kip

The kip is the unit American structural engineering actually thinks in. Open a set of steel calculations and the column schedule, beam reactions and pile capacities are in kips, while every stress is in ksi, kips per square inch: the 36 and 50 ksi yield strengths that define A36 and A992 steel are kip-built numbers. For scale, one kip is close to the weight of a concert grand piano, and it converts to 4.448 kN, so multiplying by four and a half moves a kip figure into SI within about one percent.

The kip's one public appearance is on the highway. The US federal limit on a single truck axle is 20 kips, and the AASHTO HS-20 design truck that American highway bridges are checked against puts 8 kips on its front axle and 32 kips on each of the two behind it. Those axle loads, walked across influence lines, are where a large share of all kip-denominated arithmetic actually happens.

On this site the kip feeds the structural end of the catalog: the beam bending, shear and column solvers accept force in any unit, and kips keep American inputs to three or four digits where pounds would need six. The kip-foot for bending moments and the kip per inch for stiffness in the neighbouring unit types are both built on it, and the ksi the stress side uses is this unit spread over a square inch.