Chemical anchors in an old RC beam and concrete edge breakout

[nivoo_boss]

Hey everyone!

So I'm designing a little extension of a shopping mall. It is to be made in steel. The mall itself is built in the 1970s in the Soviet era. I have the old drawings so I have a pretty good idea about the existing structures.

Anyway, I would like to support a new steel beam on a pre-existing RC beam. I would use chemical anchors to fasten the support part to the end of the RC beam. The design shear force on four anchors is around 61 kN. I'm using Hilti Profis software to design it and because of concrete edge breakout the anchor capacity is used around 300%. Now the thing is, in reality all the anchors are placed between the tightly spaced stirrups and longitudinal reinforcement - what concrete edge breakout could realistically happen here? Attached is a little drawing about the situation.

 

[KootK]

what concrete edge breakout could realistically happen here? Attached is a little drawing about the situation.

My guess would be concrete pryout or some sort of side face / top face blow out. What exactly does the software output say? The stirrups won't help except, perhaps, as supplemental reinforcement. And the contribution of the longitudinal bars will be hard to mobilize because they are unlikely to be developed within the failure frustum.

I totally get what you were trying to do with the style of connection that you choose and it's clever. Something that I've shown below might improve your anchorage situation, however, by reducing the tension on the anchor group of top.

 

[KootK]

A still better option might be to field weld a double angle connection to both the anchor plate and the supported beam. That would eliminate most of your load eccentricity.

 

[MIStructE_IRE]

Design as normal but get some site pull out tests done

 

[dik]

or something like:

for 3/4"dia A325 fasteners, 2 bolts is likely lots... you may have space for 3?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[nivoo_boss]

Thanks for the answers and ideas. The edge breakout warning comes up even without taking eccentricity into account, so my guess is the software assumes something like below, I added the red cracks:

With eccentricity a concrete breakout capacity comes up due to tension in the anchors.

You may also notice I cannot describe the beam end with the real shape it has and I cannot describe the exact reinforcement it has - and I get it, it would be very difficult to develop a software as sophisticated as this, espacially some browser-based one like Hilti Profis.

So still, I'm a little confused about the edge breakout. I think KootK had the idea that it's because of tension in the top anchors but I don't think so - it occurs with shear loading only. If it really is something like I sketched with my red cracks in the first screenshot, wouldn't the stirrups help here a lot?

 

[KootK]

If it really is something like I sketched with my red cracks in the first screenshot, wouldn't the stirrups help here a lot?

Yeah, they'll certainly help for the direct shear breakout check. But, then, the stirrups will not be mobilized until after the cracks and the breakout frustum form. Depending on how many stirrups you need to resist direct shear, this could create a significant eccentricity between your load and the resistance provided by the stirrups. That, essentially, is a version of the pryout failure mode. And I'm not sure that you'll have any, properly anchored longitudinal steel available to help with that pryout failure mode. Using the stirrups in this way makes a lot more sense to me when all of the steel that you would need to mobilize is located very close to where your anchors are delivering their shear into the concrete mass.