UmarMakhzumi

The geotechnical engineer has to provide you with soil improvement recommendations for liquefaction.

Welcome aboard

W.salaam.. Here are the answers: 1) Cold Formed Steel Structures are generally very thin sections compared to Hot Rolled Sections, therefore, limit state of buckling is likely to happen before yielding could be reached. Thus, capacities are smaller. 2) You need to see the software manual to be certain about what design aspects related to Cold Formed Structures are addressed are not. 3) For temperature loading the following may help: https://wiki.csiamerica.com/m/view-rendered-page.action?spaceKey=tp&title=Temperature-gradient+loading+for+bridge+objects Looking forward to your follow up questions. Thanks.

Please also review: http://www.sepakistan.com/topic/87-diaphragm/ http://www.sepakistan.com/topic/1480-diaphragm-flexibility/ Thanks.

You're more than welcome.

you capacity would be reduced by (Asprovided)/(Asrequired)..

I would suggest using AISC Provisions because the they are very detailed, readily available and you have tons of example. Make sure you download the handbook and examples available for free from AISC website and you are good to design anything. Thanks.

You are welcome. Anytime. Thanks.

You don't need a reference book for this. Do a simple thought experiment. When bolts are inside the flange, there is no rotational constraint. However, when bolts are outside the flange, load path is throught base plate bending and considering a reasonably thick baseplate(which is 99% of the times) bolts would create a couple offering rotational restraint. You need to ensure that your bolts can handle interaction of shear and tension. This is standard parctise. Thanks.

W.salaam.. I dont have an article at hand but I have came across multiple geotechnical reports that ask for that. Basic theory for LRFD design being that factored loads shall meet factored strength. The geotechnical report provides an ultimate bearing capacity with a resistance factor of 0.3-0.6. Structural engineer has to design for all Ultimate Limit States using (0.3 to 0.6)*Ultimate Bearing Capacity. The important thing to note here is that only settlement is a Serviceability Limit State and all other Limit States are ultimate. i will update this post with power point presentation that elaborates the discussion. This is what engineers do in Canada. Thanks.

http://www.sepakistan.com/topic/1303-spring-support-modulus-of-subgrade-reaction/?p=2398 Please see Post #9 at the quoted link above. Thanks.

I think its time that Pakistan transfers to LRFD for foundation design too. I am not sure why are we still following ASD for foundation with a touch of LRFD when almost every other country has made the switch. Thanks.

1) Mass source is related to your seismic mass. You can specify "from loads" and assign all load cases that refer to dead weight with a factor of 1 and live load cases as per code with the appropriate factor based on governing condition. It is always a good idea to confirm the total seismic weight/mass from Etabs to your manual calculation. Please see links below that discuss this aspect. http://www.sepakistan.com/topic/1310-checking-analysis-result/ http://www.sepakistan.com/topic/191-use-of-mass-source-command-in-etabs/ 2) Post your analysis log. Thanks.

Its possible. Your mass participation of that mode would be higher too unless other conditions influence like resonance. When you say "by varying the frequency of first three modes", do you mean the altering the structure? You should clearly differentiate b/w forcing function and structural response. Just saying. Thanks.

Use frame elements, period.

I am not sure what software does this question relates too but in general, I personally prefer plotting contours and designing for the maximum moment instead of using sectuon cuts. In Etabs, you can access the Shell Forces/Stresses form as follows: Run an analysis. Click the Display menu > Force/Stress Diagrams > Shell Stresses/Forces command. Hope it helps. Thanks.

There are a number of things that require discussion here:I am not sure if "Base Fixity" is the right term to use but clearly what is happening here is that with at least 2 base pin supports(the default when we model a wall in ETBAS) the shell elements "behave" as a true wall. Unlike for the case of single pin support where lateral resistance is through moment resistance only and the action is that of a frame instead of a wall. To explain further, assume a model in which only shear wall is modelled(no other structural elements) by using a single pin node at the base. The structure is unstable by itself and any lateral load would cause the wall to rotate about base. Thus for situations in structural systems where shear walls are modelled using a single pin at base, we are enforcing a "frame action" rather than a wall action. That is why your base shear per support is coming close to that of column but slightly higher, the reason for which is given below in 4th paragraph. If you increase the number of pin supports at base to more than two, you would not see any significant increase in the amount of base shear attracted by the wall. As already stated this is because minimum 2 nodes are required to enforce wall action any any further increase is meaningless other than offering a more even load for foundation design. In addition to this, I would also like to mention the case when you modelled a single pin under wall and obtained a slightly higher base shear than that to when columns were modelled (342kN vs 329kN). The reason for this is the reduced length of the beams when walls are modelled result in a slightly stiffer system and increase shear per column. Thus base fixity means nothing. The real jab lies in how ETABS calculates stiffness of shear walls and frames and interaction of shell and frame elements. What is apparent to me is that shell elements when modelled and meshed represent discrete structures rather than individual component of a structure. For interest, you should compare the stiffness of single cantilever column to that of a cantilever wall fixed at base. Apply a 1kN load to both and note the deflection. Are results different. I bet they would be. Apparently ETABS might be using some assumptions related to wall geometry to justify a higher stiffness calculation. Regarding your model, I would still suggest to model vertical elements as columns as your system is way too redundant than a shear wall system; I don't know about column count but naturally your number of columns would be >> than a typical wall system and I believe applying wall provisions in this case would be overkill. Thanks.

No, i don't agree with the explanation. You can mesh the wall finer and increase the number of supports from 3 to 6 and you should still the same base shear. Try it. Thanks.

I am not sure that why you are getting different % of base shear for case of frame elements and shell elements. The analysis should give you the same results for both cases as stiffenes for frame and shell element wouldn't differ. In general, the only advantage of modelling shear walls using shell elememts is that you can easily model wall openings and shell elements are much easier to model for a given length of wall compared to frame elements. In this case, I doubt if there are any openings. Anyhow, Walls have 2D behaviour and Shell or Frame elements should suffice from analysis point of view. Once you get the forces you can resolve them in max compression and tension forces when designing for foundation or reinforcement in walls. This answers your "10mx20m large bulky colum" wall question too. I could see why you are asking this question.1200 x 600 mm columns is not that big. Thanks.

Your warning messages are loss of accuracy. Nothing critical as you don't have any stability warnings. Normally, what you can do is that wherever you have a **Warning** message, you can go to the joint of interest; for information below, error is for UZ at Joint 3354, and zoom in to the Joint for visual inspection to see if everything looks good. * * * W A R N I N G * * * THE SOLUTION LOST 7.8 DIGITS OF ACCURACY FOR DOF UZ OF JOINT 3354 LOCATED AT X = 66.600000, Y = 30.650000, Z = 18.400000, STIFFNESS MATRIX DIAGONAL VALUE = 5.0055E+15 I haven't used ETABS in years, but, I remember that when I used to get such messages, I would check the joints, zoom in and would always find some kind of misalignment of slab with beam. The slab or beam would not align and there would be some gap or misalignment. You can spot it visually if you zoom there. You can then redraw the slab or refine your mesh there. Give it a try and post any pictures of mismatches that you find. Thanks.

I have received the following reply from my teacher, Dr.Farhat. "Personally I believe that the foundation should be taken down to competent strata using piles. If the reclamation site is near sea shore and dredged material is used then it must be assessed for liquefaction and hetrogeneity." Hope it helps. Thanks.

1200x600mm should be treated as column; It has a noticable weak axis response (espically as other members of LFRS are about same size in one axis). I don't know about your framing and column count, but % of baseshear resisted by rectangular columns would be greater in one axis and almost equal in other. If you want to get more classy, compare % of base shear distribution ratio between your rectangular and circular columns to get a feel but with the information provided at hand, best bet would be to model it as a frame element. This answers your part 1 too. Thanks.

Structural Dynamics by Mario Paz Dynamics of Structures by Anil K Chopra

W.salaam.. You should decide about which field on Civil Engineering would you like to pursue and work out a career plan with a senior engineer or your Professor. Depending upon what interests you, decisions regarding further education or working in the industry can be weighted and considered. Right now, do focus a lot on your education. That is very important. Thanks.

Naeem: Did you get an answer yet? I am trying to get my Geotechnical Prof here. Hope he comes and replies. Thanks.