More information: We are womdering about the economics of the proposal we had received from the building manufacturer which featurs a 3'6 uoset of the grade beam. This would essentially mean a 4'6, or so, momnt arm from the springline of the rigid frame to the thrust rods below the floor. Looks fairly pricey and a tad ugly.It appears to be an attempt to save money on the steel and spend money on concrete (piles and grade beam) anyone had experience with this.
Are you talking abouta grade beam, which is parallel with the ground line, or a plinth which is a vertical extension off a footing, the top supporting a column that may see both vertical and lateral loads?
I believe he is talking about a grade beam which has been cast as an upstand (i.e. it projects up above the slab rather than below).
A few things to think about.
1. how does the beam get below frost depth if it is an upstand?
2. As you alluded to, there is a horizontal reaction from the frame at the top of the beam which will cause a torsion in the grade beam, how is this taken out?
3. if the connection to the top of the grade beam is analysed as a pin then the portal columns will get shorter and the portal frame should be reanalysed with the increased EI/l of these columns. Under gravity loads these columns will attract more moment to the knee connections.
The grade beam does not extend below the frost line. The piles take care of that. The pile(s) cap is extends from the top of pile (s) to the top of grade beam (a dist of 4'6" +/-)and thus is integrated with the grade beam. The torsion in the grade beam could be taken out by using two piles (spaced at 3' c to c)perpendicular to the grade beam. The thrust bar would be extend across the building from pile cap to pile cap. The max M would occur at the top of the pile. One would obviously not want any excess deflection of the springline with resulting effect on the frame. we wrote to the building maufacturer (whose drawings show the upset) expressing concern but received no reply.
