Waqas Haider

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Everything posted by Waqas Haider

  1. This is from Book "Reinforced Concrete Design of Tall Buildings by Bungale S. Taranath" From an inspection of the given problem, determine if the design torsion, Tu, is due to equilibrium torsion or due to compatibility torsion. If it is due to equilibrium, design of member for the entire calculated torsion. If Tu is due to compatibility requirements, as in statically indeterminate structures, it is permissible to reduce Tu to a maximum value, To given by φ4λ fc ′. Sqrt(Acp^2/pcp) Why ACI is putting limit on reduction if we can reduce upto negligible values? Moreover,kindly explain the following wording by the same book. Overall Building Torsion Regarding the simultaneous application of loads in two mutually perpendicular directions, it is worth noting that for buildings in SDC B, the earthquake loads are assumed to act independently along the two orthogonal axes of the buildings. For SDC C buildings having nonparallel lateral load–resisting systems, and for all buildings in SDC D and higher, 100% of the forces for one direction are added to 30% of the forces in the perpendicular direction, the directions chosen to give the worst effect for the member being designed. Is it necessary for every type of building in SDC D and higher? I though this clause is for only non parallel lateral load resisting systems.
  2. Here I am confused regarding two things... 1st is how much we can reduce our design torsion Tu? If we reduce modifier to 0 or 0.01 it means we are reducing Tu completely... But according to ACI 11.5.2.2 — In a statically indeterminate structure where reduction of the torsional moment in a member can occur due to redistribution of internal forces upon cracking, the maximum Tu shall be permitted to be reduced to the values given in (a), (b), or (c), as applicable: (a) For nonprestressed members, at the sections described in 11.5.2.4 φ4λ fc ′. Sqrt(Acp^2/pcp) ans Section 11.5.2.4 says "11.5.2.4 — In nonprestressed members, sections located less than a distance d from the face of a support shall be designed for not less than Tu computed at a distance d. If a concentrated torque occurs within this distance, the critical section for design shall be at the face of the support." It is saying in first clause that we can reduced our torsion upto minimum of φ4λ fc ′. Sqrt(Acp^2/pcp). So reducing modifier upto negligible value is confusing me. More over how to cater this reduction upto specified limiit of φ4λ fc ′. Sqrt(Acp^2/pcp)? In 2nd paragraph it is relating torison design for distance less than d to the distance equal or greater than d. Is it a generalized statement or it is specifically saying that this minimum Tu requirement is for the distances less than d and for distances greater than d these can be ignored? Thanks.
  3. If i reduce my torsion constant to 0.01, having in mind that it is only needed for compatibility, The moment will transfer to slab and secondary beams and ultimately equilibrium will be obtained. But I m confused to achieve this equilibrium, the main beam need to twist for compatibility until equilibrium is achieved. This will induce diagonal cracks in main beam. Is it acceptable and with in serviceable limits? Are these hairline cracks dont affecting any thing especially appearance or any other design force? I have also seen an approach of providing moment release to secondary beams to avoid excessive torsion and to get same condition physically reinforcement at joints of secondary beams is provided to resemble pin support. But still i m confuse if this type of reinforcement is provided, the secondary beam section will crack. Are these cracks in acceptable limit?
  4. Assalam o alaikum, Is there any suitable software available for concrete and steel quantities take of for all members like slab, column, beam, shear wall and foundation? especially which may work in conjunction with ETABS/SAP2000/SAFE. I found suggestions of some seniors to use SAFE for slab footing beam quantities but I want some specialized software where I can export my model of etabs or safe for quantity estimation? I just want to avoid time consumption for this purpose and speed up the process. Has any one listen about BLUEBEAM REVU software? I have heard about it but dont know in details.
  5. Assalam o alaikum. My building has special moment frames and 3 to 4 shear walls. The bay where shear walls are placed, no column is in the bay and beams are coming directly at top of shear wall and for the very specific bay, slab is also resting directly at shear wall at that location. How to decide in which category, exactly, my structure comes? UBC structural systems are 1629.6 Structural Systems. 1629.6.1 General. Structural systems shall be classified as one of the types listed in Table 16-N and defined in this section. 1629.6.2 Bearing wall system. A structural system without a complete vertical load-carrying space frame. Bearing walls or bracing systems provide support for all or most gravity loads. Resistance to lateral load is provided by shear walls or braced frames. I think my structure does not lie in this category because majority of slab and walls are resting at beams connected with columns. Am i right? 1629.6.3 Building frame system. A structural system with an essentially complete space frame providing support for gravity loads. Resistance to lateral load is provided by shear walls or braced frames. Is this structure is where no lateral load is resisted by frame and shear walls resist lateral loads? How is it possible that no lateral load is resisted by frames? 1629.6.4 Moment-resisting frame system. A structural system with an essentially complete space frame providing support for gravity loads. Moment-resisting frames provide resistance to lateral load primarily by flexural action of members. If I have 7 bays in a direction at a grid and I have 5 to 6 grids in building. An only end grids have shear walls in one or two bays, Will it be considered as building MRF or Dual system? What is exact differentiation between them? 1629.6.5 Dual system. A structural system with the following features: 1. An essentially complete space frame that provides support for gravity loads. 2. Resistance to lateral load is provided by shear walls or braced frames and moment-resisting frames (SMRF, IMRF, MMRWF or steel OMRF). The moment-resisting frames shall be designed to independently resist at least 25 percent of the design base shear. 3. The two systems shall be designed to resist the total design base shear in proportion to their relative rigidities considering the interaction of the dual system at all levels. Is there any limit that at least this much of base shear must be resisted by shear wall to consider it as Dual system otherwise it is MRF? Thanks.
  6. Assalam o alaikum. I am attaching image of plan of a building. It is having wing projections more than 15% as shown in attached image. And also Vertical irregularity i.e. Only center part of building has 3rd story and rest of parts dont have. I have the following questions. 1) To consider building a re-entrant corner or of irregularity type-2 according to UBS-Table 16-M, is it necessary for a building to have both side projects of a corner greater than 15% of dimension or even one side projection is greater than 15% still it will be considered as re-entrant corner? Table says for Re-entrant corners, refer 1633.2.9 item 6 and 7 which says, item 6 : "Connections of diaphragms to the vertical elements in structures in Seismic Zones 3 and 4, having a plan irregularity of Type 1, 2, 3 or 4 in Table 16-M, shall be designed without considering either the one-third increase or the duration of load increase considered in allowable stresses for elements resisting earthquake forces." Q: WHAT ABOUT STRENGTH DESIGN METHOD? WHAT IS ALTERNATE WAY OF CAPTURING THIS POINT IN STRENGTH DESIGN METHOD? item 7: "In structures in Seismic Zones 3 and 4 having a plan irregularity of Type 2 in Table 16-M, diaphragm chords and drag members shall be designed considering independent movement of the projecting wings of the structure. Q: WHAT IS A DIAPHRAGM CHORD AND DRAG MEMBER? WHAT THEY REPRESENT STRUCTURALLY IN A CONCRETE BUILDING? Each of these diaphragm elements shall be designed for the more severe of the following two assumptions: Motion of the projecting wings in the same direction. Motion of the projecting wings in opposing directions. Q : KINDLY EXPLAIN ITS WORDING? EXCEPTION: This requirement may be deemed satisfied if the procedures of Section 1631 in conjunction with a three-dimensional model have been used to determine the lateral seismic forces for design. 2) If i divide the building into 3 parts, still it have in Part 2 and 3 re-entrant corners. Should I consider It or as it seems not exceeding much than 15% in one direction, I can ignore it. It is basically a stair case. 3) The part 1 of building has an extra story i.e. total 3 stories but wings have only 2 stories. As it can be seen roughly from plan, The lower story is more than 130% of the upper story so It is, in my opinion, vertical geometric irregularity. Should I worry for this vertical geometry? Should I go for Dynamic anlysis as suggested by reference section for the same table to consider section 1629.8.4 item 2 which says "2. dynamic analysis shall be done for structures having a stiffness, weight or geometric vertical irregularity of Type 1, 2 or 3, as defined in Table 16-L, or structures having irregular features not described in Table 16-L or 16-M, except as permitted by Section 1630.4.2. " BUT AT THE SAME TIME section 1629.8.3 item 3 says " Static analysis is permitted for Irregular structures not more than five stories or 65 feet (19 812 mm) in height." and 1629.8.4 item 3 says "3. Dynamic analysis shall be done Structures over five stories or 65 feet (19 812 mm) in height in Seismic Zones 3 and 4 not having the same structural system throughout their height except as permitted by Section 1630.4.2." Hence Item 3 of both 1629.8.3 and 1629.8.4 says only if irregular structures are less than 5 story, static analysis is permitted but 1629.8.4 item 2 says for vertical irregularity dynamic analysis shall be done. Kindly explain. PS: I m planing to divide the building into 3 parts and shown to avoid any of the irregularity. But if in certain case, i can not divide it, how to interpret the UBC conditions. Thanks.
  7. Neither I m aware of dynamic analysis procedure nor with seismic calculations by ASCE. And i dont have enough time to study for this project because of deadline of project. After understanding UBC 97 for static analysis, once this project is complete, I will obviously go through ASCE and Dynamic analysis. I m using rigid diaphragm but the checks like torsional irregularity or re-entrant corners are not specific to rigid or semi rigid diaphragms i think. I may be wrong. But i dont find any of difference between application of relevant clauses of irregularity by UBC. For what purpose sir?
  8. Thank you so much for so detailed explanations. The best thing of this forum is we come here with blank mind and get clear answers for our specific confusions instead of generalized replies. Can you please tell me from which book or study material are these excerpts of images? Now since my building has less than 5 stories and less than 65ft ht, so i m not going to perform dynamic analysis. Now how can i get this condition satisfied using static analysis using ETABS? My stair case also have same framing of column beam as SMRF and it is connected with the Part 2 and 3. But the only landing slab at floor levels will be connected with remaining concrete slab. Whether I need to consider 1633.2.9 item 6 and 7? Also as you mentioned, we need to check slab stresses in ETABS to decide whether it is greater than 0.2fc' or not or if we r using Omega factor it will be 0.5 fc'. But i think Omega factor is only used in special seismic combos which are uses if system is a discontinuous structural system having horizontal or vertical offset of lateral force resisting system. In my case, no need to use this Omega. Am i right?
  9. Thanks a lot sir for such a beautiful and explanatory answer.
  10. Assalam o alaikum. I have gone through previous posts regarding this topic but still i m unable to get the exact way to check torsional irregularity. What I have concluded through surfing and conversation regarding this topic with some of forum members, I m posting it with uncertainty that I have get it right or not. Kindly place your valuable comment. I m attaching a simple plan with the deformed floor shape under EQx loading with minimum eccentricity. I have labeled points. If X axis towards right, as marked in picture, For EQx loading with (say) 5% eccentricity, the following shape is gotten. Now UBC97 says, the building will be irregular if "Maximum story drift, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.2 times the average of the story drifts of the two ends of the structure ." My confusion lies specially in the bold part. Transverse to an axis (What axis) and two end of the structure (when two ends of structure)? Besides the confusion under wording, what i have been guided by other fellows is that, we need to check for EQx, the displacement of Point 1 under EQx drift combos (This will be the maximum displacement) and the displacement of Point 2 under EQx drift combos (This will be the minimum displacement). Then we need to compare the maximum displacement that is of point 1, to the average of this maximum and minimum displacements of point 1 and 2 respectively If this ratio is greater than 1.2, there will be torsional irregularity and if it is greater than 1.4, there will be extreme torsional irregularity. And we need to provide stiff columns or shear walls accordingly to reduce this ratio under 1.2 to eliminate torsional irregularity. Am i getting it right? Thanks.
  11. I think if Del.m is being compared, since it is maximum INELASTIC displacement, only collapse limit state will be satisfied and obviously damage and serviceability to structure will occur. Also we are designing structure to withstand seismic forces without collapse but damage is acceptable.
  12. You can assign end releases for moment to beam connecting at a joint with column, for that joint specifically. This will model the end condition transferring all the forces except moments from column to beam end.
  13. Assalam o alaikum, I need to design a college building located in zone 4 with 3 stories above ground and one basement completely buried. I have never worked with shear walls before so i m very much confused about location and design consideration of shear walls. I m attaching column and shear wall tentative layout for review from the seniors. Kindly review and put your valueable comments regarding location of walls and layout of columns. I also need to understand design of structures having frame as well as shear walls. What i know is this for zone 4, it will act as dual system with R value 8.5 and I need to satisfy all the conditions of 1629.6.5 of UBC. More over also tell whether can we provide openings for windows etc in shear walls? Thanks. To upload Model (1).pdf To upload.dwg
  14. Assalam o alaikum, I m attaching plan outline of a building quite irregular in plan. As generally, expansion joint is provided at 150ft spacing, in my case, After 150 ft, The building changes its geometry and direction of plan so Cant Decide whether it needs an expansion joint or not. It also have a big void which is an open yard in the middle of building. If expansion joint is needed, i have also marked a proposal line. Does it seem ok? Building plan outline.pdf
  15. Can u please refer me some reading material about it? Or only in code i can find required info about it.
  16. I have never applied temperature load in buildings. Kindly guide where can i find code guidelines about when it becomes necessary to apply temp load and how to apply it?
  17. I always follow the same method but while reporting of forces by etabs, it reports excessively high shell stresses near corners and joints? How to overcome this issue to get realistic value? What i did in last project i neglected the value of stress of bottom most plates near joints upto a distance equal to half thickness of the bottom slab. But i still doubt the value because it use to b higher than the manual calculations.
  18. Assalam o alaikum seniors, I am searching for some design example of TIE BEAM DESIGN.... I am having sap reactions of all the columns.. I need detail of manual design of tie beam. Or info how to design it. I also want to design it for differential settlement of columns and for lateral loads also... How shud i take these loads in my design??? If i design this beam in sap2000, i apply gravity load and support settelemnt of 25 mm at one end of beam and design it, will it be OK? Shud i take soil reaction from bottom or shud i neglect it? Jazak Allah...
  19. Assalam o alaikum, I am having a building with a semi circle shape. None of the footing is either parallel to global X-axis nor to the global Y-axis. The plan of footings is attached. The problem I am facing is, ETABS reports joint reactions along global X-axis and global Y-axis instead of along the local axes of column. Since my columns are at a degree to global axis, the joint reactions also becomes at a degree from column and footing. Since to use simple combined stress formula of stress = P/A + Mx*Y/Ix + My*X/Iy I need forces along axis of footing. Or i will have to resolve either moment of inertia of footing along direction of forces or vice versa, which is quite tough and time taking. The other option I can go for is to design footing in SAFE. But again here, there i dont find any option to rotate footing at a certain degree to match orientation of columns. If i rotate local axis of footing, It only rotates its local axes and meshing but not the footing physically itself. The orientation of footing remains same. How can i solve this problem? Either having reactions in etabs along local axes of columns can solve my issue so that i can design it manually or rotating footing in safe to match local axes of columns can solve my issue. So can any one guide me how to do either option? Thanks.
  20. Better u provide model and screen shots of ur plan showing meshing to give clear picture.
  21. Brick masonry can not be designed in these software because brick masonry is neither the frame elements which can be modelled in these software nor are the simple isotropic type materials which can be modelled easily like area elements. You will have to do design for brick masonry manually. But you can take help from these software to read results of different type of analysis u need while doing manual design. There might be other softwares which can do this perhaps. But regarding these two softwares, i hold this opinion.
  22. Only exterior walls need to be of RCC. For construction purpose, U can consult the following manual. and MUST follow its practices. But remember, it is not the design manual. So you need to have a good design of your house including footings, RCC walls, band beams, and corner columns etc.
  23. The simplest answer would be both thins are possible with equal safety.It is only you who is to decide whether you need basement for extra space or not? because ultimately it is going to affect your cost of construction. Kindly tell in which city/area this house is going to be constructed. Also tell what type of terrain/ site grading is there. Some times providing basement may reduce your cost of backfill. So tell something about soil and site grading. Is it flat, slopy, hilly or what?
  24. Hello Naqash, 1st of all welcome to this forum. And also try to spread this forum in your area/circle as much as possible, so that maximum people can take benefit of it. Naqash if you remember, in manual design of roof slab or footing slab, we use to consider a unit strip and design this unit strip for output results. Then we provide the steel and section calculated for this unit strip, to whole of footing or slab. Similarly, in Safe foundation, you are to define and assign design strip of unit width to let software show you output i.e. forces and steel etc for this unit design strip. you can also go for any width of design strip. Results will be accordingly but design will be same. Mostly a unit strip i.e. for MKS system 1 meter strip and for FPS 1 ft strip is defined and assigned. After running the model, you have two options to check your results of forces and reinforcement. 1) Based on Finite Elements In this method reinforcement and forces for each and every element, in which structure has been divided / meshed, will be reported. This is a bit different than our usual manual practice of designing. 2) Based on Design strips In this method reinforcement and forces are reported on strip based. This is quite easy to understand. You can check both methods for better idea. Another important thing is that we define 2 design strips "Strip A" and "Strip B" to read results in X and Y axis or in Shorter and Longer directions respectively. If you provide strip A in both X and Y directions, Then you will not be able to read results in X and Y axis, separately. display of results will be mixed. For this purpose we provide 2 different strips.
  25. It can happen when lateral loads are included in combinations. Stresses of reversal due to lateral loads causes any side of joint in continuous beam to have tension on lower side causing positive moment at support on one side. So check if ur combination is including lateral loads.