It's seem that horizontal irregularities geometric stability is there but a cutouts necessitate to reduce moment at the centre and avoiding tilted. This is my humble opinion. I wish to hear from you
Alex Thomas MIE REng CEng M ASCE
Original Message:
Sent: 04-27-2023 03:08 PM
From: Bill Allen
Subject: ASCE 7 Horizontal irregularity #5 - Nonparallel system
If hand calculations are done such that the stiffnesses of the shear walls are considered as well as the location and direction of those stiffnesses AND the center of force along with the eccentricities of the load (both actual and minimum) are taken into account, what else must one do to "consider the 3D behavior of the system"? IMO, if the roof diaphragm were considered as semi-rigid, i.e., the stiffness of the diaphragm considered, then I would totally agree with a 3D computer model to solve the problem. Otherwise, I don't see how one can do anything more to "consider the 3D behavior of the system". Not being critical for you have much more knowledge and experience in this area than I do. I'm sincerely curious.
Thank you,
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Bill Allen P.E., S.E., M.ASCE
Principal
Allen Designs
Mission Viejo CA
Original Message:
Sent: 03-09-2023 12:08 PM
From: Ronald Hamburger
Subject: ASCE 7 Horizontal irregularity #5 - Nonparallel system
In my opinion, a 3D model is not required because this building has a flexible diaphragm. However, even if hand calculations and statics are used ot determine the forces on the walls, one has to consider the 3D behavior of the system.
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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
Original Message:
Sent: 03-08-2023 02:52 PM
From: Manhin Lee
Subject: ASCE 7 Horizontal irregularity #5 - Nonparallel system
Great! Agree on the applications of loads.
The follow-up question would be, is it required to be analyzed using 3D model per ASCE 7-16, 12.7.3?
12.7.3 states that for horizontal irregularity 1a, 1b, 4, and 5, a 3D model is required; the only exception is provided for a flexible diaphragm structure if it has horizontal irregularity 4 (out-of-plane offset).
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Manhin Lee Aff.M.ASCE
Plan Check Professional
City of Fremont
Fremont CA
Original Message:
Sent: 03-08-2023 02:19 PM
From: Ronald Hamburger
Subject: ASCE 7 Horizontal irregularity #5 - Nonparallel system
The structure you show in plan would be considered by most engineers to have horiozntal irregularity type 5. It should be analyzed and deisnged for 100% of the forces in Direction X combined with 30% of the forces in Direction Y, and 100% of the forces in Direction Y combined with 30% of the forces in Direction X.
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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
Original Message:
Sent: 03-08-2023 12:46 PM
From: Manhin Lee
Subject: ASCE 7 Horizontal irregularity #5 - Nonparallel system
Hello fellow engineers!
I would like to get an interpretation on how to define Horizontal irregularity #5 - Nonparallel system per ASCE 7-16, Table 12.3-1.
Per the table, "Nonparallel system irregularity is defined to exist where vertical lateral force-resisting elements are not parallel to the major orthogonal axes of the seismic force-resisting system."
My question is: Which part of the vertical lateral force-resisting elements? Does it have to be the entire element on one side, or any part of the system will trigger this nonparallel system irregularity?
Please take a look at the floor plan below for a concrete tilt-up warehouse with flexible roof diaphragm. On the south side, about half of the concrete shearwall is parallel to the major orthogonal axis, the other half is skewed and not parallel.
Would you define this as nonparallel?
If nonparallel system exists here, then Section 12.5.3 and 12.7.3 are applicable.
12.5.3 tells you how the structure shall be loaded.
12.7.3 tells you how the structure shall be analyzed and modeled. For irregularity #5, a 3D model is required; the only exception is provided for a flexible diaphragm structure if it has horizontal irregularity #4.
As computer modeling becomes widely available, I do not see an issue for analyzing this kind of building using a 3D model to get a more realistic results, and I think it's a great way to learn how the building would behave in different kinds of irregularities.
Thanks!
#ASCE7-16
#Seismic
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Manhin Lee Aff.M.ASCE
Plan Check Professional
City of Fremont
Fremont CA
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