Re entrant Corners and Vertical Irregularity

[Waqas Haider]

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.

[Saad Pervez]

1 hour ago, Waqas Haider said:

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? 

A: In my opinion, both sides, this is to exclude an architectural projection e.g. a patio roof from this definition.

1 hour ago, Waqas Haider said:

Q: WHAT ABOUT STRENGTH DESIGN METHOD? WHAT IS ALTERNATE WAY OF CAPTURING THIS POINT IN STRENGTH DESIGN METHOD?

A: ACI318-11 R21.11.7.5:

 

"....collector elements of diaphragms are designed for force amplified by a factor  Ω ..."

so collector elements will be designed using overstrength factor in strength design method.

1 hour ago, Waqas Haider said:

Q: WHAT IS A DIAPHRAGM CHORD AND DRAG MEMBER? WHAT THEY REPRESENT STRUCTURALLY IN A CONCRETE BUILDING?

A: diaphragm chord and drag members are not separate elements, they are part of the diaphragm. The components at the diaphragm boundary acting in tension and compression are known as tension chord and compression chord.

Drag members have 2 types, collectors and distributors. Both are part of the diaphragm, a collector is an element that takes distributed load from the diaphragms and delivers it to the vertical element, a distributor takes force from the vertical element and distributes it into the diaphragm. See attached images.

1 hour ago, Waqas Haider said:

Motion of the projecting wings in the same direction.
Motion of the projecting wings in opposing directions.

Q : KINDLY EXPLAIN ITS WORDING? 

A: Motion of projecting wings can be ignored if dynamic analysis is performed along with finite element analysis aka ETABS model.

 

1 hour ago, Waqas Haider said:

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.

A: If your stair case is not forming part of the lateral force resisting system and is not connected to diaphragm you can ignore this.

 

1 hour ago, Waqas Haider said:

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.

 

A: Please see the attached flowchart published by SK Ghosh, it is very helpful.

 

[Waqas Haider]

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? 

 

45 minutes ago, Saad Pervez said:

 Motion of projecting wings can be ignored if dynamic analysis is performed along with finite element analysis aka ETABS model.

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?

 

48 minutes ago, Saad Pervez said:

 If your stair case is not forming part of the lateral force resisting system and is not connected to diaphragm you can ignore this.

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?

[WR1]

Why would you like to stick to static analysis? Perform the simple dynamic analysis using MRSA.

Why use UBC when you can use ASCE (Dynamic base shear=85% of static as opposed to 100% in UBC for irregular structures). You save a lot here!

Why not use some exciting concepts like scaling base shear to static base shear based on Ta if T-dynamic < Ta.

Why not use a rigid diaphragm instead of a semi-rigid one. 

Try using 0% live load in seismic mass if it is not a ware-house/permanent storage.

Try using weight/mass modifiers for beam (beam-slab over-lap) to reduce the overwall W. Think how can u reduce W even further.

 

[Saad Pervez]

12 minutes ago, Waqas Haider said:

Can you please tell me from which book or study material are these excerpts of images?

flowchart is from a presentation given to Govt of Dubai by SK Ghosh, found it on Scribd. Other images are from NEHRP Seismic Design Technical Brief No 3 "Seismic Design of Cast-in-Place Concrete Diaphragms, Chords, and Collectors"

 

14 minutes ago, Waqas Haider said:

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?

In my opinion this requirement is specific to Seismic Zone 3&4, are you designing for Zone 3 or 4?

In my understanding this relates to structures having torsional irregularity where the wings might vibrate in different directions when eccentricity is amplified as per UBC clause 1630.7. Some collector elements will have more compression and less tension if torsion is amplified.

33 minutes ago, Waqas Haider said:

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?

According to the information provided your staircase is forming part of the lateral force resisting system. And again the one-third stress increase is Zone 3&4 specific requirement, and yes if you are using overstrength factor, then your allowable stress increases. If your structural system doesn't require using Omega then don't use it.

[Waqas Haider]

3 hours ago, Rana said:

Why would you like to stick to static analysis? Perform the simple dynamic analysis using MRSA.

Why use UBC when you can use ASCE (Dynamic base shear=85% of static as opposed to 100% in UBC for irregular structures). You save a lot here!

Why not use some exciting concepts like scaling base shear to static base shear based on Ta if T-dynamic < Ta.

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.

3 hours ago, Rana said:

Why not use a rigid diaphragm instead of a semi-rigid one.

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.

3 hours ago, Rana said:

Try using 0% live load in seismic mass if it is not a ware-house/permanent storage.

Try using weight/mass modifiers for beam (beam-slab over-lap) to reduce the overwall W. Think how can u reduce W even further.

For what purpose sir?

Edited March 9, 2017 by Waqas Haider

[Saad Pervez]

1 minute ago, Waqas Haider said:

For what purpose sir?

lesser W means lesser base shear,  reduced design forces. Life is easier when forces are less.