Double Bond Beam Instead of CIP tie beam? 2

[rmartinat]

Hello! I have a project where the client is the GC & the developer. So, he is asking me to do some value engineering. The project consist of a duplex 3-story building. I have bearing load CMU walls and a CIP structural slab structural system. I have two questions:
1) Would you replace the the 8x16 conc. tie beam with a double bond beam on the exterior walls? According to the shell contractor there are some significant saving by doing that.
2) I have a quite important 8x16 conc beam that connects perpendicularly to my existing 8x16 tie beam. In the event I make the switch to a double bond beam, I would need to connect this beam to the double bond beam. I personally never done something like that. Any thoughts?

Thanks in advance for your time and help!

 

[phamENG]

1) Yes. I would never have detailed a concrete beam in the first place. Well, maybe not never, but I would have started with CMU and would have only switched if there was a very compelling reason to.

2) What kind of connection? If it just needs bearing, that's easy. If it's a moment connection...well I'd question how that was supposed to work anyway unless you have significant torsion in that edge beam. (That might be one of those compelling reasons I mentioned.) If that's the case, then you may just need to resize the concrete beam and make it larger to be simply supported. "So, Mr. Developer, if you want savings from one thing, it will cost you here." The balance may be in his favor, or maybe it isn't. Just have to check.

 

[EZBuilding]

The concrete tie beam provides for simpler load pathing in relation to the double bond beam - namely for uplift connections should you have a wood roof. It takes a bit more consideration, but it is something I have done in the past. There is often a slab turn down above the double bond beam to make up for differences in heights from top of wall CMU to bottom of slab.

The concrete beam would need to bear on top of the masonry wall with the masonry wall vertical reinforcement and bond beam reinforcement through the concrete beam. Depending on the magnitude of the loads, I would consider returning the concrete beam an additional cell on either way to get it additional bearing/vertical reinforcement through this section.

 

[rmartinat]

It is a moment connection, and of course we have a very tight clearance that prevent a deeper beam. Thanks for your reply!

 

[JLNJ]

You can get more strength from a concrete beam than a masonry bond beam, so be careful about guaranteeing a 1:1 replacement.

I’m also leery of intersecting CMU beams. Be careful about the detailing and shear calcs there.

I really like the way Pham brings the business end of things into it. Why should you spend your own money to put money in the developer’s pocket?

 

[EZBuilding]

I would agree with phamENG's concerns regarding the fixity that the edge tie beam provides. I expect that most of your rigidity from that assembly would be a product of the flexural stiffness of your masonry wall and not so much the torsional stiffness of your beam. Accordingly, I'd expect some similar levels of fixity provide by a full height masonry wall versus a full height masonry wall with a tie beam at the top of it. A plan view of your condition could help us look at this a bit more holistically

If you substantially need the fixity for the beam, you could consider utilizing a reinforced concrete column within the profile of your masonry wall.

 

[phamENG]

rmartinat - what was going to resist the moment where it hits the outside wall?

JLNJ - not exactly what I meant, but you're right. This should absolutely be an additional service if the OP's contract allows for it, which it should. I was referring more to tradeoffs in the design. If the perimeter beam is concrete to resist torsion developed by a moment connection in a transverse beam that had to be fixed at the ends to get strength and deflections to work with a shallow beam to meet height requirements...well then he either has to pay for that or he has to pay for making the building taller to make room for the deeper, simply supported beam. No free lunch, as they say.

A lot of these contractors and developers focus in on a tree and forget that it's part of a forest. They want to cut out a tree here and there to save money, but don't realize that without making another one bigger, the prized wood pecker will never live there. (Sorry...I was grasping for straws to finish out that metaphor.)

 

[rmartinat]

Great! Thank you everybody for your help! I think I can provide a more educated response to my friend the developer now!