Anchor Shear Capacity in Double Wythe Brick

[lutein]

Does anyone know where could I find the allowable shear for anchor bolts embeded in the collared joint at double wythe bricks wall?

ACI 530 Commentary Section 2.1.5.2 states that the shear bond strength of collar joints with mortar could vary from 5 psi to as high as 100 psi. I was trying to find some publications that address the anchor bolt shear breakout strength in this kind of construction.

I would appreciate if you could shed some lights into this matter.

 

[Lion06]

Is the shear in the plane of the wall or out of plane? Is it a high load? If it isn't a high load, just check it for breakout of the collar joint and neglect the wythes.

 

[lutein]

It is out of plane and the loading is approx. 400plf. What is the breakout allowable stress for collar joints? Thanks in advance.

 

[Lion06]

First, let me make sure I know what a collar joint is. That is the grout placed between two wythes of CMU, correct? If that is correct, just use the shear breakout equations from ACI 530 and see if you have enough capacity just in that joint.
400 plf is pretty small. If the load is not reversible, then place the anchors such that the edge distance in the direction of the load is whatever you need it to be to work.

 

[hokie66]

Reading your posts, I still don't know how your bolts are oriented. To be in shear and solely within the collar joint, the bolts would either project from the top or end of the wall. In either case, and loaded perpendicular to the wall, I think breakout of the brickwork would be the controlling case.

 

[lutein]

Thank you all for the quick response. The bolts are embeded from the top of wall into the 'space' between the 2 layers of brick, i.e. the collar joint.
The shear is out of plane pushing away from the building.
I am looking at the mortar joint bonding failure, 5 psi per ACI 530 2.1.5.2.2. What do you think?

 

[dougantholz]

In my discussions with Hilti, the brick wall values in the HY-20 are based on failure of the mortar. The problem with the design values is typically the age of the masonry. Mortar looses strength over time due to lime leaching out of the joint. Quality of old masonry can also be in question. You can't really trust the look of mortar becuase some cheap tuck pointers repair only the last 1/2" and do not fix the entire joint.
The fall back would be to have your anchor mfr come out and pull test the anchors. I swear I don't work them, but our local Hilti rep has often offered to come out and pull test. You can use Ch 17 of the IBC to determine test requirements.

 

[lutein]

dougantholz - I concur with you about the age of masonry and the governing mortar strength. This is actually an existing building that I am trying to determine the original designed capacity of the bolts embeded into double wyhte bricks wall from top of walls. I could not find any literature that discuss this design.

 

[dougantholz]

the UBC had tables for embedment into masonry - and I believe that goes back about 20+ years. Look in the masonry chapters, but of course you need an old IBC.
Your most reliable is to test 10% or so of the anchors for 2.5 or 3 x the load in pullout (the way all anchors fail).

 

[hokie66]

Applying lateral loading to the top of the wall as you describe would bend the wall perpendicular to the bed joints and also bend one wythe horizontally. I don't think load testing the bolts would prove anything, as the resisting force would negate the bending.

 

[apsix]

On a slight tangent;
"You can't really trust the look of mortar becuase some cheap tuck pointers repair only the last 1/2" and do not fix the entire joint."

The whole point of repointing is to replace only the external 1/2" or so. This is effective and hopefully affordable. 'Fixing' the entire joint would mean rebuilding the wall.

 

[dougantholz]

Aspix, anyone who has ever walked up to a wall, pulled out a brick by their bare hands and watched mortar run out of the joint as sand won't trust an old wall. A lot of them even look good on the outside becuase they've been tucked. Structurally we depend on the whole cross section and not only a 1/4" of mortar, so we need it.

 

[lutein]

This is an existing building which was built 50 years ago.

I think the shear breakout of 1 single bolt should be 5psi (per ACI 530 2.1.5.2.2) x shear cross section = area at bottom and 2 sides of 1 brick

I could not find any other literature that discuss the shear stress of a mortar joint, other than using 5 psi from ACI 530 specified as the shear bond stress between collar joint and brick face.

 

[apsix]

dougantholz
Yes, in that situation repointing obviously isn't the answer.
I agree that the condition of the mortar should be checked beyond the external face.

 

[youngstructural]

Interesting post... One point I'll comment on.

Dougantholz & Apsix:

My standard spec for working with old buildings addresses this issue by nail point test. I say something along the lines of:

"When the mortar is being re-pointed, if removing 1/3 of the mortar bed does not reveal a sound middle third (sound being defined by capable of sustaining six firm blows of a standard carpenter's hammer driven 3mm headed nail set while yielding to 5mm or less), deep pointing will be required. It is suggested the stability of the wall be maintained by a combination of propping of the overall wall and metal shims in the mortar joints. Stability of the wall remains the responsibility of the contractor at all times, and the contractor is advised to engage an engineer to prepare formworks and falseworks procedures and drawings if they feel professional engineering input would be warranted."

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[apsix]

While on the topic of old brick walls; If it is a cavity wall the condition of the wall ties should be checked, corrosion can be a problem.

 

[csd72]

Look at the hilti catalogue, it may be some help.