Muneeb Badar

Dear mhdhamood, I am happy to help............If you still not clear just let me know.....I ll send you some literature regarding this. What Rana Waseem summarized is absolutely perfect.........I just wanted to add in his Point # 1 that during this out of plane design we need to satisfy ACI 21.13.6 for Flat Slabs. In case of Beam supported slab no need to consider ACI 21.13.6. Thanks Muneeb

Dear mhdhamood, My dear as I told you before that you are 100% right about this.... .....But these are inplane forces not out of plane forces. The picture you showed me is for inplane forces. As I told you before that you need to design for gravity for out of plane by satisfying 21.13.6 and then inplane forces (which you showed me) you nedd to design as well, which itself is a very complex kind of design. You not only need to design for collector forces as you showed but also for Shear Friction, Shear Reinforcement and Tension Reinforcement.......BUT ALL OF THIS DESIGN IS FOR INPLANE FORCES NOT FOR OUT OF PLANE FORCES You are confusing inplane and out of plane design. I ll summarize again: 1- Out of Plane: You need to check the thickness of slab (for flexure and shear )by satisfying 21.13.6 using gravity loads 2- In Plane : You need to check the thickness and provide shear reinforcement and flexural reinforcement for inplane design uding Lateral Loads. These reinforcement includes Tension (Chord) Reinforcement, Shear Friction Reinforcement, Shear Reinforcement, Retaining Wall Distribution Reinforcement: Retaining Wall Vertical Reinforcement The second type of design (for inplane forces) is not easy to handle. It involves a lot of knowledge and engineering judgement and is not common at all. If you want to design for this I can provide you guidelines. Hope it will be more clear now Thanks Muneeb

Dear mhdhamood, Sorry I forgot to add one thing. You can design the Flat Slab based on the gravity loading and then design the diaphragm for lateral forces. BUT there is one condition you need to check. In case of Flat Slab, if you are designing it on gravity loading then you need to check the story drift ratios. Please check ACT 21.13.6 (b ). We need to satisfy this condition if we want Flat Slab to be designed as gravity member. Thanks Muneeb

Dear All, It is a good guideline, but, in my opinion these provide highly conservative results. Secondly, you need to select that are you taking bottom as fix or pinned. Because everything depends upon your selection. If you take bottom as fix then it will be conservative. Secondly, in case of pure cantilever wall, it gives much forces for the distribution reinforcement. This is my experience anybody can share his own............ Thanks Muneeb

Dear Rana, Please check the reference from ETABS Concrete design manual. This is what I am saying. Thanks Muneeb

Dear Rana, I partially agree and partially disagree You are right about the explanation. This is much informaticve for me. but when ETABS will check the torsion, how it will check. In ETABS its is not coded to check the beam torsion by which way. Neither ETABS nor we can ask it to do what we want. So ETABS takes it conservative. I am also not 100% confirm but this is my opinion. About the modifier you are saying, I think no code allows to reduce it to 0.001. This is just a practice to handle ETABS issue. Actually, I am not saying it by myself, I asked this question from the person that used to work for CSI & ETABS as you know that was my last office. Thanks Muneeb

Dear mhdhamood, in your next comment you are asking which axial force to use, top shear wall or bottom shear wall whcih will cause rotation. Dear these shear forces can not be used to check the slab thickness as these are inplane shear forces when transferred from shaer wall to slab or from slab to shear wall. Even IF YOU PROVIDE THE THICKNESS FOR THIS SHEAR, IT WILL NOT BE EFFECTIVE IN CASE OF EARTHQUAKE AS YOU PROVIDED THE THICKNESS WHERE IT IS NOT REQUIRED. You have to increase the thickness in the basis of inplane bending not on the basis of out out of plane bending. Thanks Muneeb

Dear mhdhamood, Yes I am saying this to check the shear for gravity. Actually, what you are saying is 100% correct but this design should be inplane. I ll explain again. When earthquake comes and there will be rotations then these rotations will not be out of plane. These will b e inplane. So thats why you need to design the slab for inplane forces for lateral loading and thickness of slab for out of plane forces. Thats the concept of code. This is the reason the ACI chapter 21 allows us to design the slab member on gravity for out of plane forces and then design the slab for lateral forces for inplane forces. So there are two types of design for Slab in case of earthquake if building is in high seismic zone 1- Out of plane design which is based on gravity analysis which includes the ordinary flexural reinforcement and shear reinforcement if required 2- Inplane design which includes the Tension reinforcement, shear reinforcement and shear fricktion reinforcement. Seconldy if your building is in high seismic areas you can not use these same load combinations which you wrote above. If as per ASCE if your building is in seismic deisign category D, E and F then you need to modify these load combinations.

Dear Rana, I can not understand your first comment clearly. Actually the lateral forces transferred to vertical elements through beams and slabs as axial forces and shear and moment. We are only required to design the slabs for gravity not for these lateral loadings but we need to check the punching by some other way which is not code based. So what you are saying to check horizontally, then I think he is not asking for this. He is asking to check the slab shear for lateral loading which code does not require. Sorry if I can not clarify 100%

One more thing I want to add, although I am not sure about this. In ACI code there are two methods of calculating the torsion for a beam in ACI 11.5. One is slab resting on beam and there is torsion in the beam ACI Fig R 11.5.2.2 and second method is slab cantilever from beam Fig R 11.5.2.1. ETABS normally design the each beam by the second method which is the conservative one. So this the reason beams mostly fails in torsion effect. This is the reason we reduce the torsion modifier otherwise no code allows to reduce modifier to such extent. This is my concept any body can comment...... specially Rana Waseem

Dear mhdhamood, 1- In case of seismic loading, the shearwall will take high moment and high shear. In case of seismic, it is not necessary that the design combination will be tension. If you talking about alone seismic load case, it can be tension. but when you design for load combs, it can be tension if seismic increases from DL and compression if not. 3- First of all you need to check the punching of slab for gravity loading. The thing which you are saying is 100% correct. But what your senior saying is also correct. I ll explain. What your senior are saying: This is right but not for code based design. You do not need to design the thickness of slab for seismic as in ACI code, you can design the members for gravity loading for the structures which are in high seismic areas ACI 21.13. You are designing the slab for out of plane forces. AND these out of plane forces as per ACI 21.13 we can design for gravity loading including shear and flexure. What you are saying: BUT you need to design the slab (diaphragm design) for inplane forces which are based on seismic demand. So yiu are not required to provide the thickness of slab for seismic loading as it will be out of plane forces. But you need to design this slab near shearwall are for inplane forces Now what you are saying is the next step. After providing the thickness of slab on gravity loading, we need to check the thickness of slab by checking it on slab beam behaviour and checking its rotation which is the next step and its details are not given in any code. There are guidelines which can be used to check these details like ATC 72. Thanks Muneeb

Dear Baz Just a discussion You are almost right. Area does not matter too much to resist the seismic. The main area which resists is adjacent to shearwall. If area goes on increasing the Mass source is increasing and this increasing the lateral forces. Secondly, Column will not contribute too much as these have small dimension along x and y direction. Mainly shear force is resisted by shearwall. For example x direction shear force is resisted by shearwall along x direction and y direction shear is resisted by y direction shearwall. Because we do not design the shearwall for out of plane bending. In shearwall shear design the depth used for design is the length of shearwall. so thats why by increasing the thickness of shearwall you can not satisfy the shear in shaerwall. For this you need to increase length. Same is the case with beams. In beams if you increase the width of beam, it will not change your Time Period significantly but when you increase depth, it will contribute more. If Base Shear percentage by weight is more then every force will be more. For shear this can be satisfied by increasing either no. of shearwalls or by increasing the length of shearwall.. Thanks Muneeb

Dear The reason is when we suddenly increase the size of column for example from 600 to 300, the forces accumulated at these locations. Actually at each joint the lateral forces normally distributed to adjacent members and also to the below columns. If you suddenly reduce the column size there are much chances of improper stress/forces distribution. So a gradual change is required. Secondly in your case you are suddenly increasing the size at Slab on Grade level. This situation is much more complex. Your structure in soil (structure from slab on grade to foundation level) act as one unit. It means there will be much inplane shear force at slab on grade level. Your complete structure will vibrate starting from slab on grade not from foundation as you provided much thicker columns below slab on grade and made this portion rigid. If you increase the size and do not model slab on grade, I think it will not be good. You can model slab on grade and check the inplane forces distribution at slab on grade level. In this case you can be sure that increasing the column size is affecting or not. This is my concept, anybody can disagree........

Just one thing I want to add that if this is highrise building then you need to check the axial forces in the adjacent beams if you increase it eccentrically and if there are some grade beams. It will not effect in all cases but better to check. ; Secondly if you increase it eccentrically then take care how you will continue the reinforcement from this column to above column as everything depends upon the reinforcement. and in this case you need to model slab on grade and check the inplane stresses if there is soil outside because your total burried area will act as one unit One last important thing, you said the column creating problem in punching and when you increase size it is solved. you can not suddenly increase the size in that small length too much. You need some curtailment for column size. For example if the column above slab on grade is 300mm in diameter then you can not provide 600 mm dia below slab on grade. So please take care of this. These are just my opinions anybody can comment of not agree...... Thanks Muneeb

Dear How r u designing the shearwall in shear? If shearwall is failing in shear then you can do following things as told by Mr. Umar 1- Try to increase the length of shearwall. The thickness increase will not matter too much. But if you can increase length that will be helpful to design shearwall in shear. 2- Try to increase the shear reinforcement in shearwall. 3- Try to increase fc' 4- Try to add some more shearwalls to reduce the total shear demand by structure. I think by these points of these four points you will be able to solve the issue because everything depends upon the shear force in shear wall. You also need to provide confinement in the confined zones as well for flexural reinforcement. Thanks Muneeb

Thanks alot dear Wasim, Its really quite a good place to know the things and sharing the knowledge. Please change the sign of "Newbie" from my display profile......

Very Well Explained by Wasim, very nice approach towards PBD

Dear Umar Makhzumi, Thanks for your reply. Well, truly speaking Performance Based Design is not necessarily meant for Tall Buildings. Actually, its about the performance of structure. In some countries we done some projects of 2 or 3 story concrete and masonry buildings. So, we can use these any where. For small buildings it is also of great importance due to the effect of brick masonry walls which can only be predicted by performance based design. So both techniques are not equal. Actually, code based design has too many flaws when considered for the design of a little non rectangular or non square building. PBD is really quite good for undergrads for projects on a small scale. Thanks Muneeb

Dear Engr. Uzair, many thanks. But I do not need internet search. I just need to discuss these details on this forum. This post is just for knowledge sharing. Thanks Muneeb

Dear All, What do you think what are the advantages or disadvantages of code based design And What are the benefits/drawbacks of Performance based design which is also Non Linear Design practice. A short discussion is welcomed. Thanks Muneeb

The best option is Hilti. If there is steel shed then do not use normal drilling. As there will be severe lateral loading due to wind. Ask Hilti, they are costly, but they provide very good and reliable solution as in Lahore on all roof top towers, normally hilti solution is applied. Thanks

Well explained by Mr, Waseem. The first type of Non-Linearity, the Geometric Non-Linearity can easily be incorporated into the structural model. Adding Material Non-linearity is quite difficult. Even there is no code which provides this method. There are many guidelines which normally Structural Engineers follow to add these geometric non linearity. The parameters given in the code are basically derived by considering these non linearities but these parameters do not give 100 percent reliable results, because everything depends upon the shape of structure. The code mostly covers the buildings which has symmetric shape. For example in case of Wind if the structure is not symmetrical or rectangular in shape we can not use the code based factors with 100 percent reliability. Thanks Muneeb

What is the difference between Linear and Non Linear Analysis..............What are the advantages of Non Linear Analysis.........Why do we need Non Linear Analysis........Can anybody explain????

Thanks Dear Waseem and Umar................... I ll add a picture soon................

Assalam u Alaikum, This is Muneeb Badar. I am UET graduate. My session was 2001. I am currently working in Saudi Arabia. My experience is in Linear and Non Linear Time History Analysis. I used to work on Performance Based Design of Tall Buildings. I am happy to find such a fantastic page. Thanks Muneeb