Tall Masonry Wall 1

[jechols]

I have 45' tall 12 cmu wall. This wall supports a roof and stage rigging system. The loads I have calculated are:

Dead Load - 1000 plf
Roof Load - 1000 plf
Live Load - 1620 plf (for the stage rigging equip.)
Wind Load - 17 psf

I have determined the wall should be reinforced with #6 at 16" on center, each face. The wall has brick veneer which I will support with a relief angle at 30'.

The deflection meets L/600 without the 0.7 reduction factor.

I am using the NCMA design software.

I have never designed a load bearing wall this tall before, so any thoughts would be greatly appreciated.

Thanks,
J

 

[civilperson]

You forgot to list self weight with the Dead Load. I get a moment of 27.54 ft-k/ft from wind loads and thus a required moment strength of 367,200 in-lb. The #6 at 16 inches is 0.33 sq in/ft so is not sufficient assuming a d of 10.1". the brick is not available for resisting moment. (0.33 sq.in. x 60ksi x 10.1"= 200,481 in-lb, NG).

 

[shin25]

The induced moment will be acting with compressive load on the wall. So, you need to construct an interaction diagram for the wall.

Is the wall braced? If not, you have to consider slenderness effect.

What kind of wind velocity are you considering for design? Depending on the location of the wall in the building, 17 psf wind load sounds too low.

 

[JAE]

Questions:

1. Did you include in the loading the eccentric DL moment from the brick veneer sitting on the relief angle?

2. Did you include some provision for additional Pdelta effects due to the lateral wind plus vertical axial loads? According to a spreadsheet I wrote, this could add about 4 to 5 psf of additional "equivalent" lateral pressure on the side of the wall. The ACI 530 has some information on this in the strength design section (section 3.3.5.5)

3. Would intermediate bond beams placed at some distance apart - say 12 feet o.c. help with uneven load distribution?

4. Did you include an eccentricity in the axial loads? Especially the rigging?

5. Does the rigging loading have any lateral load to them?

Hope these help.

 

[msquared48]

I realize that you are using CMU for your wall, but there is an old PCA manual for "Tilt-up Load Bearing Walls" - from 1979 I believe.

It has tables that are probably not applicable in your case, but the methodology should be the same. If you can get your hands on one, it might help you formulate your method of attack to the problem.

There is also a similar publication from PCA - "Notes on 318-77", Appendix 22A covering a "Design Method for Lightly Loaded Walls". The current PCA notes might have a current rendition.

Good hunting.

Mike McCann
McCann Engineering

 

[jechols]

Thanks everyone,

I have decided to use a steel tower to support everything. The P-Delta effects mentioned by JAE were not being accounted for by the NCMA program as far as I could tell.

Thanks again!
J

 

[WillisV]

The NCMA program does account for P-Delta effects provided the IBC strength design provisions are chosen as the design method. The masonry code does not require a P-Delta analysis for allowable stress design methods and therefore the program does not perform this check for ASD methods either. This is a gap in the code and will be fixed in the future though.