ASCE 7-02 2.3.2

[jak20]

I noticed none of the strength design combinations in section 2.3 have combined Fluid loads, F and Earthquake loads, E. The allowable stress combinations of section 2.4 do include these provisions (Load combinations 5 and 6). Does anyone no why this combination is not included in the strength design combinations. This would seem unconservative to me in the design of liquid storage tanks (where much of the design mass is from the liquid stored). Thank you in advance for any thoughts or guidance.

 

[JoeTank]

I wonder if it might be the fact that AWWA D100 and API-650 already have specific rules for seismic loads? Maybe building-type engineers prefer to let the tank specialists write the Codes and Standards for such matters. Since about 1980, both API and AWWA documents have contained the seismic provisions for tank design.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[jak20]

Thank you for your response Steve. My interest lied in the difference between the Allowable Stress and Strength provisions (I was using liquid storage tanks as an example). ASCE 7-02 Section 9.14 does include provisions for the seismic design of nonbuilding structures (tanks and vessels are included). This section provides the references you cited and many more. Most of the industry standards for seismic loads are based on older lower seismic force levels for tanks and vessels. ASCE 7-02 updates many of the provisions of the industry standards to the force levels of NEHRP 2000 (C9.14.7.3). Thank you for your reponse once again.

 

[JoeTank]

FYI... Tank codes are already based on plastic action of the tank bottom plate (for unanchored tanks). So in a way the force levels were already in some aspects of the seismic provisions. Shell buckling is normally governed by elastic buckling,;therefore, I doubt there is much room for the force levels of the new loads. Although the tank design formats keep changing to match the current thinking of UBC, ATC, NHRP and ASCE codes, the net change has probably been minimal. Of course the seismic map has changes and boundaries vary, but the design loads are similar.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com