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Seismic Sanity Check - Washington State

Seismic Sanity Check - Washington State

Seismic Sanity Check - Washington State

(OP)
I am working an alteration to a steel tower (non building structure) that was originally built in 1988 per the 1988 UBC. Using the 2012 IBC the members pass the typical load combinations for a SCBF, but the IBC 2012 & AISC 341 put additional requirements for member and connection strength. With these additional requirements the connections aren't even close. Am I off base here? What are my options?


RE: Seismic Sanity Check - Washington State

If this is a cell or radio tower, it will fall under the design guidelines of TIA-222G, as well as the IBC.

Mike McCann, PE, SE (WA)


RE: Seismic Sanity Check - Washington State

What type of alteration? Was the building original designed as non-building? What was the Rw? Likely the tower was not designed as SCBF, Rw=8, and using an Rw=5 or lower. Check the tower as OCBF.

RE: Seismic Sanity Check - Washington State

And chances are that wind, or wind + icing will control, not seismic, although you will have to run through the seismic calculations.

Mike McCann, PE, SE (WA)


RE: Seismic Sanity Check - Washington State

What kind of steel tower is this? Monopole, lattice, guyed?

Mike McCann, PE, SE (WA)


RE: Seismic Sanity Check - Washington State

(OP)
The tower supports a scrubber at a power plant, is over 200 feet tall and has several levels that are enclosed. The alterations will be the relocation of one of the upper braces and some additional weight from a new duct. The original documents do not specify Rw. Due to the height it cannot be considered an OCBF unless an R of 1.5 for non building structures is used. Using an R of 1.5 creates massive overstresses of 80% in the footings and members. There does not seem to any good way to comply with the connection requirement of RyFyAg for the braces.

The tower is 40'-0x40'-0"x210'-0" braced frame on four sides.

RE: Seismic Sanity Check - Washington State

Just out of curiosity what is requiring you to update it to the new IBC?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com

RE: Seismic Sanity Check - Washington State

(OP)
The reasons for using the IBC 2012 are alterations to the framing and the additional duct load that is to supported.

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