Questions on CMU to wood diaphragm design 1

[jsand10]

I've been reading on these forums a lot over the past few years and it's a wealth of information. I typically do wood frame design but have a project I'm working on that is a CMU car wash building with a wood frame truss for the roof. The structure is essentially 16' wide x 50'8" long. The wall heights are 22' to the top of the parapet and the plan was to utilize an open web truss set approximately 4' from top of parapet and have a ceiling height of 14'.

My question is regarding the diaphragm design. Based on wind/seismic seismic is controlling and in our area has an Sds of 0.510g giving a Cs value of 0.255. With the weight of the fully grouted 8" CMU block I'm essentially getting an 800#/ft load in the long direction for the diaphragm design. Since I'm use to most wood frame designed this seemed high to me. So just wanted to check with you guys if this seems reasonable.

Thanks.

 

[JAE]

Seismic demand in ASCE 7 is an ultimate effect (i.e. 1.0E) vs. 0.7E for allowable stress design which is typically in wood.
When you derived the 800 plf was that 1.0E or 0.7E?

 

[msquared48]

Cs of 0.255 seems high.

I am used to closer to 0.15.

What are your R and I values? Where are you?

Mike McCann, PE, SE (WA, HI)

 

[jsand10]

JAE, this was at the 0.7E so would be a bit more going to the 1.0.

msquared48, I’m in Northern CA. R value of 2 and I of 1.0

 

[Aesur]

I calculate a lower force:

 

[msquared48]

Why R =2 ?

Mike McCann, PE, SE (WA, HI)

 

[jsand10]

Thanks for the responses. I had chosen 2 for ordinary reinforced masonry just as a quick pull from the table. But after your responses I went back to it and since I’m in seismic D need to go to special reinforced which will allow me to go to R of 5.

I appreciate you guys taking a look at it.

 

[radiocontrolhead]

How are you justifying the detailing required for special reinforced walls?

 

[jsand10]

Not really a choice being in site class D

 

[radiocontrolhead]

jsand, is this a new or existing wall?

if existing, how do you justify it has adequate detailing?

 

[r13]

Is the 800 plf load due to the roof mass, or from the CMU end wall? Can the end wall resist the seismic force without the roof?

 

[jsand10]

Radiocontrolhead, this is new construction.

Retired13,

I haven’t yet looked to see if the end walls can take it without the roof. The mass was mainly from the cmu block. I was just starting preliminary numbers as I have mostly done wood frame design, but will be diving more into it next week.

I appreciate the responses guys.

 

[r13]

Yeah, I got similar number with roof load included.