Live Chat Session by Dr N. S.

Posted May 12th, 2009 by Dr. N. Subramanian
Author: 
Dr Subramanian Narayanan
Posted On: 
2009-04-21
Article Summary: 
Live Chat Session
gayatriajit(Q) I live in seismic zone IV. Most of the structures here are reinforced frame structures. Building height is restricted to 11.5 mts. Here the practice is to use 5" thick brick infill wall even on the outer walls. How safe is this?
drnsmani(A) Half brick walls are not safe in any zone. Note that research in Infill walls is still continuing. It is better to have reinforced brickwork for safe structures.
vijaya_vittal(Q) What is the support condition for the columns supporting the crane girder. Is it be taken as cantilever or horizontally supported at the crane gilrder level. The sliding friction of the gantry girder on the crane girder induces horizontal support
drnsmani(A) In my opinion it is better to design it as cantilever, though there may be some restraint.
himanshu_patel(Q) Can you give me my method of modeling in Staad for Shear wall is i am adding column member at center of wall and given actual size of wall, then i increase the modulus of elasticity of this member (column- shear wall) by 100 time the normal elasticity of concrete connection beam member either sides of column member for making rigid element. this try or give me comment
drnsmani(A) Sorry. I am not in a position to answer your question.
vijaya_vittal(Q) What is percentage of the tension area below the foundation is acceptable
drnsmani(A) I do not remember the exact percentage. But the foundation should be safe for overturning condition. May tell you in email after referring
vijaya_vittal(Q) Can we relay on the expansion bolts in the concrete where the reversal of stress do occur.
drnsmani(A) Hope it also relates to foundation. Are you talking about anchor bolts? Anchor bolts are effective in tension. For compression it is not required. Unfortunately IS Code does not contain provisions for the design of Anchor bolts. Refer ACI 318-App.D
amitav(Q) Dr NS, Good evening. I had followed your discussion and Mallick sirs on tensile strength of concrete. IN the end I got very confused whethe concrete 's tensile strength is actually considered in any code/calculation or its just ignored.
drnsmani(A) Mr Amitav, good Evening to you. Tensile strength is not used in ordinary concrete flexure theory; But in concretes such as fibre reinforced concrete, where the tensile strength may be more, it is considered. Also for calculating the moment at which cracking occurs tensile strength is used. In general it's use is not much in design
vijaya_vittal(C) The mechanical anchor will become loose when ever the movement is included due to the stress reversal like in the wind condition
amitav(Q) Sir I made the the frame of my house with columns & beams with steel and trying to make a big panel from lightweight concrete using perlite, lime & cement for the walls (which have no loads on them) what is your advice? Can I use the panels (with reinforcement) in the roof? Do I need to plaster the walls and paint them to keep rain from them?
drnsmani(A) Light weight concrete is good for such application; in fact ACI 318-08 has modified all the eqns to cater to light weight concrete(LWC) also. It will reduce the foundation load. But I do not know about the porosity of such wall. After testing it you may abopt it. In my opinion plastering is for beautifying the surfaces only!
sudip_sefi(Q) sir I have done be from NIT. I want to persue a career in design in India. Some at SEFI suggested that M.Tech is must for any design job. I am trying to do that this year confused if steel design should be taken or Concrete design. Today I get an an opportunity to ask you , directly , please could you share some of your inputs. What is best to start a independent consultancy design office. I may be asking wrong thing please ignore if so. I am a big admirer of you. Thanks sir.
drnsmani(A) Thank you for your words. To tell you frankly, unless you have a GOD father, it is not advisible to start consultancy alone- as the fees are low and many even do not pay in India. But you can join in big firms after doing M.E. Now there is a boom in India. In India Concrete is given more imp. and steel designers are less- that me and it is advantageous to be a steel designer, if you get a position. But concrete designers will get a job in any place.
vijaya_vittal(Q) Thank you Dr. Subramanyam. H.N.Vijaya.
drnsmani(A) Dear Er Vijaya, how are you? I am answering your question now. Prof. Eligehausen of the Univ. of Stuttgart, Germany has done considerable work on anchor bolts both in cracked and uncracked concrete. He is also one of the authors of App. D. of ACI 318. I think it considers cracked concrete. Please read the commentary to App.D. in the code to get more info.
sudip_sefi(Q) Many thanks sir. I will follow your advice. I am really more relaxed on to hear from person of your experience. One last query is it advised to do Ph.D, will it add to value as for as industry is concerned. Sorry to bother you too much.
drnsmani(A) No Problem. Actually I am happy that I am able to guide budding Engineers. Doing Ph.D is good for you. But in India Ph.D. means you are good for teaching position only. I think I was the second or third to start my own firm with Ph.D. Now the situation is changing. Many companies are hiring Ph.D.s. IN USA Ph.D.s are found everywhere-even in RMC plants!
P.K.Mallick(Q) Good morning from India.
drnsmani(A) Good Evening Er Mallick! Congratulations again!
sudip_sefi(Q) Many thanks sir. Indian realities are different from what its in US. your suggestions have boosted my morale. I may meet you some time in my industrial career as I begin. Its my wish. I have read your computer books and steel design as well. Good Night Sir.
drnsmani(A) Thanks. Surely we may meet. Good night to you. For me it is 11.19 am!
imtiyaz_ahmand(Q) Respected sir can you please guide me on the design rules of steel plates structures I could not find codes etc other than thumb rules..
drnsmani(A) I think I cited a ref. for steel plated structures in the regular SEFI forum. In fact all steel members are made of plates only and the main think is that we should follow the guidelines given in codes to proportion them such that local buckling of plates is avoided. What is your exact problem?
imtiyaz_ahmand(Q) I will post my problems on SEFI in more detail. Chat box has limited space. I have fundamental doubt though. One more nagging doubt sir. Does the corrosion of steel reinforcement actually increase bond strength between steel and concrete or it degrades the bond.
drnsmani(A) OK. When people were using plain bars they were encouraged to use slightly rusted bars to increase bond. Noe with bars with ribs, bond is mainly provided by the ribs. Hence now there is no necessity to use slightly corroded bars. You may read the classical book by Park and Paulay(1975) for more info. on bond strength. This book is out-of-print but surely available in Lib.