Unreinforced masonry design & Enercalc software 1

[inju]

Hello folks:

I came across this clause in ASCE 5-05/ACI 530-05 sec 2.2.32
"Allowable tensile stresses for masonry elements subjected to out-of-plane or in-plane bending shall be in accordance of table 2.2.3.2 "
Table 2.2.3.2 gives allowable flexural tensile stress values
for clay and cmu.

My question:
1. For cmu wall subjected to net negative internal pressure(suction) which value of flexural stress do you consider in design/check of unreinforced masonry- Fb=(1/3)*f'm ..(Eq 2.4) or the one specified in table 2.2.3.2

2. When using Enercalc software ASD design what is the recommended value of Load duration factor (stress increase factor)..1.333?

3. Is Enercalc software capable of design/check unreinforced masonry.

Any thought would be greatly appreciated

Inju

 

[DaveAtkins]

For unreinforced CMU, the allowable flexural tension stress will govern. (1/3)f'm is the allowable flexural plus axial compression stress.

The 1.33 increase is no longer allowed for the design of CMU.

DaveAtkins

 

[structuresguy]

Actually, the 1.33 increase is allowed for masonry using Allowable Stress Design, if you don't also take the 0.75 load reduction for (wind + live) load case (per IBC or FBC). So if you're limiting load case is D+W, then you can take the 1.33 stress increase. This is per ACI 530-05 paragraph 2.1.2.3 The allowable stresses...shall be permitted to be increased by one-third when considering load combination C, D, or E. So per ACI 530, these three load combinations are D+L+W (OR E), D+W, and 0.9D+E.

Per Florida Building Code, they allow the same usage of the 1/3 stress increase, as long as the 25% load reduction for combined live and wind loads is not used.

Also, Enercalc can not handle unreinforced masonry. Sorry.

 

[DaveAtkins]

structuresguy,

IBC Section 1605.3.1.1 specifically forbids the use of allowable stress increase except with wood. Am I missing something?

DaveAtkins

 

[WillisV]

Dave - your not missing anything - ACI 530 permits it but IBC overrules it and permits it. If you read the commentary of IBC they basically say that though they permit it, many of the building codes do not.

 

[WillisV]

errr...IBC overrules it and doesn't permit it...

 

[inju]

structuresguy,

Great thanks for your valuable and concise answer.

DaveAtkins and WillisV :

Thanks for your suggestions.

How could IBC Section 1605.3.1.1 and ACI 530-05 paragraph 2.1.2.3 be reconcilled?

I would really like to use 1/3 increase in allowable stress.
(I have exterior wall around stair, 15ft tall, 8"cmu, 65psf wind load, even with 1/3 increase #6@16" reinf does not seem to work)

Do you guys consider reinforcing interior non-load bearing wall in huricane-prone area?

Thanks again,

inju

 

[DonPhillips]

I am not sure the confusion. 1605.2 is LRFD and 1605.3 is ASD.



Don Phillips

http://worthingtonengineering.com

 

[sandman21]

1605.3.2 allows the use of a 1/3 increase for both wind and seismic.

 

[structuresguy]

would really like to use 1/3 increase in allowable stress. (I have exterior wall around stair, 15ft tall, 8"cmu, 65psf wind load, even with 1/3 increase #6@16" reinf does not seem to work)

This is getting pretty near the limit of 8" masonry walls. However, I checked with Enercalc Slender Wall module and it seems to work with #5 @ 16" o.c. Though since I don't know what your DL & LL requirements are, I can't say for sure.

For walls with very high wind loads, they are typically governed by flexure. In this case, you want to use the Enercalc Slender Wall design module, which uses the Strength Design Equations, rather than allowable stress design. When using strength design, you can not use the 1/3 stress increase. But you won't need to, as you can use teh full strength of your 60 ksi steel. ASD restricts the steel tension to 24 ksi max, so even with 1/3 increase, you only get 32 ksi; compared to 0.9 * 60 = 54 ksi for strength design.

Do you guys consider reinforcing interior non-load bearing wall in huricane-prone area?

This would not be typical, but it is done sometimes. Is the wall subject to the wind loads? If not, then I would only check for the minimum pressure required (5 or 10 psf) for interior walls. However, if you are trying to compartmentalize a structure, then you could design the interior walls for wind loads. I have done that where the client wants a hardened room inside the structure, or where part of the structure is a critical facility, such as an emergency operations center.

 

[DaveAtkins]

1605.3.2 DOES allow the 1/3 increase, but you have to increase wind load by a 1.3 factor, so you don't gain much.

DaveAtkins

 

[ChipB]

An old rule of thumb for maximum wall height is:
1.5 times the nominal width of your block, is your max height in feet. So an 8" block wall should be about 12' tall. Otherwise you have to reinforce the bejesus out of it (as you're finding out).

All of my engineering friends in Florida, have switched over to masonry strength design as structuresguy has suggested. Too much steel in ASD. I think he has given you your best option for the least amount of steel.

I have used the intermediate landings, as diaphragms to brace the wall, and distribute the lateral load into the shear walls. The only potential problem with that is, do you design the masonry as your MWFRS or is it now "components and cladding"? I've heard opinions (arguments) on both sides. I design it as MWFRS. You could also distribute bond beams such that it gets two-way action.

Sadly, yes, they have taken our 1/3 increase (kind of) away. We get some of it back with multiplying any combination of two (or more) variable loads by 3/4 with full dead. Inverse of 3/4 is 4/3=1.333 and there it is. Unfortunately, it doesn't apply to D+W.