TelfordWannabe
Structural
- Mar 14, 2014
- 2
Some thoughts (right or wrong) on resisting combined shear and tension in holding down bolts...
I am aware that it is not recommended to take out shear and tension forces in a column baseplate via the holding down bolts only, the preference being to recess a pocket in the foundation or by providing a shear key.
Looking at guidance in BS 5950 for steel bolts with combined shear and tension, I find that bolts are capable of taking high shear and tensile forces (taking account of reduced shear due to packing i.e. grout) adopting the equation Ft/Pt + Fv/Pv < 1.4.
The problem seems to be in achieving sufficient bearing capacity in the concrete surrounding the bolts due to the lateral force. I have seen recommendations referring to Concrete Society Publication TR34 with respect to designing dowels in slabs in order to best assess the resulting bearing stress capacity, with a maximum bearing length of 8 x bolt diameter. The key phrase here is maximum, with some software packages using significantly less than this, maybe because of the differences in stiffness between a 150mm thick slab and a 750mm wide pile cap in allowing the bolt to engage with the concrete over any significant length. In addition, this philosophy would suggect that by utilising the bolt cone insert (assuming filled with high strength grout) would increase the area of bearing on the foundation concrete.
Additionally, how is even distribution of lateral force in every bolt justified, even if welding of washer plates is adopted?
Thanks in advance.
I am aware that it is not recommended to take out shear and tension forces in a column baseplate via the holding down bolts only, the preference being to recess a pocket in the foundation or by providing a shear key.
Looking at guidance in BS 5950 for steel bolts with combined shear and tension, I find that bolts are capable of taking high shear and tensile forces (taking account of reduced shear due to packing i.e. grout) adopting the equation Ft/Pt + Fv/Pv < 1.4.
The problem seems to be in achieving sufficient bearing capacity in the concrete surrounding the bolts due to the lateral force. I have seen recommendations referring to Concrete Society Publication TR34 with respect to designing dowels in slabs in order to best assess the resulting bearing stress capacity, with a maximum bearing length of 8 x bolt diameter. The key phrase here is maximum, with some software packages using significantly less than this, maybe because of the differences in stiffness between a 150mm thick slab and a 750mm wide pile cap in allowing the bolt to engage with the concrete over any significant length. In addition, this philosophy would suggect that by utilising the bolt cone insert (assuming filled with high strength grout) would increase the area of bearing on the foundation concrete.
Additionally, how is even distribution of lateral force in every bolt justified, even if welding of washer plates is adopted?
Thanks in advance.
