trigger for revised lateral systems on existing structures

[muncher]

I have consulted with other "very-experienced" structural engineers (design and plan check) to determine precisely what triggers revisions to a lateral load-resisting system under the provisions of IBC 3404.4 (2012, 2009, etc) and read the code slightly differently. I am hoping to get a wider range of interpretation via this forum. (For those respondents, please indicate your interpretation of the code language - not just what you've done in the past.)

As an example, for a simple project that requires a solitary 10'-long shear wall to be reduced by 6" due to an enlarged opening, here is my thought process...
Considering the triggers:
"...where the alteration increases design lateral loads in accordance with Section 1609 or 1613..." [not applicable as no new exposure for wind or weight for seismic increases]
"...where the alteration results in a structural irregularity as defined in ASCE 7..." [presume that this does not apply]
"...where the alteration decreases the capacity of any existing lateral load-carrying structural element..." [this applies as the 9.5'-long wall has less capacity than the 10'-long]
Therefore,...
"the structure of the altered building or structure shall be shown to meet the requirements of Sections 1609 and 1613".
However, an exception exists stating...
"Any existing lateral load-carrying structural element whose demand-capacity ratio with the alteration considered is not more than 10 percent greater than its demand-capacity ratio with the alteration ignored shall be permitted to remain unaltered.
At a glance, this seems like we might be ok as we've only reduced the wall's capacity by 5% (= 6"/10'); however, the next statement clarifies the term "demand-capacity ratio"...
"For purposes of calculating demand-capacity ratios, the demand shall consider applicable load combinations with design lateral loads or forces per Sections 1609 and 1613"
To me, this seems to indicate that the analysis must always be done and, in this step, the engineer must determine the demand based on CURRENT code provisions and this is the problem. Many of us are aware that the evolution of the codes have had significant impact on the lateral loads that we must impart on structures in the design phase. Just the fact that the "newly-determined" lateral loads may be higher than those used in the initial design of the structure (many years ago?) could trigger the need for the addition of supplemental support with ANY capacity reduction, no matter how slight, of the existing wall.
There is NO allowance for the "belief" that "because the wall is still there it must indicate that it's historical demand-capacity ratio must be a maximum of 1" (which would then allow our 5% reduction to be less than the code-allowed 10%).

 

[wannabeSE]

The way I look at it when checking the change in the demand to capacity ratio, the demand is the same before and after the modification so it cancels out. See below with subscript n is new and e is existing
(D[sub]n[/sub] / C[sub]n[/sub])/(D[sub]e[/sub]/C[sub]e[/sub]) < 1.1
D = D[sub]e[/sub] = D[sub]n[/sub]
therefore (D/C[sub]n[/sub]) / (D/C[sub]e[/sub]) = C[sub]e[/sub]/C[sub]n[/sub] <1.1

 

[muncher]

In response to wannabeSE, I would argue that, in most cases, the capacity remains the same over time so that C[sub]n[/sub] = C[sub]e[/sub]. However, if I determine the wind or seismic load on a building using the 1994 UBC, I'm probably going to get a smaller design value than if I subject the same structure to the provisions of the 2012 IBC. Therefore, because of the evolution of our knowledge and the resulting changes in the codes, the demands are greater now so that D[sub]n[/sub] > D[sub]e[/sub]. If this increase is more than 10%, then just by the nature of us touching the existing lateral force-resisting item, we are required to provide supplemental support. That is the nature of my question...is this correct?

 

[msquared48]

I think this could apply to OSB, Plywood, CMU and concrete shear walls, and not to sheetrock or gyp sheathing walls in high seismic areas. Low seisimc areas - well that's another matter...

In the late 80's or early 90's there was a code provision introduced, with a lot of good documentation driving the de3cision, to reduce the allowable shear by 50% of the tabularized value for all walls controlled by seismic loading dut to the working of the nails during a seismic event, lowering the capacity of any wall that did not fail. Since then, in my area, I have not used another sheetrock wall in shear, and have totally negated the lateral effect of these walls in any remodels, taking all the shear with one of the other shear wall options mentioned.

Mike McCann
MMC Engineering

 

[wannabeSE]

The IBC is pretty clear. It says "For purposes of calculating demand-capacity ratios, the demand shall consider applicable load combinations with design lateral loads or forces per Sections 1609 and 1613." It does not say the existing demand should be calculated using the code when the building was first permitted.

The code is a minimum standard. An engineer still needs to apply some judgment and decide whether or not the alteration is safe.

 

[muncher]

To clarify my point, I'll use some numbers. Let's say seismic loading controls the design which was originally done under the 1994 UBC...

Presume the original base shear, V = W x (Z x I x C) / R[sub]w[/sub], resulted in something like 0.15W.
Now, per Section 1613 which dumps us off to ASCE 7, the calculated base shear becomes V = W x C[sub]s[/sub] and results in something like 0.185W.

Barring a situation as described by msquared48 where a system is now considered to be weaker than previously assumed, the capacity of the element is unchanged.

However, the demand under current code is already 0.185/0.15 = 23.3% over what was originally determined ~20 years ago - thus, the evolution in the code requires strengthening of the system because this exceeds the 10% limit. And, this hasn't even taken into account our 5% reduction from making the 10'-long shear wall a 9.5'-long wall which would increase the demand-capacity ratio to around 1.3.

I can't tell if this puts me in agreement or disagreement with wannabeSE but I appreciate the feedback.