get involved in whether or not the concrete beam itself is strong enough, but you should be able to design hanging stirrups to deliver the 150 kip force to the top of the beam. This situation is appropriate for strut and tie design, and is similar to the design of concrete corbels, where the hanging bars are one of the key elements.

 

[JAE]

Don't forget to check the concrete beam for torsion as well.

 

[ec29]

Thanks hokie66. When you say hanging to the top of the beam it seems to me looking at the breakout cone in the sketch that there is not much of the beam left at the top to hang from. We would be taking the 150 kips with that small section of concrete left at the top. It doesn't seem possible. Am I missing something?

 

[hokie66]

Using a truss analogy, i.e strut and tie method, the "cone" is not applicable. The hanging steel takes tension, and concrete struts take compression. Suggest you study concrete corbel design. I think you US guys get too hung up with this "Appendix D", whatever that is.

 

[Ingenuity]

hokie66
Using a truss analogy, i.e strut and tie method, the "cone" is not applicable. The hanging steel takes tension, and concrete struts take compression. Suggest you study concrete corbel design. I think you US guys get too hung up with this "Appendix D", whatever that is.

Pun intended?

 

[hokie66]

No, but wish I had thought of it.

 

[ec29]

In the attached british standard for design of headed anchors in concrete (which we the US guys don't use) they talk about the concrete "cone" failure as being one of the modes of failure. (concrete cone failure doesn't happen in the typical corbel unless you are using anchors). The document says that if you don't have enough capacity in the concrete to prevent this cone failure then you can use supplementary steel.
From page 8:
d) The supplementary reinforcement should be anchored outside the assumed failure cone with an
anchorage length lbd according to EN 1992-1-1.
e) A surface reinforcement should be provided as shown in Figure 2 designed to resist the forces arising
from the assumed strut and tie model, taking into account the splitting forces according to 6.2.6.

It seems to me that you have to prevent this cone failure, if you don't have the capacity in the concrete you provide steel to take the full load (in this case the "hanging stirrups") and you have to anchor the steel outside of the failure cone (in this case what is left of the top of the beam). I don't see how that small area at the top of the beam can resist the 150 kip load.

 

[CELinOttawa]

No. Listen to Hokkie... The advice is sound.

You need a code that applies strut and tie. Pick a code, any code...

 

[hokie66]

Not sure where this idea of a cone came from. A conical failure is when an anchor is in tension. These anchors are in shear, and they each deliver a force to the concrete which in turn is carried by the hanging bars in tension, which in turn is delivered back into diagonal compression struts, which in turn are supported by other vertical reinforcement. To make this all happen, you may need more than one width stirrup in the beam.

 

[ec29]

Thanks hokie. The failure mode that I am referring to is the concrete edge failure shown in section 6.3 of the british standard that I attached.

 

[hokie66]

ec29,

While not familiar with that standard, I perused the Section 6.3, and did not find anything which disagrees with the advice I gave. It says that if the vertical component is taken into the concrete by supplementary reinforcement, the reinforcement must resist the entire force. I agree. It still works in a strut and tie manner as I tried to describe.

 

[dhengr]

Hokie:
You’ll explain this much better than I can, but, doesn’t the strut action on either side of the embed area cause a compression field which literally prevents the failure wedge (trapezoid) which the OP’er first showed?

 

[hokie66]

That's correct, when combined with the tension steel in the bottom. The hanging steel in the form of stirrups has the effect of delivering the force of the supported beam to the top, so this is like any other concentrated load.

 

[AELLC]

See attached