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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.

Some one asked me abt dynamic analysis and when we need to perform dynamic analysis and what about seismic irregularities and how to base our design on that. I couldnt reply but right now there is power shut down in my office (im enjoying it). So thought to share the reply on sepakistan.com so its openly available for everyone. I took snap shopts from my mobile. This document is the seismic symmary from SK gosh. Then there r some pages abt irregularities from my fav book abt tall structures. I think these pics speak more than i know. So have a look at them.