Masonry Wall Design

[jonathanwilkins]

Looking at designing a load bearing, masonry wall in a high wind area. Wind Load is 37psf, wall is 17' tall. For the sake of simplicity for my question, I'm treating the roof load as negligible and only using wind loads.

Going through the TEK manual, I get a #5 @ 24" o.c. from the allowable stress design tables.

**16,039lb-in/ft required, 16,420lb-in/ft provided

When I do strength design per ACI530...I get (without any reductions, yet)...33,374lb-in/ft provided. The formula I used is Mn=(Asfy)(d-(a/2))

I thought maybe instead of fy=60,000psi, I should've used Fs=24,000psi but that yields 14,118lb-in/ft. Do the 2 methods yield these drastically different results or have I made a mistake? I also cannot seem to "back calculate" the allowable stress tables.

Thanks for the attention. JEW

 

[ToadJones]

For Masonry I do believe you should be using Fs=24,000 psi.
Are you sure you are using the proper equation from ACI-530?
I think you should be using EQN 2-17 or 2-18.

 

[slickdeals]

Toad, he is talking about strength design. Ch 2 is for ASD.

 

[ToadJones]

ah...my apologies.

 

[jonathanwilkins]

The equation I used is a strength design equation (chapter 3), not an asd equation (ch 2). I used the table for the asd reinforcement.

I, too, thought I should use Fs, not fy, however eq 3-27 & 3-28 both use fy. **3-27 and 3-28 are the equations I used to get the numbers above. 33,000 lb-in/ft is using fy and 14,000lb-in/ft is using Fs. Makes a big difference!!

 

[ToadJones]

I was thrown off by ..."I also cannot seem to "back calculate" the allowable stress tables"

 

[slickdeals]

In strength design, you use ultimate loads (1.6 factor for wind). In ASD you use a 1.0 factor for Wind. I have not run the numbers, but the strength design and allowable design numbers are supposed to be different.

 

[Lion06]

You would use fy in the strength design equations, you use Fs in the ASD equations. I would expect you to get a significantly higher moment capacity for strength design because you're comparing it to a factored load instead of a service level load.

 

[ToadJones]

I'm not so sure you can eliminate 'Pu' from your equations.
Ref. section 3.3.4.1.1

 

[jonathanwilkins]

I can't eliminate Pu. I am just doing this for the sake of education. I am just trying to figure the best, most efficient way to design and build this wall.

I too see why the strength design is "stronger" than asd. I realize it's not when you add in the factors. Thanks for everyone's help.

 

[Lion06]

For what it's worth, unless the wall is load bearing (and carrying a lot) the axial load willl generally HELP the strength analysis (at least for strength design).

 

[SteelPE]

I asked the same question here on the forum about a year ago.

http://www.eng-tips.com/viewthread.cfm?qid=252451&page=2

http://www.eng-tips.com/viewthread.cfm?qid=252182&page=3

Never did get a good answer.

 

[jonathanwilkins]

The wall is load bearing, the axial load, though, is insignificant. (short span roof load)

 

[JedClampett]

There is a benefit in using the "strength" design vs. ASD. There are also a bunch of P-delta restrictions, maximum reinforcing restrictions and axial stress restrictions. Once you fight through all of them, it's pretty much free capacity.
I only use the strength design allowables when I'm in pretty desperate straits.

 

[Gumpmaster]

LRFD
-More "accurate"
-Much harder to do a quick calc by hand

ASD
-Proven to work for a long long time
-Easy to do a quick calc

They're not really related to each other at all. It's an analog to concrete design (or steel design for that matter). They're just two different methods of doing the same thing.

With a taller, more slender wall (h/t>30), the strength method may be the only option as the ASD method starts to get pretty conservative for slender walls.

Yes, the do yield different results. As slickdeals said though, for strength design you have to multiply your wind by 1.6. 25,000 lb*in/ft demand versus 33,374 lb*in/ft capacity. For ASD it was 16,039 vs 16,420. Not as drastic a difference as you had in your OP.

 

[abusementpark]

We need to establish here that the difference between LRFD and ASD in masonry design is more than just load factors - it is also in the analysis methodology.

LRFD is based on the ultimate strength methodology similar to concrete where Whitney's stress block is assumed and both the steel and masonry are brought to ultimate capacities to determine the nominal strength. ASD is based on a simple elastic stress distribution using conservative allowable stress values.

I recently compared design results and there is a surprisingly large disparity between the two methods. Apparently, you can get by with a lot less if you use LRFD to design masonry. It prompted me to started a thread about the noted difference as well.

http://www.eng-tips.com/viewthread.cfm?qid=258356&page=2

I still like to use ASD because the quality of masonry construction is so poor. We have been making a significant efforts in recent months to crack down on special inspection requirements and idiot-proof some of our typical details to circumvent some problems. However, there is only so much you can do when the masonry sub barely looks at your drawings. You can't be out there every day.