Uplift on masonry wall 2

[jechols]

I have a general question about uplift on a fully grouted masonry wall. I am looking at FEMA 361, Second Edition / August 2008, page 6-26 (166 of 374) Figure 6-4. Here is a dropbox link to that page:

https://www.dropbox.com/s/5zjiqca8qo8v1it/FEMA 361-Figure 6-4.pdf?dl=0

This is a diagram showing continuous load path from roof to foundation. See position 3 in the diagram. It shows a steel roof beam bearing on an embed plate with hooked anchor bolt(s) into a masonry bond beam. It also shows vertical reinforcing extending into the bond beam.

Question: Would you consider this an interruption in the continuous load path? It seems to me that the vertical reinforcing would have to develop past the embed plate.

Secondary Question: If you agree that the vertical bar should develop past the embed plate, would you consider the point of beginning for the development length, the top of the hook of the hooked anchor bolt or the bottom of the embed plate?

My concern is that during the tornado the roof and single course bond would detach from the rest of the wall failing the storm shelter.

 

[mike20793]

Yes, there is an interruption of load path if the bar is not developed. I always make sure my dowels from the roof (topping slab with precast or slab for cast in place) lap the vertical reinforcement in the FEMA Shelters I work on. Doing that will adequately transfer uplift and shear from the diaphragm.

 

[jechols]

Thanks for the reply Mike. Where would you measure the development length from? The hook on the hooked anchor bolt or the bottom of the embed plate?

 

[mike20793]

Sorry, I replied without actually referring back to the figure you mentioned. I still think there is a load path issue, unless you could have dowels at the top of the wall with a 90 degree hook in the line of the wall essentially anchoring down the bond beams. If your CMU is reinforced in every cell (I always reinforce every cell whether I need it or not), then there can be serious congestion at the top of your wall. Is your system composite slab or just form deck? The presence of steel deck on top of the beam makes it difficult to dowel into the wall, but it can be done. I would try to cast a concrete beam in between the steel beams and have dowels into the slab above that lap the CMU reinforcement. Then the steel beam is transferring uplift to the slab by 'bearing' (assuming noncomposite action for the uplift condition) and the slab is doweled into a concrete beam at the top of the wall on either side of the beam. I use a similar detail/load path when I use double tees at the roof. The people at FEMA really like it and have commented on several of the peer reviewed shelters I worked on.

Alternatively, you could try a precast system (hollow core planks or double tees) with a topping slab. Construction is quick and simple and the detailing isn't too bad.

 

[KootK]

In my opinion, whatever uplift is tributary to the steel beam ought to be transferred through the connection between the steel beam and the masonry wall. A couple of options to evaluate the situation:

1) If you think that you'll reliably have vertical bars right under the embed plate, then you would develop the bars beyond the point where they intersect the pullout cone for the anchor bolts. This would be very similar to how you would normally deal with anchors and pier reinforcing at a column base plate connection in tension.

2) You can anchor the bolts to the bond beam, transfer the tension along the length of the bond beam to where there are vertical wall bars, and then anchor those vertical bars into the bond beam from below. This seems to be what the FEMA document is suggesting. Trouble is, it's really an anchorage problem rather than a development problem and I don't really know how to assess that in a masonry/rebar connection.

I'd like to use a two course bond beam and use embedded deformed bar anchors on the bearing plate to get a more convincing "lap" with the adjacent vertical bars. That might be tough to sell to your contractor, however, as it will tend to look unconventional.

FEMA really doesn't mess around with spelling out the load path continuity thing, do they?


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.