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Waqas Haider

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Everything posted by Waqas Haider

  1. Respected seniors, It is my 1st post. I have got help from this forum many times but posting 1st time.. i am a junior structural engineer.. i am to model a steel column resting at 8 ft concrete pedestal of isolated foundation... The base of steel column is attached resting at pedestal with the help of a hinged type base plate... but i am confused what type of constraint i shud provide at the connection of steel column base at pedestal.... i even dont know exactly what a constraint is... i am to design foundations of column. so kindly let me know how to model it properly and accurately...
  2. From engineering point of view, it is really very amazing to see a complete city builtup in water ways, in a lagoon. How did they make the foundation system of buildings completely all the time underwater? Really interesting article here is. It also explains some modern challanges to the city i.e. flooding due to raised levels of sea and an inteseting solution to this problem. https://sites.google.com/site/engineeringvenice/
  3. R/f at top in isolated footing

    Top reinforcement is needed in isolated footing majorly for two reasons. 1) Due to negative pressure under some or whole part of footing. In part of footing where there is positive pressure, the footing is in complete contact with soil and tension is in bottom side of footing. But in part of footing where there is negative pressure the footing is no more in contact with ground. Either it is designed as it is (and reduced contact area is used for calculations) or it is made to become in contact with ground by help of over burden loads. In either case, the bending of footing is in such a way which causes tension of footing at top of foundation demanding top reinforcement in footing. Mostly top reinforcement is less than bottom reinforcement but for simplicity, if it is not affecting economy much same reinforcement can also be used for top and bottom. 2) Due to temperature and shrinkage control. Code says we can provide temperature based minimum steel either in one layer at center or at any face or we can divide total steel in two layers i.e. half at top face and half at bottom. According to Zahid Ahmad Siddique (Professor at UET Lahore) in his book concrete structures mentions it is better to provide temperature steel in two layers if thickness of footing increases 18 inch.
  4. Assalam o alaikum, i m confused whether collectors and chords are only required in Flat plates/slabs or also required in beam slabs? because in beam slab we are having beams at perimeter of every panel and they can act as chords and for diaphragm bending and as collectors at junction of slab to shear wall. Usually, in seismic zone 4, i dont design diaphragm exclusively. i just design it as simple slab and design my beams in SMRF or Dual Frame in etabs. Also i m confused in excerpt from a document stating The Special Seismic Load combination is also indirectly identified in Chapter 18, Section 1809, Foundation Construction – Seismic Zones 3 and 4. 1809.3 Superstructure-to-Foundation Connection. The connection of superstructure elements to the foundation shall be adequate to transmit to the foundation the forces for which the elements were required to be designed. For instance, since Section 2213.5 Column Requirements specifically identifies the Ω0 factor, Section 1809.3 requires the column-to-foundation connection to be designed for a load combination which includes the Ω0 factor. What does it mean? In seismic zone 3 or 4, we need to design foundations for special seismic combos ALWAYS or only if we r having discontinuous system? The full document is also attached. Thanks. OmegaFactorDiscussion.pdf
  5. Import CAD file to Etabs

    I use etabs version 9.7.4 so i can tell u about importing plan from cad to etabs in this version. Define a layer in CAD for example GRID LAYER. then draw ur whole grid in this cad layer.. after you finish marking grid in cad drawing, save the file as DXF FILE instead of DWG extension. Please note that the insertion units of cad drawing (you can see insertion units by UN command) and in etabs while importing this grid must be same. One u saved drawing as dxf file. open new etabs model, delete existing grid, import file as DXF file, set units before importing the file in drop down manu. Select layer of Grid and import this layer. U will get your grid import. since there are a lot of grids usually in a cad drawing, it is better to shift whole grid before savin as dxf file into a new cad drawing with origin set 0,0,0. Copy grid from old drawing as a block into new drawing in desired layer, open new cad drawing, paste and put insertion coordinates as 0,0,0 Then explode the block and save file as dxf and import this file as said above. More over Perpendicular an inclined grids can be made in etabs but i prefer to go for importing grid because it is less time consuming and easy.
  6. Assalam o alaikum, If a retaining wall is retaining horizontal backfill, It will experience soil presure due to full height of backfill because we are forcing soil to retain at an angle larger than its angle of repose and hence failure wedge will cause force on full height. But if a retaining wall is retaining backfill having sloppy face( for insulation purposes lets say), then it should not experience the pressure due to full height in my opinion. Because soil is having slope on opposite side, (away from wall) and hence if a failure plan would occur, it will cause soil to slide away from wall, not towards wall. This is what i think. Pictures are also attached. In my case the wall height is 30 ft but it is retaining a backfill with slope. So i wonder if i should take load due to full 30ft height. I think i can take 25 ft height(or some other reduced value). Can any one guide me how to calculate backfill load due to such type of backfill? Thanks.
  7. Manual and Etabs result don't match

    In manual calculation we do 2D analysis of structures using different method of analysis than stiffness method. So somewhat difference will always be there while you compare results of manual vs Etabs in Systems.. For individual elements, they are 100% same.... More over, be sure to make modifiers of slab to 0.0001 to nullify slab stiffness because in manual analysis, might be u are assuming only rectangular beam and applying line load to this beam. but in etabs you are modeling slab with beam making system a bit stiffer due to slab stiffness. If you will reduce slab stiffness to negligible value, than results will be near to each other. Still there will be some difference because we might be using force methods idealizing 2D structure while etabs performs 3D analysis using direct stiffness method.
  8. Backfill load with backfill in negative slope

    As per discussion with my senior in office, we have concluded that since soil slope angle is already lesser than angle of repose of soil, so soil will not slide to that slope. Wedge of soil will be exerting pressure on wall because at wall we are forcing soil to stand at larger angle than its angle of repose. So i was wrong than we should take lesser backfil load than total height. We should take ful height or at least the height of soil which cantilever heal is bearing.... This is what we think.. If we concluded wrong, kindly correct. thanks.
  9. Ads for Guests

    Yeah. I always use to amaze who is funding this website except you sir. It really need fiances, time and extreme effort to arrange and manage such a website having a good interface along with updating options of social world. It is really good sir. You have initiated a chain and it is really making change in Pakistani community of engineers who rely on internet a lot for learning. We try our level best to spread this forum as much we can. At end, heartily thanks to the moderators and all other members of the forum who are sharing their knowledge and time with people. Thanks.
  10. RCC Dome Design

    A very interesting and explanatory document along with example..... design of dome.pdf
  11. Design For Shear And Torsion Using Etabs

    This is what i found in "Structural Concrete theory and Design by Nadeem Hussen" Here it also reports the same thing that we should design for torsion for atleast φ4λ Sqrt(fc ′) A2cp / Pcp Which is also specified by ACI that we can reduce our torsion upto phi.Tcr i.e. cracking torsion and not below this. Even if we neglect the remaining torsion for compatibility, we should atleast perform design for phi.Tcr or should at least provide minimum longitudinal and transverse reinforcemet for torsion. This is to control crack width to satisfy servicability. ACI, Nilson and Nadeem hussen all quote the same thing. Hence reducing the modifier upto such a little value is not good at start. At start we should go for a value of 1, 0.3 or any other suitable value which user think is small enough to release majority of compatibility torsion and will sustain only smaler torsional moments. After designing, if still some indeterminate beams are being failed, then for the specific beams we can reduce value unless we get Tu equall to phi.Tcr or a bit larger than that. Because putting Torsional modifier to 0.001 would not report any torsional reinforcement and hence adding no torsional reinforcement at all would cause excessive crack widths affecting serviceability of the structure.
  12. Building Joint Displacement/ Drift in Etabs

    please go through this thread. Thanks.
  13. Shear wall design

    Last month, i design a structure with shear walls. I will attach more documents. What i know/remember i will share with you. In simple, Pier is similar to column and Spandrel is similar to beam. Shear wall is modeled in etabs as shell element. You are to assign it either pier label or spandrel label (Depending on situation of shear wall) to design a shear wall. If you want to design a shear wall, it mostly needs peir label to be assigned. But for the portions of shear wall above openings, as its behavior is somewhat like beam resting on two supports, you need to assign it a spandrel label. I have attached screen shots of my model where you can see i have assigned pier label to the vertical elements and i have assigned spandrel labels to the elements spaning horizontally above openings. The reason behind is detailing based. The detailing in vertical elements of shear wall is similar to wall (or column) and its design is based on considering it as an element resisting majorly AXIAL FORCES + MOMENTS along with inplane shear forces. (Theoratically it acts as a cantilever beam spaning vertically which again represent behavoiur similar to column). I have also attached screen shot of design detail of pier which reports total vertical steel in wall with respect to section area for resisting moments, horizontal steel for resisting shear and some times boundry elemtns (i.e. special detailing at ends of shear wall exactly as in column i.e. shear rings confining vertical bars). I have also attached screen shot of design detail of spanderal which reports top steel, bottom steel, to resist moments similar to beam and vertical steel to resist shear along with DIAGONAL reinforcement some times needed to account for reversal of forces. I will attach more documents throwing detailed light on shear wall design i found few weeks ago. Also my model is attached (it is in seismic zone 4 from where you can get idea about pier labeling and spanderal labeling) One important thing is that, pier label and spanderal label also meant to integrate forces. i.e. if you assign same pier label to all walls in a floor connected with each other, the software will report results only for one and critical wall because all walls were assigned only one pier label. If you want to have multiple outputs at multiple walls, you should assign them different pier label. in image number 5, note that i have not assigned any pier to the selected portion of wall. hence no reporting of results is done. Etabs Center Portion.e2k
  14. Structural Draftsman needed

    Assalam o alaikum, There is a job opening for structural draftsman in Kashif Aslam and Associates, Lahore, with minimum of 3 to 4 years of experience in structural drafting. Interested candidates may send their CV at waqas.haider@kaa.com.pk Please forward to the concerned person in your circle also. Thanks.
  15. Design For Shear And Torsion Using Etabs

    I am confused regarding this approach of reducing torsional modifier to such great extent. 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 Sqrt(fc ′) A2cp / Pcp It says that we can reduce torsional moment upto a specified limit. Not to zero or 0.001. In its commentary it says, For this condition, illustrated in Fig. R11.5.2.2, the torsional stiffness before cracking corresponds to that of the uncracked section according to St. Venant’s theory. At torsional cracking, however, a large twist occurs under an essentially constant torque, resulting in a large redistribution of forces in the structure.11.34,11.35 The cracking torque under combined shear, flexure, and torsion corresponds to a principal tensile stress somewhat less than the quoted in R11.5.1. When the torsional moment exceeds the cracking torque, a maximum factored torsional moment equal to the cracking torque may be assumed to occur at the critical sections near the faces of the supports. This limit has been established to control the width of torsional cracks. Also according to Nislon, this distribution is only possible after extensive cracking as highlighted in below pic. So I doubt the approach used to neglect torsion upto 0.001 level What i got from the 2nd attachement of Zain Saeed the author is dividing Tcr with Tu to find how much reduction in Tu is needed to reduce torsion upto Tcr which is, as mentioned above, is necessary to keep torsional crack widths in control. and hence using the modifier for each section defined for beam. It might be a bit lengthy task to evaluate for each type of section ( most loaded members of a type of beam may be checked only), but the approach seems more realistic. Kindly comment as I think if even we reduce to 0.001, still this redistribution in torsion is not possible without large twisting which is not possible without excessive cracking. So reucing upto such a low value does not seem good.
  16. Sir link is not working. Their website is not being loaded. Is there any other alternate to know the details?
  17. Torsion: Reinforced Concrete Members

    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.
  18. Torsion: Reinforced Concrete Members

    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.
  19. Torsion: Reinforced Concrete Members

    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?
  20. Quantity Takeoff software

    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.
  21. 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.
  22. 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.
  23. Re entrant Corners and Vertical Irregularity

    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?
  24. Re entrant Corners and Vertical Irregularity

    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?
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