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Thoughts on SW W/ Cant Columns as collector elements

Thoughts on SW W/ Cant Columns as collector elements

Thoughts on SW W/ Cant Columns as collector elements

I recently received a plan check with comments regarding my choice of response modification factor R=6.5 for a resisting SW element for the upper floor of a 2 story single family residential structure.

Basically a 350sf 2nd floor(18.75ft sq box)and a 1350 sf 1st floor. The master bedroom is located on the 2nd floor and the owner wants one wall line open with windows. All of the material is relatively light weight, standing seam metal roof, carpet no tile so I thought I would be creative so he could have his windows without introducing a steel moment frame and added expense.

So using an R=6.5 value I obtained a seismic shear load of 920Lb along this line. See attached pdf of my design.

Basically I am using the 6x6 wood posts as collector elements and designing the shear wall segment below the windows based on the reactions from the pinned support along the top of the SW blocking.
The plan check engineer wanted me to use an R value of 1.5, for a cantilever column system for the design of the shear wall. I told him that the wood cantilever columns are not the resisting system, but the collectors. I even designed them with the Omega factor of 2.5. He didn't want to hear any of it and was not even listening to me when I tried to explain it to him.

Just curious as to your thoughts. Also something to consider that this is a light weight wood framed residential structure. If this was a multi story large residential structure with heavy finish everywhere I wouldn't choose to use a system like this.

RE: Thoughts on SW W/ Cant Columns as collector elements

While in the English language they "collect" lateral shear from the upper roof and deliver it to the shear wall system below the windows, I guess I don't see them as strictly defined collectors.

Because it appears that the vertical posts are also gravity bearing columns, they need to be treated as a cantilever column system. The concern here, requiring the very low R value, is that if you lose column capacity to carry gravity load, you have collapse and an unsafe condition.

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RE: Thoughts on SW W/ Cant Columns as collector elements

The response of the structure will be dominated by the behavior of the cantilever column. An R of 6.5 assume fairly ductile behavior of the lateral system which is not what you have.

RE: Thoughts on SW W/ Cant Columns as collector elements

Ok, I see why he was insistent on using R=1.5. Makes sense now.


RE: Thoughts on SW W/ Cant Columns as collector elements

Hmmm.... I see what you're getting at, but as described it sounds to me like a cantilever column system.

Now, let's say you had defined the roof diaphragm as a cantilevered diaphragm that brings the roof shear back to a another shear line, and you accounted for the moment caused by this. In that case, the columns are there exclusively for gravity. But, they'll have some significant drift. So, maybe you still make them cantilevered so that they can take the P-Delta effect from the diaphragm deflection. I might even add an omega of 2.5 since it's something of a soft story mechanism.

Maybe then you could get away with the larger R value for the overall structure. But, the logic is still a little convoluted and the plan checker could still insist on the R of 1.5.

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