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





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