Uplift on CMU wall

[jike]

I have a net roof uplift load of 15 psf (adhered roof) (06.D+W). The 40 foot steel roof joists bear on a bond beam in a CMU wall. The net uplift load on the wall will therefore be 40/2 x 15 psf = 300 plf. Without counting on mortar tension, it seems like I would have to engage (1/0.6 x 300 =) 500 plf of masonry to provide enough dead weight (or is it 1.5 x 300 = 450 plf?).

What is the best way to accomplish this?

Some options that I thought of:

Provide #4 vert @ 48" oc in grouted cores

Grout upper courses of block needed for dead weight.

Provide lower bond beam at elevation necessary to engage that much masonry dead weight. Provide vertical rebar from joist bearing elevation to lower bond beam.


Any other suggestions? What have others done in this situation? How many of you have ignored this requirement in the past?

 

[TFL]

as far as i am aware seismic requirments require vert bars @ 48" o.c. in walls in sesmic catagory c or greater. therefore i always have #4 @ 48 min. = uplift not a promlem. does your wall work for out of plane wind loads unreinforced? i would recomend reinforcing the wall for the uplift W/ vert bars

 

[JAE]

With 0.6D + W (as with any of the required load combinations) you have to follow the full load path through the building.

So the joist must be designed for the net wind uplift.

The joist connection to the wall must also.

The weight of a reinforced bond beam at the top of the wall (usually required by code anyway) can be used as counter weight to the uplift.

Reinforcing inside the wall to engage more of the wall below.

 

[Lutfi]

If you have that much net uplift, then you must have wind pressure on the walls. How did you design the walls for the wind? I live in Florida and we provide vertical bars in the CMU wall to resist the bending moment created by the wind on the walls. These bars also tie the tie beams (we use CIP tie beams in lie of bond beams in the most part here in Florida. These vertical bars hook into the foundation and into the tie beams. Therefore, you have continuous load path to resist the uplift forces.

JAE is right on target in his statements.

Good luck.

 

[jike]

Building is not in a seismic zone. The wall will work as an unreinforced CMU wall to resist wind.

The situation wasn't critical until the architect eliminated the stone ballast on the roof. The weight of the bond beam alone is not sufficient to resist uplift.

Thanks, JAE, for your confirmation to follow the load path thru!

The architect will not be happy, but then again he never is!

 

[JAE]

One thought, jike, is that you could place the bond beam at the second course down from the top, that way engaging two courses of weight which might tip the balance and get you what you need. Just be sure that the joist connection extends down into the lowered bond beam.

 

[Ron]

jike....as usual our colleagues JAE and Lutfi are right on target. One additional consideration since you mentioned it to be an "unreinforced" masonry wall......your dead load resistance must be balanced to the point that tension in the mortar is no longer an issue. That could be several courses down as JAE noted, but you need to check it...otherwise complete tensile load path to foundation is way to gain adequate resistance. Also, don't forget to consider that wind blowing on the wall will create tension the inside mortar joint; which is also under tension from roof uplift. The forces are additive, though they actually only occur at one transient point in the load application.

 

[haynewp]

If you hook the vertical reinforcing bars at 4' o.c., you have a good chance a joist will be attached to the bond beam between vertical bar locations.

Remember to design the bond beam to resist the bending due to uplift between vert bars locations. I would not count on the mortar that attaches the bond beam to the next course below. Then I would just continue these vertical bars into the footing.

Is there shear on this wall?

 

[bjb]

In my opinion another point to consider is development of the vert wall reinforcement into the bond beam. As I see it, the top bond beam spans between vert rebars which provide the holdown reaction. I have seen some designs require hooks in the rebar into the bond beam. Also, providing a 2 course deep bond beam can give you more length to develop the bars.

It's my understanding that your wall works unreinforced for out-of-plane loads from wind. Don't you at least need some rebar to resist shear wall overturning, and also some jamb bars at openings?

 

[jike]

Yes there are jamb bars at openings. There is no bars at end for shear wall since it is a very long wall.

I like the idea of the 2 couse bond beam.

Thanks to all!

 

[sda97]

When you calculated the uplift on the joists, did you use component wind load analysis? Is your anchorage to the wall based on this compnent analysis?

 

[jike]

Yes