Masonry Design LRFD vs. ASD 2

[bouk715]

I know this topic has been discussed before, but I just wanted to touch on it again...

I'm working on a building project that utilizes CMU shear walls for lateral stability. The walls have large overhead doors, so the piers between are not especially long. The shear walls were designed using ASD and the job is now in construction. Once the walls were already built, the owner decided he wanted to strap a large tower antenna to the building. This added a lot of additional lateral load to the building (the tower loads seem very conservative, but that's not really the point of this thread). Anyway, I updated the ASD design calculations and the walls were roughly 30-40% overstressed.

I decided to take a closer look at it and used the LRFD design spreadsheets provided by the The Masonry Design Guide #7. Lo and behold, with the same wall geometry, reinforcement, etc. the wall now worked and even had roughly 10% reserve capacity. Not believing that LRFD could make that much of a difference, I verified the spreadsheet numbers by hand and they are accurate.

I saw that a few people had previously posted on this site about the large difference between ASD and LRFD masonry capacities. To me, it almost seems like one of the approaches is "wrong". I assume that the ASD approach (e.g. limiting yield stress in 60 ksi rebar to 24 ksi, etc.) is just very conservative. Does anyone know if ACI or TMS plans to address this? Seems like an awfully big discrepancy and the only justification I even see to use ASD going forward is for familiarity reasons.

Curious if anyone has anything to add here.

 

[AELLC]

When you say 30-40 overstress (ASD) vs 10% extra capacity (LRFD) did you mean allowable shear only, or allow shear, allow compression in CMU , allow tension in rebar, whichever controls?

I find that the smallest difference between ASD and LRFD methods is in the calculation of tension rebar required.

Another thing, when you ran it as ASD, did you include the rebar in the compression zone of the CMU shear wall to be effective? If you left that out, that may describe part of the huge discrepancy.

If possible, show me one small example of wall geometry and working (unfactored) loads, and I will run it on my Excel ASD worksheet.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[CELinOttawa]

Are you being careful to include combined actions? You need to be careful about areas where you have significant bending and compression...

Effectively I'm questioning whether you are appoint the whole of each code (thus comparing similarly sized apples and oranges).

 

[bouk715]

Thanks for the quick responses...

AELLC - I'm not at work today, so I'm going off memory a bit here (always dangerous). The wall is 7'-4" long, 14'-0" tall, and has #6 bars at 16" o.c. It is a solid-grouted 8" CMU wall with f'm = 1900 psi. The bars are set 4" in from each end and then 16" o.c. for a total of 6 bars. In-plane shear load at the top of the wall is roughly 14 kips from wind load. Design is per ASCE 7-05, so this is not ultimate wind. So for LRFD I used a load factor of 1.6. The maximum vertical load on top of the wall is roughly 90-100 kips spread evenly along the length (~12 kips/ft). Vertical load is unfactored and is due to dead and snow load. I don't remember the exact breakdown.

Both ASD and LRFD indicate the wall is OK for shear with no shear reinforcement. The difference is in the moment capacity. If I recall correctly ASD calculated the allowable moment to be ~150 kip-ft. LRFD calculated it to be ~400 kip-ft. Even dividing the 400 kip-ft by 1.6, it is still much greater than the ASD moment. These are the calculated "acceptable" moments with zero axial load. With some axial load, the wall can take more moment in both the ASD and LRFD scenarios.

The spreadsheets (LRFD or ASD) I use discount any rebar in the compression zone since it is not tied. So I would assume this is apples to apples. That would be great it you could run independently in ASD. I'm curious if I made an error somewhere. The hand calcs I ran were to check the LRFD spreadsheet, not the ASD one. Let me know if I left any pertinent info out.

CEL - I checked the wall with zero axial load and with maximum axial load to determine allowable moments. Both were within with moment-axial load curve, so it checked out OK.

 

[JedClampett]

bouk, I've noticed the exact smae thing. Walls that don't have a chance of working for ASD work swimmingly using LRFD.
Now for the bad news. With an 8 inch wall and #6's at 16 inches I think you're over the reinforcing ratio allowed, at least in the out of plane directions. See section 3.3.3.5.1 of ACI 530.

 

[AELLC]

Are you sure the uniform gravity load is 12 KLF? That looks unusually large.

Give me the dead load and snow loads separately.

Unfortunately, this old ASD method includes only the rebar at the very end of the wall, which is ridiculously conservative. At= 0.44 in^2, for one #6 bar.

The tension in rebar governed, and works only if there is at least 4610.plf dead applied along the top of wall.

Look at this pdf of my Excel (Magnify to 400% for visibility)

Wait until Monday, no problem.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[AELLC]

Here is a wall design for wind perpendicular to wall, and the #6 @ 16" vert seemed to work fine by ASD.

This is an old UBC Code method, probably UBC 1997.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[bouk715]

AELLC - I can double check the loads, but they are larger than normal since the wall has overhead doors on either side of it. There is a tube lintel over each door and runs the entire length of the wall (was easier than using two separate pieces and leaving a small gap in between). So this load is mostly due to point loads distributing into the wall.

Jed - I'll take a look at that section. Like I said in the original post, I originally designed this wall using ASD so the out-of-plane rebar wasn't checked using Chapter 3.

 

[AELLC]

Sorry, wrong file. Use this instead.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[bouk715]

Thanks again for the check AELLC. I can try and post the MDG #7 ASD spreadsheet on Monday. It allows you to input multiple lines of reinforcement.

 

[AELLC]

My guess is that when CMU was allowed to be designed by LRFD, the Code writers didn't "calibrate" the factors to result in approximately the same end result, comparing ASD to LRFD, as they did in wood, concrete, and steel.

The old masonry design methods may have been way over conservative.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[Hokie93]

The code (ACI 530) writers have recognized the discrepancy and have attempted to bring the two methods (ASD and LRFD) somewhat into alignment in ACI 530-11. To that end, the allowable tensile stress in steel reinforcement has increased to 32,000 psi (from 24,000 psi). Similarly, the allowable compressive stress in masonry due to flexure or flexure in combination with axial load has increased to 0.45f'm (from 0.33f'm). The article "Harmonization of Allowable Stress Design and Strength Design of Masonry" by Ed Huston, PE, SE in the May 2013 issue of STRUCTURE Magazine (

http://www.STRUCTUREmag.org)

discusses this in more detail.

 

[msquared48]

Interesting.

I went to school with Ed. He has his own firm in Seattle now. Small world...

Mike McCann
MMC Engineering

 

[AELLC]

Hokie,

Thanks, very useful. Those Excel sheets I posted above were all based on 0.33*f'm comp in CMU and 24000 psi tension in rebar.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[bouk715]

Thanks Hokie. I was hoping they had addressed it in some way.

 

[JLNJ]

Which version of the masonry code are you using?

There have been some significant changes recently which should bring the ASD and LRFD capacities closer together.

 

[bouk715]

Attached are the tables/graphs created by the MDG spreadsheets. I did update the ASD version to include steel stress of 32 ksi and masonry compressive stress to 0.45*f'm. That obviously improved the ASD calculated capacity. ASD is roughly 200 kip-ft (with no vertical load) and LRFD is roughly 425 kip-ft.

JLNJ - my original post was based upon the 2008 masonry code, which uses a steel stress of 24 ksi and masonry compressive stress of 0.33*f'm.

 

[bouk715]

Previous post has LRFD spreadsheet. Here's the ASD one.