Anchoring to top of CMU wall

[msdengineer]

Following the ACI 530 section 2.1.4 or the corresponding LRFD section, the design of anchorage to resist tension and shear can be designed. It appears that one of the (2) equations for each direction of load resistance (tension and shear) take into account the masonry construction, while the other (2) take the strength of the bolt into consideration. Concerning the masonry construction, the tension strength uses the projected area of the bolt and the shear strength uses the "makeup" of the masonry construction.

My question is, how do you account for a continuous bond beam at the top of the wall or vertical reinforcing. In my mind, this will create a continuous load path to the foundation, but I am unable to find any technical justification for the design of anchors utilizing the supplementary reinforcing in this way. Thanks in advance.

 

[Den32]

ACI 530 anchor equations are basic (and probably really conservative) in comparison to ACI 318 Appendix D equations. ACI 530 doesn't account for reinforcing. If you are using a Post-Installed Anchor - you can use Manufacturer's data for 'Grout filled Stem Walls'

Regarding your load path question - without knowing more details or loads - the anchor would transfer load to the bond beam, then the bond beam would transfer uplift/tension load to vertical wall reinforcing (so depending on loads, bond beam would need to be checked to 'span' to vertical reinforcing - plus you should check 'development length' of vertical reinforcing in bond beam if loads are high). if the loads are small, the bond beam self weight, or the mortar bond to wall below may resist the uplift loads.

 

[a2mfk]

I think I know what you are getting at... If you have a relatively high uplift that you need to transfer into a CMU wall, one way to accomplish this is to align a vertical filled cell with that location. Then you can extend a threaded rod from your connection into that reinforced filled cell and then design this as a lap splice between the all-thread rod and the rebar. I know this may not be specifically spelled out in ACI 530 but...

Regular shear cone failure from ACI 530 won't get you much with edge distance reductions. Like Den said, the first option for me has always been to check an epoxy bolt product like Simpson or Hilti, since they establish their values through testing rather than analytically. And their tables are much easier to use and try multiple iterations...

 

[msdengineer]

Thank you guys for your responses. Both very helpful.

I agree that a post installed anchor is a quick and easy solution to use when loads are relatively small.

In this particular case, I am anchoring a shear wall holddown for flat strapped x-braced shearwalls on top of a CMU foundation stemwall. The loads are higher than the allowable design values for typical post installed anchors or a simple failure cone analysis from ACI530. I am trying to investigate an alternate solution to having to extend all threaded rod thru the foundation wall into a concrete footing. The constructibility of such a detail is questionable.

Do you have any more recommendations for designing the anchor to properly tie into the bond beam?

 

[a2mfk]

For a stem wall I would go straight to the foundation with an ATR like you are thinking, you know exactly how the load gets to the footing that way. They can coordinate this at a few limited locations, not that they will like doing it but....

 

[southard2]

When designing joist embed plates or any anchor bolts to the top of CMU walls this is how I approach the problem.

1) First I see if using the ACI 530 equations work as is. Usually I need to spread the anchors for an embed at least 8" apart so I get some good shear values.

2) If the the studs or anchors fail in tension then I do one of two following solutions


A) Pour a 16" concrete tie beam at top of wall and then use ACI to figure my capacities or,

B) I put NELSON D2L deformed bar concrete anchors on the embed plate 8" apart and demand that they be lapped with the vertical reinforcement in the wall. I use this a lot with joist girder tie downs or where I have really high uplift loads. For individual joist this is cost prohibative so I usually go with option A. The D2L anchors come in length of 24 to 36 inches. So you can get the proper lap splices in order to create a continuous tie of the embed plate down to the foundation. The D2L anchors are deformed just like rebar but they are made of a high strength steel. With the D2L anchors you only have to worry about shear then.



John Southard, M.S., P.E.

http://www.pdhlibrary.com