If you are truly concerned about the building's performance, then use ASCE 41 and upgrade the entire building to conform to appropriate performance objectives. Otherwise, just use ASCE 7 and conform to its requirements.
Original Message:
Sent: 07-24-2023 03:09 AM
From: Almontasir Mohammed
Subject: An addition to existing RCC structure which was designed for seismic-resistance force as building frame system (Ordinary shear wall) & the addition as per the code require the addition to be special moment frame system
Ronald, thanks for your feedback.
For your point no.1: The use or selection of R is relevant to the seismic design category which is classified as "D" for the addition & this requires to comply with Table 12.2-1 ASCE for the selection of seismic force-resisting system which also will end up with using special seismic force-resisting system accordingly higher R & here is our concern regarding the performance of the overall building seismic force-resisting system with different ductility behavior between the old existing part & the new addition.
For Point no.2: I am trying to avoid upgrading the entire existing structure due to a small addition
------------------------------
Almontasir Salah
Se.Structural Engineer
EHAF Consulting Engineers
UAE-Abu Dhabi
Original Message:
Sent: 07-20-2023 01:39 PM
From: Ronald Hamburger
Subject: An addition to existing RCC structure which was designed for seismic-resistance force as building frame system (Ordinary shear wall) & the addition as per the code require the addition to be special moment frame system
Almontasir - You are correct that under ASCE 7-16, upgrade of the entire structure would not be required, assuming that the new addition conforms to the applicable requirements of ASCE 7 for new construction and that the demand on existing elements of the structure does not increase by more than 10%, unless the elements have the capacity to resist the increased forces. Note that in the U.S., local building codes often adopt requirements for additions to existing buildings that are different from those in ASCE 7, in which case the local building code requirements would apply. Because of this, in the 2022 edition of ASCE-7, Appendix 11B was deleted.
My primary concern with placing a new special moment frame on top of the existing shear wall is that the design forces for the special moment frame would be substantially less than the stucture is capable of transmitting to the new strucutre, which could result in performance issues. You could "manage this" in several ways:
1- Use an R factcor for the new addition that matches the equivalent R factor used in the design of the original structure (but not less than that specified by ASCE 7-16, presently. This would give sufficient strength to the addition to delay onset of premature damage, relative to the balance of the structure.
2- Rather than using ASCE 7 for design, use a performance-based approach, such as ASCE 41 to assure that the addition will perform as anticiapted under the building code. Note that you may find the existing building is inadequate under ASCE 41. In that case, it would be responsible to upgrade the existing structure.
------------------------------
Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
Original Message:
Sent: 07-20-2023 10:32 AM
From: Almontasir Mohammed
Subject: An addition to existing RCC structure which was designed for seismic-resistance force as building frame system (Ordinary shear wall) & the addition as per the code require the addition to be special moment frame system
Dear valued ASCE team & members ,
Kindly we need your kind technical advice on the following case of study:
The case is an existing building located in Abu Dhabi-UAE which was designed as building frame system (ordinary shear wall system) & as per old design code seismic parameters, currently there is new addition needs to be added above the existing building.
According to the updated seismic maps & parameters in Abu Dhabi this new addition part is classified as a special moment frame (IV SDC "D") (occupancy category HOSPITAL).
Below sketches for easy understanding the case:
According to ASCE 7-16 & after my investigation on the increased seismic force due to the addition I found this case is falling under the exception of that if the addition is not increasing the seismic force of the existing building by more than 10% accordingly, the existing structure does not need to meet the seismic force-resistance requirements for the new structures & therefore no need to retrofit the existing structure lateral resisting system.
The following general statement mentioned in APPENDIX 11B.3 ASCE7-16
- The addition shall comply with the requirement of the new code parameters "design as special moment frame"
- I think that we have to match the existing building ductility behavior (ordinary) for the new addition & not follow the new parameters & that's to achieve the compatibility and homogenous behavior between the existing and the new addition avoiding making strong addition above the weak existing structure which was never designed to meet SDC "D" (see the above sketch-compatibility system)
- If I follow the code regarding the addition part & make the system of the new added floor as special moment frame the doubt will be on the workability of the full seismic system of the building with different ductility behavior specifically how the special moment frame (for the addition) above the existing will behave joining the ordinary supporting system. (see the above sketch- ductile system supported on weak system)
I appreciate your help and kind technical advice for this particular case.
Almontasir Salah - Sr.Structural Engineer
Member I.D.# 11128356
Regards,
#Seismic
------------------------------
Almontasir Mohammed Aff.M.ASCE
Sr.Structural Engineer
EHAF Consulting Engineers
Abu Dahbi
------------------------------