ASCE 113 discrepancy between USD and ASD?

[Daniel Backstroem]

Hi fellow engineers,

ASCE 113, Substation structure design guide, allows design using either “Ultimate strength design” USD (otherwise commonly known as LRFD) and Allowable stress design, ASD.

When it comes to seismic design, I can’t seem to arrive to similar utilisations using these two approaches, which confuses me. I wonder if you guys could enlighten me.

A simple example. Let’s consider a ceramic post insulator, carrying some compact equipment. If I understand correctly, the seismic load part of the load combination becomes

USD: 1.1*D+1.25*E*I
ASD: 1.0*D+1.00*E*I

where D is dead weights, E is seismic action and I is the importance factor.

The seismic action can be taken as

E=Sa/R*W

assuming a unit static coefficient, which is reasonable for this type of structure. Here, Sa=Sds is 2/3 of the maximum 5% damped MCE spectral acceleration, R is the response modification factor and W is the structure weight.

ASCE 113 suggests using R=1.00 for USD design of post insulators, and R=1.3 for ASD, respectively. Hence, the load from the load combination becomes

USD: …1.25*Sa/1.0 times weight
ASD: …1.00*Sa/1.3 times weight

It follows that the USD seismic design load is >60% greater than the ASD design load.

But, the “strength resistance” of porcelain insulators is given in section 6.9.4 as

USD: 0.5*cantilever rating
ASD: 0.4*cantilever rating

Hence, the design load using USD seems to be 60% greater than that of ASD, but the strength allowance is only 25% greater?

Any guidance, explanations or comforting back-patting is greatly appreciated

Best,
Daniel

 

[HTURKAK]

Although Station Post Insulators are Porcelain or Composite , assume that the post is steel .. The allowable stress for steel is Fall =0.6Fy ..
When you repeat the calculation for steel post with the procedure described in the ASCE 113, Substation structure design guide, you will see that USD and ASD will give similar results..

-Most of the codes allow to increase the allowable stress 30% ( that is , adjustment of R =1.3 for ASD)

-Seismic Load Combination USD: 1.1*D+1.25*E*I

-Seismic Load Combination ASD: 1.0*D+1.00*E*I

- For seismic load USD /ASD = 1.25 and USD: 0.5*cantilever rating (The strength resistance factor ,0.4 is increased to get
similar results btw USD und ASD..










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