Bearing Length for Steel Channel Pinch Beam

[dabomaman123]

Good Day,

I am currently working on a project where the client would like to create a 13'-0" opening in an existing 8" thick CMU wall (unknown if fully grouted). I have decided to proceed with designing a steel pinch beam. In my case, I will be using two C12 steel channels that sandwich the CMU with tightened bolts.

My question is how much of bearing length would be required at the ends of the pinch beam? I believe the correct design would be to have the bearing length of the steel beam to be equal in length to a CMU pier to resist the same weight. Then the bolts would need to be checked for similar bearing.

If someone could help direct, it would be greatly appreciated. Thank you!

 

[WARose]

ACI 530 gives allowable bearing stress (0.25f'm, if memory serves). You are also given guidance as to how to spread the bearing load out over the supporting masonry.

 

[birneys]

I would proceed with great caution. There are multiple issues with using something like this for long-term support. I know contractors use them for temporary shoring, but, personally, I won't use them for a permanent lintel. You're relying on a handful of bolts to support CMU of unknown quality across the opening. I'm not saying it can't work, but there are several places where I would check my numbers very carefully if I was forced to use a "pinch beam". I would typically install a W-beam lintel within the wall, and leave the shoring up to the contractor to install it.

Sorry, I'll get off my soap box now.

 

[dabomaman123]

Thank you for your responses. I would like to proceed with caution as well.

It would be difficult to install a single W-Beam of such long length in the space required. Perhaps it is not impossible though. Perhaps two small W-Beams. I found some youtube videos of really nifty ways contractors have been able to shore a wall to install a new beam.

Back to the pinch beam. I am not as concerned with the CMU above the pinch beam, I am more concerned with the bearing at the end conditions. Meaning that the total end reaction is going to a handful of bolts. Bolts may be strong enough, but the bearing support on the existing CMU is so minimal to be concerning.

Additional input is appreciated. Thank you

 

[MotorCity]

Maybe I am not understanding the arrangement, but what are the channels going to bear on if they are clamped to the outside faces of the wall?

 

[dabomaman123]

Here is a previous thread that includes the pinch beam detail:

https://www.eng-tips.com/viewthread.cfm?qid=434595

I believe a lot of discussion is based on the design of the bearing bolts. But I am concerned with the bearing length of the channels at the ends of the steel. For example, when I design a precast lintel, we like to have 1" of bearing for every 1'-0" of clear span (so a 13'-0" opening would require 13" of bearing at each end). What would the procedure be for the steel pinch beam bearing design?

 

[bob33]

You could install bearing plates at both ends first. The plates would cantilever out to pick up the channels. Then cut the opening.

 

[dabomaman123]

bob33: that sounds feasible. I am concerned with the installation. Also, should not the CMU that the plate is bearing on be fully grouted?

 

[bob33]

Yes, CMU is normally grouted under bearing plates, but in this case it may not be necessary depending on the load. I am assuming you were just looking for tips and that a structural engineer will be involved in the final design.

 

[T_Bat]

We usually break out the face shells above the ends units and fully grout. You can also install vertical channels on the sides of the openings installed flush under the pinch beam to bear. Then the steel jambs provide the bearing.

 

[HouseBoy]

Much will depend on the actual loading and "the numbers" but.... I don't recall ever using the bolts alone to provide the support for such a pinch beam solution. I probably have done that but it would be unusual and the loading would have to be slight.
Ordinarily I use a bearing plate at the end that projects out from the face of the wall to support the steel channels.
Usually are bearing plate is relatively thick (to provide sufficient stiffens) and the cells where it is bearing are grouted solid.
I'm pretty sure that is what bob33 is describing too.
In extreme loading cases I use T_Bat's idea of channels on the sides -OR- I have installed a steel column within the plane of the wall with a wide plate on top (sometimes with gussets, then the pinch beams and then cut the opening.

Seems like the 12:1 ratio is a good starting point but there are many factors that contribute to whether or not the numbers work out.