Can someone please confirm or provide directions for using unit weights for structural elements with zero thickness in the model? 1D and 2D. Let us assume the following:
1. Vertical Concrete embedded beam in soil (say 24 inches in diameter): use 150 pcf - 120pcf = 30pcf? To account for the fact that there is soil in the place of the actual beam and you don't want to double count.
2. Vertical Concrete Diaphragm wall (say 2 to 3 feet thick): Same as above? 30pcf?
3. Horizontal Concrete tunnel station roof (say 12 inches thick): 30pcf?
4. Steel, Thin Tunnel liner (say 0.5 inches): 490pcf - 120pcf = 370 pcf? I feel 490 is more appropriate.
Dear Kostis,
The concept is that in a zero thickness element we only account for the weight difference between soil and structural element to, as you say, not double count.
It's not a perfect solution though, because by using only hte weight difference the structural force due to self weight are not correct since the weight in the plate element is only the weight difference and not the real weight. For for instance piles this is not a problem as the load is usually far higher than the axial force due to self weight. But for for instance a thick raft foundation the weight of the raft could have significant influence on the bending moments for instance. On the other hand, giving the plate the full raft weight may influence the bearing capacity....
Furthermore one could reason that if one side of the structural element is going to be excavated, only have the weight will be double counted.
So in case of the concrete tunnel station roof you could choose to use 150 pcf - 0.5*120pcf = 90pcf
Case 4 is not different from the other 3 as that 0.5 inch width is soil in the model where in reality it's tunnel liner... So the choice to use the full weight, half compensation of full compensation for the soil weight is in principle not different.
Basic conclusion is .... it's not so straightforward.
With kind regards,
Dennis Waterman
Answer Verified By: Kostis Syngros
Thank you Dennis!