Applying Orthogonal Combination Procedure as per ASCE-7-05 for SDC C in Etabs program. 2

[ayelamayem77]

Dear all,
Structures that have horizontal structural irregularity Type 5 in Table 12.3-1 (non-parallel system), ASCE-7-05 requires to take take 100%of seismic force in one direction + 30% of seismic force in the other direction.
Example Ex+0.3Ey.

for the case of doing dynamic analysis ( response spectrum) in etabs and after defining respose spectrum function.

my questions in response spectrum case data menu as follows.

1-modal combination do we use CQC or SRSS ?

2-Directional combination do we use SRSS or (ABS + ORTHOGONAL self factor = 0.3)

3-input response spectrum do we combine the two directions of seismic forces (U1 & U2)in one response spectrum case or we do it separatly for every seismic force direction?

4-what is the required value of input respose spectrum Uz as per ASCE-7
Best regards.

 

[MrHershey]

Would reference you the analysis reference that comes with the program for a lot of this, but for short answers:

1. I use CQC. CQC can account for coupling of modes. SRSS cannot. Both are acceptable per ASCE 7.

2. You can do either. I usually use both just to make sure I'm covered. See below.

3. I usually do five cases:
A. 100% U1 (ABS combination, though could be SRSS since it's only one direction)
B. 100% U1 + 30% U2 (ABS combination)
C. 100% U2 (ABS combination, though could be SRSS since it's only one direction)
D. 100% U2 + 30% U1 (ABS combination)
E. 100% U1 + 100% U2 (SRSS)
I typically find that either B and D governs.

4. I generally don't do input a response spectrum for Z (vertical) direction. Vertical response spectrum accelerations tend to be very site specific, depending on many more factors than a standard response spectrum can hope to address. As far as I know, there are no response spectrum functions for vertical acceleration provide in ASCE 7. Vertical acceleration is accounted for in ASCE 7 through amplifying the dead load by 0.2*Sds. See 12.4.2.2 and 12.4.2.3

 

[ayelamayem77]

Dear MarkHirschi,

really i am very Grateful for you help,but i still have some questions if you don't mind.

Regarding item no 4 in your valuable answer,
please check the following link when doing ritz vectors

http://www.mediafire.com/view/y66ntait6y5ibh5/Acc+...

is that right?

Regarding item no 3 in your valuable answer,
please correct me if i was wrong
i understand that both dynamic and static forces should be the same before design
base on the previous information i noticed that the starter value of the scale factor = g/(R*I)
where g=acceleration,R =response modification factor and I = importance factor am i right?

when doing the following:
B. 100% U1 + 30% U2 (ABS combination)in Etabs
Directional combination ABS , ORTHOGONAL SF = 0.3
U1=Function 1 , scale factor = g/(R*I)
U2 =Function 1, scale factor = 0.3*g/(R*I)


note: after running the analysis the scale factors should be modified to make (static force = dynamic force) before starting design of the structure.

again I appreciate your support.

best regards

 

[wannabeSE]

To start, the scaling factor should be Ig/R. The dynamic base shear must be at least 85% of the static. See:

https://wiki.csiamerica.com/display/etabs/Scale+factor+in+RSA

. I knew an engineer that had a building where scaling with Ig/R yielded a higher base shear than static. I think he was stuck with the higher base shear (I always wondered if something was wrong with the model). The code does not say you can decrease the dynamic base shear because it is higher than the static.

 

[ayelamayem77]

many thanks for wannabeSE for his helpful post