Loads and load paths in a building

Every load in a building must travel, member by member, down to the soil. This route is the load path. A floor tile's weight goes into the slab, the slab bends and passes it to beams, beams carry it to columns, columns to footings, and footings spread it into the ground. A designer who can trace that path for every type of load rarely makes a fundamental error.

Loads are grouped by nature. Dead loads are the self-weight of the structure and permanent finishes, partitions and services; they are calculated from dimensions and unit weights. Imposed (live) loads depend on use: a residential floor, an office, a library store or an assembly hall each has a code value per square metre. Wind pressure depends on location, height, terrain and building shape. Earthquake forces come from the building's mass and stiffness and the seismic zone. Snow, temperature, soil and water pressure, and construction loads complete the list.

Because these loads do not all peak at the same time, codes specify combinations with partial safety factors, larger for less certain loads. Lateral loads need their own path: diaphragm action of slabs carries wind or seismic force to frames, shear walls or braced bays and then to the foundations. Weak links in the lateral path, such as a soft storey with open parking, floating columns or missing ties, are among the main causes of collapse in earthquakes.