elevated mechanical platform SDC D 2

[Prestressed Guy]

I have been asked to design a small elevated mechanical platform in a SDC D seismic region. They would like to construct the platform with perimeter WF beams and WF columns with knee braces for lateral support. The Sds is pretty moderate at 0.62.
I am not sure as to how to classify this type of structure. I am confident that it will need to be per Table 13.5-1 but am undecided as to what to call it.

 

[bones206]

I would classify that as a non structural component (chapter 13) supported by a non building structure (Chapter 15). Chapter 15 has a criteria based on weight of component relative to total combined weight of structure + component. If the ratio is greater than 25% then it triggers certain analysis requirements.

Note that the 25% threshold is supposedly changing to 20% for ASCE 7-22, according to the committee task group chair whose presentation I attended this afternoon.

 

[Prestressed Guy]

This stand-alone platform is mounted to CIP footings at grade and separated from any structures. The intent is for the columns to be pin connected to the footings.

 

[bones206]

Understood. The platform itself should be designed as a “non-building structure similar to buildings” per Chapter 15 of ASCE 7.

 

[dik]

If you can add knee braces at the center columns it will help. To analyse this you likely need to do a computer run because of the knee braces so add a couple of more. Your guards, 3'6" high, need verticals at 4'max on centre to use the smallest HSS 1.66 dia x 0.125 (Canadian code). AISC in their Stair Special Publication has a means of checking your guard posts to stair stringer channel connection. You may need a toe plate. You need a footing at the foot of the stair in case soil movement, else a 6" or 8" slab. Just about designed. I maybe design a couple of these a month and guards and stairs a couple of times a week... but no seismic, here.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Prestressed Guy]

What I am stuck on is what is the structure type. I do not plan to make any of it moment frame. It needs to be a totally bolted together structure. The project owner would like use knee braces, but I do not see anything that fits that bill in table 15.4-2.
It is not a moment anything. It doesn't really come out as an eccentrically braced frame.
Would you call this an Elevated tank, vessel, bin or hopper on symmetrically braced legs (not similar to buildings) R=3
Would you call this an Elevated tank, vessel, bin or hopper on unbraced legs or asymmetrically braced legs (not similar to buildings) R=2
Or just go to
All other self-supporting structures, tanks, or vessels not covered above or by reference standards that are not similar to buildings. R=1.25 That seems a bit extreme given that it is down there with unreinforced masonry!
What should be the proper R, Omega-0 and Cd?

 

[HTURKAK]

This is a typical non-building structure on supporting structure similar to buildings.. Apparently ,weight of equipment relative to total combined weight of structure + component is greater than 25% and (15.3.2 Greater Than or Equal to 25% Combined Weight condition ) is applicable..

Mr BONES has provided a clear respond which deserves a BPS.

Regarding the use of knee braces , no need to stuck ..

Knee braces or similar which delivers a large unbalanced force into a column are not allowed in areas of
high seismic risk or for structures in SDC D or higher in accordance with AISC 341..

It would be better if you choose one of the suggested frame at (Table 15.4-1 Seismic Coefficients for Nonbuilding Structures Similar to Buildings ) rather than considering the client or MBA desires ..

 

[bones206]

Knee brace frames are typically designed as OMF. I believe you can design either per AISC 341 or AISC 360. Not the most seismically resilient setup but it's permitted by code.

The simple path for this application is to design as OMF per AISC 360 with R = 1.

 

[SJBombero]

See ASIC 341 Commentary E1.2 OMF Knee-Brace Systems. This would be the basis of design if you were to try to use R=2.5 per ASCE Ch. 15. Typical practice is to use R=1 and avoid having to design for full plastic moment and any other requirements of AISC 341. R=1 keeps you in the scope of AISC 360.