development of hooked dowels at the top of masonry walls

[Gopher13]

In buildings constructed with masonry walls and wood truss roofs, I have seen a typical detail used again and again at the top of the masonry walls. Please see the attached sketch. The masonry wall is reinforced and there is a hooked dowel at the top of the wall into the bond beam. There is a double 2x top plate on top connected to the bond with a cast-in headed anchor. With the bond beam being nominally 8 inches deep, the cast-in anchor can only be embedded just over 5 inches before it would hit the bottom of the horse-collar bond beam. My question is: for the wall to be properly tied together, shouldn't the hooked dowel have to be fully developed by the bottom of the cast-in anchor? If this is true, the bond beam would have to be 16 inches deep to allow for a deeper embedment of the anchor and the correct development length of the hooked dowel. Thanks for any guidance you can provide.

 

[DaveAtkins]

I think most engineers don't even provide a hooked dowel at the top of the wall, like you have shown.

Don't forget the mortar joints themselves have tensile capacity, probably enough to resist the uplift from the wood trusses.

DaveAtkins

 

[MotorCity]

I have never used that detail or similar. It can be a real pain for the mason to run verticals through the bottom of a bond beam.

 

[BridgeSmith]

It would seem that with the head on the anchor bolt overlapping with the hooked reinforcement would provide pullout strength at least close to steel capacity. I would suspect that if the connection was tested to tension failure, the first thing to fail would be the wood top plate, either by splitting or by the bolt pulling through. In reality, the connection of other components to the top plate are unlikely to be attached securely enough to apply anything close to the force necessary to fail the connection of the top plate to the wall.

 

[WARose]

I've done that a few times myself. The answer to your question about should it be done depends on the uplift. Assuming your anchor bolt gets it into the bond beam.....you then have to get it into the wall. That detail could do it. (Even if you only use partial development length.)

In fact, sometimes I've seen people use a 180 degree hook (around the horizontal bond beam reinforcement).

 

[Gopher13]

WARose - What is your preferred detail? Is it the 180 hooked dowel? Maybe if the head of the anchor is below the bond beam reinforcing and the 180 degree hooked dowel is hung from the bond beam reinforcing, there would be a proper load path.

 

[jayrod12]

How much uplift are we talking about? Sometimes I've had it preferred to put a strap over the top of the wood nailers extending down the wall on each side with concrete screws. It usually depends on the finishes etc.

 

[BridgeSmith]

For the configuration shown in the sketch, it seems the only area of concern for pullout failure of the anchor bolt would be whether the hooked reinforcing bars are placed at the same locations as the anchor bolts, so that reinforcing bars are fully within the potential failure zone (cone) of the anchor bolt. If not, you'd likely need a washer under the head and drop it enough to lock it in under the bond beam reinforcement to get the full tension capacity of the anchor bolt.

I would start with how much tension capacity is required by the weakest link in the load path, and then look at what embedment depth and/or reinforcement is necessary to provide that capacity.

 

[WARose]

WARose - What is your preferred detail? Is it the 180 hooked dowel?

It's certainly less obtrusive in the cell. But the transfer (from the hook to the wall) is tricky. I've heard a lot of different opinions as to how that goes.

 

[Gopher13]

jayrod - The wood trusses are spaced at 2 feet on center and span 50 feet with a 6 foot overhang on each side. So a good amount.......or 582 pounds per truss using allowable stress design load combinations.

 

[jayrod12]

To avoid the complaining by the masons of trying to get the wall rebar through the bond beam, I have previously specified that they use knock-out web blocks in lieu of bond beam blocks and paper stops.

All in all it's not fun one way or the other. Honestly, if I was the guy on site building it, and the finishes on the interior/exterior allowed, I'd prefer to use something like a simpson twist strap with concrete screws to deal with the uplift. Then you only need to worry about connection for the lateral loading which is much more manageable.

 

[BridgeSmith]

"...582 pounds per truss..."

Peanuts for the anchor bolts...unless your anchors are spaced well over 10 feet apart. Per the AASHTO bridge design spec, at the minimum 6" embedment depth for a deformed #4 hooked bar (comparable to a 1/2" headed anchor bolt), the tension capacity is in excess of 8 kips per bar. That's breakout force for 3.25 ksi concrete without confining reinforcement.