We are carrying out design of an above/below ground FRP piping system and have some queries re the seismic design (which we are looking at for the above ground piping only).
We have the option of designing the piping to either BS7159 or B31.3. When we use B31.3 the seismic design to this code is fine.
However when we design to BS7159 the seismic design is proving a little more difficult. BS7159 mentions wind loads but does not appear to mention seismic loads, so we decided to use the same occasional load combinations as we would use in B31.3 as a starting point,ie sus+U1, sus+u2 etc. However doing this we have run into some problems.
In some areas of the FRP piping the code allowable stress is 30MPa and in others it is 60MPa (due to the properties of the FRP pipe). We have a node point with a code allowable stress of 60MPa and a sustained stress of 45MPa. However when we define the occasional seismic loadcase as sus+U1, we are getting an occasional code allowable stress of 39MPa (30+30% occasional load factor).
We then do not comply with the code at this point as the sustained load is 45MPa whereas the sus+U1 allowable is only 39 MPa. We are already exceeding the allowable stress with just the sustained component, before we even begin to consider the seismic loads.
Any body with any ideas? Any suggestions/advice would be greatly appreciated.
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Miss Itchy