Non-building Structure Supported by Other Structures

[Nick6781]

Hi Everyone,

I’m currently evaluating an existing concrete roof due to modifications made to the roof’s mechanical supporting structure.

I’ve outlined my thought process and questions in the sketch below. Feel free to share any comments or suggestions. Any help would be greatly appreciated.
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[HTURKAK]

Two questions,
- is the rooftop structure ( 20 ft tall steel braced str. ) framed by extension of the building frame ?
- what is your plan for calculation of Hf ( Factor for force amplification as a function of height) ,
which expression are you planning to use (13.3-4) or (13.3-5)?

 

[Nick6781]

Two questions,
- is the rooftop structure ( 20 ft tall steel braced str. ) framed by extension of the building frame ?
- what is your plan for calculation of Hf ( Factor for force amplification as a function of height) ,
which expression are you planning to use (13.3-4) or (13.3-5)?

The rooftop structure is sitting on the roof slab (12" concrete flat plate) and is not an extension of the building frame.

ASCE 7-16 applies to this project, so I only need to refer to Equation 13.3-1.

After reviewing Section 15.3 again, the first paragraph states that only nonbuilding structures identified in Table 15.4-2 (nonbuilding structures that are not similar to buildings) are applicable to the section. Since a braced frame is listed in Table 15.4-1, Section 15.3 does not apply.

Section 13.5.11 of ASCE 7-22 provides some clarification. It states that penthouses and rooftop structures can be designed in accordance with the forces specified in Section 13.3.1. However, I cannot find a similar section in ASCE 7-16.

 

[HTURKAK]

However, I cannot find a similar section in ASCE 7-16.

AFAIK In ASCE 7-16 and earlier editions , penthouses were designed to the requirements of Chapter 13 as a nonstructural component, without any restrictions on their lateral force-resisting system.

Section 13.5.11 of ASCE 7-22 provides some clarification

Pls look C13.5.11 Penthouses and Rooftop Structures. The relevant clarification ; (....... For ASCE 7-22, the requirements for rooftop structures have been revised. The seismic design force for rooftop structures is now dependent on the lateral force resisting system selected for the rooftop structure. Force-resisting
systems may be selected from Chapter 12 or 15, and are subject to the system limitations and detailing requirements for the system selected. The seismic design force is determined in Section 13.3, using design coefficients obtained from Table 13.6-1.....)

Good Luck..