ACI 530-99, Masonry Design Fb

[ofthesea]

Our office is currently updating our masonry spreadsheets to conform with ACI 530-99 in order to make the transistion to IBC 2000 code.
We have come across a question in regards to the Masonry Fb value. It appears that for an f’m of 1500 psi the Fb would be = 2.5(code increase) x .8(code phi for flexural) x .33 x f’m, or in other words an f’m of 1500 would yield a Fb of 1000 psi. Is this correct? Is there an Fb max?
This is a substantial increase from the special inspected 1997 UBC value of 500 psi.
Also, does all ACI 530 require special inspection?

Thanks for any help to a west coast designer.

 

[whymrg]

According to 2.3.3.2.2 of ACI530-99 "stress in masonry due to flexure or due to flexure in combination with axial load shall not exceed (1/3)f'm." Or for f'm=1500, Fb=500psi.

Good luck.

 

[ofthesea]

whymrg,

I appreciate that information on maximum Fb given in the reinforced section of the code.
What I found confusing was that the section is for ASD and yet they had factor increases as well as phi factors, which reek of utimate design. Why are they in the code under ASD?

Thanks

ofthesea

 

[v2]

ofthesea

will you be able to give me a copy of your masonry spreadsheets with ACI 530-99?

thanks

V2

 

[MatthewC]

The section where you are referring to phi factors and strength design (I believe) is under section section 2.1.1.2.3.1 and is only required when structures are NOT designed in accordance with section 1.1.3 (see 2.1.1.2.3). Section 1.1.3 tells you that you can design using chapter 2 ASD. Therefore, use an allowable Fb per chapter 2 of 1/3*f'm, and you may be allowed a 4/3 increase per 2.1.1.1.3.

In IBC, however, beware of the ASD basic load combinations (1605.3.1) as they do not permit the 4/3 increase. The alternate basic load combinations (1605.3.2) DO permit the 4/3 increase, but note the omega factor of 1.3 times the wind load (if using IBC 1609.6 or ASCE7). This effectively eliminates any allowable increase. Nice, huh?

 

[ofthesea]

Thanks,

We also discovered that 1.3 increase on the wind combinations and questioned one of the code writers (Gerry Neville) of ICBO.

He informed us that the wind increase is their way (ICBO) of forcing the elimination of the ASD combination all together.

I don't think any of these code writers have ever designed.