Masonry Strength Design - LRFD

[abusementpark]

I was designing a CMU Lintel Beam as a flexural member recently and decided to compare the results using the tradional elastic analysis (ASD) approach (which I have always used) and the ultimate strength design (LRFD) approach now allowed by code which considers Whiteney's stress block.

I was startled to see that there was a huge difference between the ASD and LRFD approaches. The stress ratios I was calculating using LRFD were significantly lower than the ASD approach (more than 40%). I expected the LRFD results would yield somewhat smaller stress ratios (perhaps 10-20% difference)since it is considering the true ultimate state, but I didn't expect the difference to be so pronounced.

Has anyone else noticed large differences in design using ASD & LRFD for masonry??

 

[sdz]

Can you post your calcs as an attachment?

 

[DaveAtkins]

I have noticed you get a big advantage using ultimate strength design with masonry. I think this is because the assumptions for allowable stress design of masonry are so conservative--triangular compressive stress block, low allowable compressive stress, low allowable steel stress, etc.

DaveAtkins

 

[SteelPE]

I have also noted large advantages when using Ultimate Strength Design of masonry. I noted it on this forum a few months back without much discussion on 2 different posts (kind of related to the same question).

http://www.eng-tips.com/viewthread.cfm?qid=252182&page=1

http://www.eng-tips.com/viewthread.cfm?qid=252451&page=1

I verified all of my calculations by and with the NCMA computer program… so I hope they are correct

 

[abusementpark]

OK, thanks guys.

I just needed a sanity check. It seems absurd that a code would present two different methods with such differing results.

Does anyone know when strength design was introduced in the masonry code?

 

[SteelPE]

2002 was the first ACI 530 to introduce strength design.