WR1

1. How are you calculating slab deflections from ETABS? 2. Are deflection elastic or long-term? 3. Perform long-term non-linear deflection analysis in SAFE. 4. Then if deflection is excessive, try increasing top and bottom reinforcement. 5. If nothing is working and you cannot increase thickness or decrease loads, then try using another type of slab. For example ribbed slab, hollow cores etc.

Dear Rumann, Based on the image you have attached, and as you have asked me to comment, please read below; In mode 1, 2 and 3, I see almost equal mass participation in all three modes Mode 1 is translational in Y direction with 60% mass (T=2.7s) Rz=1% Mode 3 is translation in X direction with 58% mass (T=1.7s) Rz=1% Mode 2 is torsional mode around Z with 61% mass (T=1.9s) Ux=Uy=1% Your building seem to be more rigid (more stiffer) in X direction (check framing) than Y direction. The building does not seem to have torsional irregularity (~CR=CM). The building is regular vertically and does not seem to have higher modes effects. This is because I do not see coupling between rotation and translation in any mode. See above, mode 1 & 3 have no torsion (1%) and mode 2 has no translation (1%). That implies the building is regular.

Dear Ahmad, Please follow forum rules. A simple search of old forum posts would have been helpful. For example see below;

Yes, nothing wrong in it. It depends on design. More reinf at upper levels could be due to more moment and less axial loads.

Hi, I am looking forward to make a website with four main sections for commercial and education purposes; 1. Structural wiki + Book or Mag 2. Tools 3. Courses 4. Consulting In (1), I am planning to make a wiki on the same format as in CSi ETABS Wiki, where I intend to contribute to the point and practical approach articles on structural engineering and its softwares, like RSA, SAFE and Punching, Seismic etc. These can be combined in form a hard-copy book or a periodic magazine which can be sold. (2) will contain tools and softwares I am planning to build and sell. (3) will contain courses like in OStruc. (4) for consulting business and outsourcing. Unfortunately, I don;t have a team with IT, software skills and I can't do it alone. I need some volunteers who share my idea on partnership basis. First of all I need someone who can make the website in a good, easy format. If you like the idea, please PM me.

I guess you are talking about modal super-position, but that is not my question. Yes and according to SK Gosh (http://skghoshassociates.com/SKGAblog/viewpost.php?id=5), we need to amplify dynamic torsion because; ".... accidental torsion is not determined as part of the dynamic analysis, but as the result of a separate static load applied at an eccentricity. The only way we can use the exception [to ignore amplification in dynamic] is to incorporate the accidental torsion effects into the building model itself by defining a floor mass distribution that is not uniform so that the center of mass has a 5% offset from the centroid of the floor area." Agreed. Now, the next step is to how to do it practically. We are not just talking about the concept but to actually do it. Now, see CSi ETABS Wiki (https://wiki.csiamerica.com/display/etabs/Accidental+eccentricity) 1. In the first method, accidental torsion is included in dynamic analysis by actually shifting CM as SK Gosh suggests that would change the dynamic properties, natural characteristics and stiffness matrices for each eccentricity and so we do not need to amplify further, because accidental torsion has been calculated through dynamic analysis. Con: The main drawback is that as the properties of model change for each eccentricity, hence 4 separate eigenvalues analyses must be performed for each eccentricity and then finding a way to envelope the maximum response of these 4, which is not possible in majority of software. What is the simple solution? See point #3. 2. Second way is to model a static torsional load [or a static force applied at an eccentricity] at each story for each eccentricity to approximate these effects. Then static + dynamic response is combined. This is where we need to amplify accidental torsion. Because accidental torsion has not been calculated dynamically. 3. An "efficient and practical" approach is adopted in ETABS. After the analysis of MRSA cases; a. Acceleration at each node is multiplied by tributary mass and given eccentricity so the result is a torsional force = m.a.e = F.e b. A static response is generated under these torsional loads and added to MRSA dynamic results. Now, strictly speaking, eccentricity was not directly analyzed in dynamic analysis in method 3 but atleast there is a satisfying globally used practice. So we dont need to increase accidental torsion by this method. 4. There is another method developed by Fahjan et al. and quoted by CSi wiki. It also contains good background information. You can view the paper from csi wiki page.

I am involved in project where I urgently need to have good answers to following questions related to response spectrum analysis for my clear understanding and to assert to client that my approach is right. I am not designing shear walls under RSA load cases but only for static cases. Scale factor between static and dynamic is 3 in X and 2 in Y so the dynamic forces are lot lesser than the static ones. Also I am not amplifying torsion in RSA. And combining orthogonal effects with SRSS approach and not by combinations. 1. Do we need to amplify accidental torsion according to Table M of UBC-97 regardless of seismic zone? In ASCE you need to do it only for SDC C and above. 2. If RSA is done using ETABS (shifting of mass 5%, then determining acceleration at each node, then multiplying tributary mass to get torsional force), then do we still need to amplify RSA eccentricity by Ax (Amplified torsion)? In my understanding you need to only amplify static seismic cases. 3. According to FEMA, UBC-97 and Wilson, there are 2 methods of orthogonal effects. 100% X+30% Y or SRSS. That means I can define a scale factor in U1 and U2 both in the same spectrum load case and select SRSS, instead of defining U1 and U2 (2 separate cases) and then combining them in load combinations. FEMA and Wilson (2004) prefer to use SRSS (100%X and 100% Y to have same strength against earthquake in all directions). 4. Do I need to consider U3 in RSA for vertical direction equal to 2/3 of horizontal or i can ignore it? It is already considered equal to 0.5CaI (UBC-97) or 0.2Sds (ASCE) in static load cases by the way. 5. Do we design shear walls on RSA? Because would'nt it be too conservative (unrealistic) to design huge shear walls just for pure tension instead of tension-compression couple from lateral loads? Because results from RSA are abs (+ or -) but not both. If not then what is the role of RSA in seismic design other than to get the maximum response only to have an idea of total base shear and its distribution. 6. How do we say that dynamic effects are not governing rather static seismic forces are governing? By comparing values of base-shear at each story? Most of the times in regular low seismic zones, dynamic analysis does not govern. What is the meaning of "does not govern"? How do we practically specifically check that in ETABS? By comparing forces as I have said?

From geotechnical report.

Well, it should be equal. Are you comparing the sum of reactions on the same load case? You should not check dummy load case. Also you should compare reactions excluding any slabs you are putting in SAFE.

OStruc is down till further notice.

Pickup columns or planted columns are structural cols supported on transfer slabs or beams (which you are referring as pickup beams i think). For seismic design you have to overdesign collector elements like transfer beams or slabs by omega factor. Issues with pickup cols is loads in them depend on stiffness of slabs/beams supporting them. If supports are flexible they are literally just hanging threads from slabs above carrying little or no load at all, you cannot just manually transfer loads to them by distributary area. Another thing is to check deflection at upper and below slabs as well as punching on both slabs. Make sure the transfer slab has sufficient shear strength. Another issue might be consteuction sequence and long term deflections due to these columns. Also make sure you check for irregularity clauses of seismic code.

First you must be clear about requirements (arch, mep etc). Have a clear understanding of dwgs, are the cols masonary, non load bearing? Have you prepared 2, 3 different struc schemes? If you think you can value engineer and reduce cols, check with client req if they too want it. Then you make sure the scheme is working, deflections, load paths, foundations, framing, cost. Reducing cols mean, bigger beam and slab spans and so more deflections. Have you checked that? Also more local load on foundations if cols are far, and more sway in wind/eq bcz less bending stiffness of vertical elements (if you are not increasing col/wall sizes to compensate that). If above is okay, you can eliminate cols that are not required. As a struc engr you have to be proactive, pitchin and promote the "most efficient" scheme that is "simple" and "necessary".

There is nothing wrong in designing slabs from the program if modelled properly. Yes in 32x12 slab main reinf should bd along 12 feet direction. But 1 way or 2 way doesnt only depend on just geometric properties. It also depends on boundary conditions. Basically the way a slab bends (1 way or 2 way). For m11 contours you will put reinforcement along 1-1 local direction (red) and m22 along 2-2 local direction (white). And you can rotate local axis if orientation of reinf is different (e.g. at 45 deg) or you can manually resolve components. Hth.

Moment redistribution. We used to increase bottom by 20% to account for moment redistribution in case the beam becomes simply supported due to plastic hinges. But with this 20% increase, we used to keep pattern live load factor of zero.

You need give it a shot. Think about it... For stability checks you need maximum moment..right..Now think about what combo in your case gives maximum moment. Also for the shear in case of sliding...There wont be many combinations that give you maximum results. Try checking by contours for example if you cant judge.

Ramadan Kareem!

I dont think so there is such an option in SAP. You can do this in SAFE or by hand or any software/sheet.

In addition to above, in my previous practice we used to provide the same reinforcement for top and bottom. Whatever is the max of top and bottom, put that at top & bottom.

Yes 68 stirrups of dia 8mm spaced at 125. 850mm and 900mm are the distances from ends for the bottom middle bar 2T12 which is clear from the above picture.

What is a floating column? You mean the planted columns? In my understanding you have columns supported on a beam which is then supported by a wall at one end and a column on the other end. Yes you check the beam with special omega eq load combination (2.8 factor as you said). What is the beam size btw that is supporting planted columns of 8 stories? Is shear okay at wall-beam junction? Also try inserting the beam into the wall a little further (not node-node connection). Did you check the wall stresses at beam connection? Is the supporting beam a deep beam?

You have to perform the modal analysis under options menu>dynamic analysis>then select eigen/ritz modes and number of modes to get their time periods.

The best thing is to model the deep beam as a shell element in elevation. This is due the fact that stresses in deep beam are not linear. You cannot view stresses in a line element. This can only be done in FEA of a shell element. Remember that the deflection of a deep beam will be flexural + shear.

A shell will transfer load to the 4 nodes into the supporting column/support. In order to get the same results as membrane, use 0.25 modifiers in "m" and also mesh it every meter so the shell is divided every meter and the supporting beam gets loaded as UDL every meter.

Just model the strap as a line element or as a slab element with more thickness.