Question:
It is stated that when a multi-storeyed building has columns & shear walls as supporting members, forming a combination of load sharing system between framed parts & shear walls, for taking care of lateral forces (like EQ & WIND), then the FRAMED part of the structure can be assumed to take care of a maximum of 25% of the lateral loads and the remaining75% of the load by the shear walls. But how to do this type of analysis, in practice, using commercial commercial software is the question. Also what modification in the load inputs is required to share th ese lateral forces between frames and shear walls. The load carrying capacity of a structure depends on the stiffness of members and therefore how can one predetermine that a certain percentage will be taken by framed parts and the balanced portion will be shared by the shear walls. Can we force the structural system to do such type of sharing in the analysis , when the system has certain inherent stiffness properties properties of its own. If anybody in SEFI has carried out such an analysis on buildings buildings following this procedure, using ETABS or STAADPRO , a request is made to those , to please please share their experience and and explain the procedure. There are experts in Sefi, li ke Er. Rajan , Er. Sukatha ADHIKARI, Er. Kumar Abhishek singh, Er. Rahul, Er. Rengarajan , Er. Prabhakar to mention a few, and other experts to explain the procedure for the benifit of others , if it is possible. I request experts in this forum forum to express their comments comments on this and offer their valuable opinion in the matter.
Answer: Dear Sir, I have posted the method I suggest a while back, but I will post it again for your benefit. First of all, the 25% : 75% sharing system (Dual System) is not mandatory - you can design a building with Ductile shear wall and SMRF without satisfying the above if you adopt R=4 instead of R=5. This is brought out by note 4 b in table 7 of IS 1893. Further, if you are in Zone II where ductile detailing is not essential, you can even design the shear wall to take full eq load and your main frame as OMRF and have R=3. However, it is recommended to go for Dual System (for zones 3 and above, you will get R=5 instead of R=4 since ductile detailing is anyway mandatory for both the wall and the frame). Secondly, we must understand the requirement of dual system. As per the code, Buildings with dual systems consist of shear walls ( or braced frames ) and moment resisting frames such that: a) the two systems are designed to resist the total design force in proportion to their lateral stiffness considering the interaction of the dual system at all floor levels, and b) the moment resisting frames are designed to independently resist at least 25 percent of the design seismic base shear. In simple terms, if total base shear is 100 kN, consider the following two cases: a. If walls t ake 80 kN and frame takes 20 kN, we need to boost frame component to 25kN, but walls
should still take 80kN (overall, system takes 105kN in t his case) b. If walls take 70 kN and frame takes 30 kN, frame takes more than 25% by itself and hence, no need to make changes. Now, the next question is how to implement the same in design. The method adopted by me is explained below. Do note that STAAD as of date does not design ductile shear walls. Hence, we should go for ETABS or design the shear walls manually. Step 1: Create the standard model and run t he analysis and design. Revise sections to ensure all sections adopted for frame and wall are safe. Step 2: In post processor, look up total reaction in EQX and EQZ corresponding to (a) all supports, and (b) supports for the shear walls only. Let us call total reactions to all supports as RA and total reactions to supports of shear walls only as RB. Step 3: If RB is more than 75% of RA, we need to boost the earthquake forces for frame design. If RB is less than 75% of RA, design can be finalized. Step 4: If required by step 3, boosting the eq forces of frame design is carried out as follows (i) Calculate factor as 0.25 x RA/(RA-RB). In above example with RA=100 and RB=80, the factor will be .25 x 100/20 = 1.25 (ii)Create a new set of load combinations for frame design and in these combinations, multiply factor for earthquake case by the above factor. For example, 0.9 DL + 1.5 ELX will become in the above case to 0.9 DL + 1.25 x 1.5 ELX or 0.9 DL + 1.875 ELX. (iii)Alternatively, we can create copies of the load cases EQX and EQZ and apply l oad factors from (i). Then, the new load combinations will be copy of existing load combinations, but will use different load case number for earthquake code. (iv) Design the wall t o old load combinations and frame to new load combinations. I dont have any project softcopy right now to highlight above. If some body shares a model i n ETABS completed upto Step 1 (create dual system model and apply general loads, run analysis and design to check safety of frame and wall sections), I will post the methodology above with values so that you can do a similar check... Hope this helps. Yours sincerely, Arunkumar
Follow up: Dear Arunkumar Thank you very much for the prompt response. I went thru' your reply and I have a request to you to explain the problem with reference to a model, we are doing with frames & shear walls , which will be forwarded to you shortly. The procedure & methodology , if you can explain with the model ,will give better understanding of the problem.
I have certain doubts like i) whether it is possible to take the base shear in EQX & EQY , corresponding to supports for shear walls only, in the post p rocess in ETABS. It is possible to take base shear in EQX & EQY for all supports including columns and shear walls; for walls alone, I have not tried for separate base shear values, and for shear walls alone in EQ cases, is there a separate option in ETABS. Another thing which needs clarification is in your statement under ( iii) Alternatively, we can create copies of the load cases EQX & EQY and apply load factors from (i) . Then , the new load combinations will be copy of existing load combinations, but will use different load case number for earthquake code. This is not very clear. FOR this, is it enough if we change the scale factor in the small column that appears along with theresponse spectrum analysis . For Zone three with response reduction factor 5 , we usually provide a scale factor of 0.975. ( g/2R) Instead of 0.975 , we can in this case,provide a scale factor of 0.975 x 1.25 [1.25 is the calculated factor in your step (i) procedure] Can you please elaborate this part more explicitly. Statement in (Iv) is clear I will be forwarding to you a model shortly, direct to you, if your email id is given . Then you can explain w.r.t. the model many thanks & warm regards K.Gangadharan
Answer:
Dear Sir, 1. Regarding sending a model, please send a relatively small model so that it will be easier to work with and easier to explain/understand. 2. To take base shear in EQX and EQY, you can get these from Display -> Show Tables, export the table to Excel. Then, in the model, you can look up the list of node numbers corresponding to the shear wall supports and use it to identify and collect shear wall contribution to base shear. 3. Regarding the factor, you are correct. In STAAD, it is simpler by specifying in the load case itself the factor (instead of 1893 X 1 we can give 1893 X 1.25). In ETABS, you have to "corrupt" one value by boosting it and not entering its actual value. For Stati c method, perhaps Importance Factor can be changed to introduce the factor. Yours sincerely Arunkumar
Follow up: Dear Mr.Arunkumar, As per your suggestion I tried one model of simple structure and applied the load factor multiplication so that the SW takes 75%. But it is obvious without trying out a model that either increasing or decreasing the X fator the moment frame also shares its forces proportionatly since the structure is combinationation of frame and wall. This is because we are not doing SW alone but the SW is combined with the moment frames. If t he SW is alone analysed then your suggestion may work out .As such this is of not understood by me. As I went through many articles but no one paprer suggests this idea. The above is true whatever software i.e STAADPRO,ETABS,NISA etc is made use of. Also note that while doing Dynamic analysis the cracked sections of wall,column and beams are to be taken into account. As such better option is find out by applying the EQX load case only the Seismic base shear for the wall and frame. If the SW takes less better to increase the wall length and thickness as the height is kept to the floor height or decrease the sizes of columns. The vice vers is in case it takes more base shear. Experts and yourself can comment more on this.
