FEM vs Empirical PTI Methods

[akastud]

All,

First time facing this issue. Usually FEM analysis yeilds a thinner slab in practice for us so this issue hasn't arisen. We recently designed a PT SOG uniform thickness slab for a large apartment complex (6-8 units per building). The soils have large edgelift conditions andcomplex geometry. We designed the slab using a FEM program and came up with 14" thick slabs that barely work. The owner has hired another engineer for a second opinion and he (using the empirical PTI method) comes up with 10" slabs. I understood the PTI method to be conservative compared to FEM analysis. I do not know how to justify our thicker slab except to say that we have designed to the exact site conditions while the other engineer is using less exact solutions. Have any of you had this happen? What did you do or recomend doing?

akastud

 

[woodman88]

Sounds strange, on most of my three to four stories wood framed apartments I did, we used PT SOG per the PTI method. When one of the principals of the company had some questions, we did a few FEM spot checks and except for some minor reinforcing changes they were very similar. Then one of Building Departments would not accept the PTI method so we ran a full FEM analysis again with minor changes in the reinforcing. It was than that the company decided to go with only a FEM analysis for the PT SOG designs, but it turn out that the program we were using was very difficult to check alternate slab depths/tendon spacing on. So we did a PTI method for the depth and tendon spacing for input into the FEM program. The only time we had a problem was when there was a input error of the soil parameters in the FEM analysis. So I would agree that the PTI method is typically more conservative than a FEM anaylisis.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.