Unreinforced Masonry Building Upfit

[colengineer]

Looking for some guidance on the 2015 IEBC as it relates to what would amount to be a Level 2 alteration to a URM building in a high seismic region (New Madrid).

IEBC Section 807.5 is the focus of my question, specifically the calculation of demand-capacity ratio.

IEBC states that demand shall consider applicable load combinations with design lateral loads or forces in accordance with IEBC Section 1609 (wind) and 1613 (seismic; with ASCE7 provisions invoked). Calculation of seismic demand is itself obviously a function of a number of things, including the response modification coefficient (R) of the building's SFRS. That's easy to understand. But what if the URM building in question resides in a seismic design category C or worse as would be the case with NW Tennessee? Are we to take this to mean that, regardless of R, the fact that the URM building has a "not permitted" limitation results in a non-starter?

IEBC does seem to try to throw the designer a bone by allowing for some increase in D/C ratio (10% or less) before retrofit/remediation is imposed, as well as allowing the designer to use "reduced" IBC level seismic forces. But again, is the designer even in a position to carry out the demand-related calculations that would potentially benefit from these provisions if the building itself, as it sits, carries the NP scarlet letter? How does one determine whether the D/C ratio increase is below the 10% threshold if the demand component is contingent upon system attributes that are prohibited from use?

A big thanks in advance for your help.

 

[jittles]

I'm not precisely familiar with the various levels of alteration, but as long as the governing code section doesn't explicitly require a complete new lateral analysis, then...

I think the 10% provision is still valid/applicable. As long as you're consistent on both sides of your check, using the same "R" factor (no matter the exact number), then all you're looking for is the relative increase.

If you're outside of that, I think you will need to head towards ASCE 41 for analyzing and evaluating older, non-code compliant seismic systems.

In the 10% increase case, exercise your engineering judgment to determine if this is actually reasonable. Blindly allowing a 10% increase just because the code allows it is not always the prudent thing to do. However, if you have a bunker of a CMU building, perhaps you can fairly easily justify that there is ample additional capacity there.

We often use ASCE 41 to justify old reinforced CMU that does not meet current code special wall detailing. Ample available capacity for the loads, but prohibited in our design category. ASCE 41 provides the guidelines that let you rationally check the walls, with discounts/penalties for the non-compliant or unknown bits of the equation.