Blast Design of Reinforced Masonry Walls 1

[steellion]

I'm trying to determine if the reinforcement provided (#5 @ 32" oc) is sufficient for a masonry wall with a punched opening supporting a window. Conventional Standoff distances are met, so minimum blast pressures apply.

I have two types of software at my disposal capable of doing dynamic analysis: HazL and SBEDS. The problem is once I have the dynamic results, I don't know what to do with them. HazL will give the edge shears to design the window support for but won't design the supports themselves. SBEDS will design a masonry wall, but there's nowhere to put in window size.

 

[structuresguy]

What is your blast pressure? How high is the wall? #5 @ 32" o.c. is pretty light reinforcing for a masonry wall supporting any significant pressures. That reinforcing won't even work for moderate hurricane wind speeds, and normal floor to floor heights. And these pressures would be about 30-50 psf. I know blast pressures can easily get to 1 psi (144 psf) or even much higher.

 

[steellion]

The 8" wall is only 12'-8" high, non-load bearing. Standoff distance is met, so only the windows need to be considered for blast design (no hardening). Therefore, the static equivalent pressure is about 1 psi, but that's only acting on a 4'x4' window, not the whole wall.

So I have my dynamic loads, but I don't know where to go from there. Can I convert this to static somehow? How much of the wall can be considered to be resisting this pressure? How do I figure out a masonry wall's "ultimate" capacity for a small region? I know how to do it for steel or CFS studs, but the confusion comes when it is a wall, particularly a masonry wall.

 

[jike]

The blast pressure that I have delt with is normally much higher than normal wind loads. A 4' x 4' window is not much of a blow out panel to relieve the pressure build up in the room.

If I recall correctly, NFPA is a good resource for calculating pressures.

What is conventional standoff distance refer to?

 

[steellion]

Jike, I am referring to AT/FP blast loads, not internal blasts such as a chemistry lab. Sorry for the confusion.

 

[structuresguy]

I must be confused, becuase I don't know why the pressure on the window would be any different that the rest of the wall. If you had 1 psi pressure on the wall, the reinforcing you list would not be adequate. I know you may be concerned more about ultimate failure of the wall, rather than allowable design values. But in this case, with 1 psi, you have exceeded the design capacity of the block wall by about 2.3x and the capacity of the steel reinforcing by about 2.7x. The stress in the steel would be about 87 ksi, which is higher than the yield strength. Whether any of this would be high enough to actually cause the wall the "fail", I don't know. Only testing would tell for sure. By fail I mean, complete rupture of the wall, not just extreme cracking, which would most certainly occur. This pressure may be enough to cause the wall to have explosive spalling on the inside as well, which can be a problem with block walls.

 

[KBVT]

I think the OP is referring to UFC requirements, where if a building is located at least the "conventional construction standoff distance" away from either a controlled perimeter or roadways/parking within a controlled perimeter, than the blast design is limited to windows, skylights, etc. A blast analysis of the entire facade is not required.

 

[Gumpmaster]

I'm following you steellion.

I use SBEDS to design the jambs. You kind of have to fake it and determine an equivalent reinforcing spacing to account for your jamb steel. I've had to increase the normal reinforcing in my jambs to take the glazing reaction.

Steellion is on the right track. He's following the UFC 4-010-01 article B-3.1.4 methodology.

 

[steellion]

Thanks Gump! That was the answer I was looking for. I was hoping to not have to "fake" the calcs, but it seems like that's the best option. I think SBEDS results are what the govt. is looking for in the calcs, so hopefully it goes through.