30ft metal building on top of 100ft structure 1

[AYPE]

I am designing a 30 ft high metal building on top of another steel structure that is 100 ft tall (by others),the site is located on SDC D, The baseplate will be pinned and bolted to a steel beams of the 100 ft structure, the purpose of the structure is to enclos machinery. According to Chapter 11, this structure can be designed under Chapter 15 for nonbuilding structures. However, there's an exception in Chapter 12 allowing the use of single-story OMF with unlimited height for this type of building. Additionally, Chapter 15 permits OMF with unlimited height but with seismic coefficients equal to 1.

Some questions I'm grappling with:

-Which chapter is more suitable for this structure?
-Does the upper portion qualify as a single story and therefore be designed according to Chapter 12?
-Designing it as a nonbuilding structure will result in a seismic load 3.5 times higher than designing it as a building structure (R=1 vs R=3.5). Does that seem accurate?
-This is my first time designing a structure with this scenario. Are there any special design considerations I should be aware of?

 

[JStephen]

Is the 100' supporting structure already designed? If so, how did that designer treat the situation?
The situation sounds a lot like that described in 15.3, but by reference to Table 15.4-2, that section is limited to the not-similar-to-building items.
Will seismic actually govern the design, or will seismic be dwarfed by wind loads anyway?

 

[HTURKAK]

Q-Which chapter is more suitable for this structure?
A-This is building type structure and be designed according to Chapter 12

Q-Does the upper portion qualify as a single story and therefore be designed according to Chapter 12?
A- NO! you can not qualify the upper portion as a single story bldg.
Q-Designing it as a nonbuilding structure will result in a seismic load 3.5 times higher than designing it as a building structure (R=1 vs R=3.5). Does that seem accurate?
A- The upper and lower portion shall be analyzed together to see the effects of new upper portion.
Q-This is my first time designing a structure with this scenario. Are there any special design considerations I should be aware of?
A-I would suggest you to contact the designer of lower portion , find the as built docs . and design calculations and probably would be better to ask services or consultancy of an experienced engineer.



Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[Deker]

Your structure is a mechanical penthouse and should be designed using Chapter 13 provisions for nonstructural components. If the weight of the penthouse is greater than 25% of the effective weight of the structure, you are permitted to design the penthouse using Chapter 15 provisions but additional requirements apply, such as modeling the combined structure. See Sections 13.1.1 and 15.3.2.

 

[AYPE]

Thank You for your input

Deker (Structural)8 Feb 24 18:35
Your structure is a mechanical penthouse and should be designed using Chapter 13 provisions for nonstructural components. If the weight of the penthouse is greater than 25% of the effective weight of the structure, you are permitted to design the penthouse using Chapter 15 provisions but additional requirements apply, such as modeling the combined structure. See Sections 13.1.1 and 15.3.2.

I'd like to provide further clarification. The upper portion of the structure comprises an 87ft wide steel moment frame (my company is PEMB and we design OMF only). Would it qualify as a nonstructural component or a nonbuilding structure Not similar to a building (which type will it be in Table 15.4-2)? Alternatively, should it be classified as a nonbuilding structure similar to a building with an Ordinary Moment Frame (OMF) and unlimited height (considering the total structure height 130ft >100ft) (as per Table 15.4-1)

Thank You!

 

[Deker]

Did you read Sections 13.1.1 and 15.3.2? Is your structure less or more than 25% of the effective seismic weight of the combined structure? You'll likely need coordinate with the EOR to figure this out. You might start by assuming it's less than 25% of the weight, design using Chapter 13 provisions using the ap and Rp values for Penthouses in Table 13.5-1, and come up with a seismic weight that you can send to the EOR to verify that it is in fact less than 25% than that of the overall structure. You'll need to use z/h = 1.0 in Equation 13.3-1.