Framing Systems

• Layered Framing: In bridges and buildings, members supporting a surface are commonly arranged in layers where each layer spans longer and is spaced wider than the layer it supports, and is oriented perpendicular to that layer.

Tributary width

• Assume load distributes to nearest support: The tributary width assumption says that a member transfers a distributed load to a each support by dividing the load into areas bounded by the halfway points between supports.
• Approximate!: This assumption is an approximation that gives good results for simple spans and several roughly equal spans, but incorrect results in several cases, including highly unequal spans for continuous beams, and large overhangs.

Load Evaluation

• Live and environmental loads commonly modelled as uniform pressure: Building codes typically express live loads and environmental loads (e.g. wind and snow) as pressures distributed uniformly over an area.
• So, approximate dead loads as uniform pressure too: It is convenient to express dead loads in this form, even if the loads are not precisely uniform.
• Load "smearing":This is done by load "smearing" using the following steps:
  • Identify a repeating construction module.
  • Calculate the weight of the module.
  • Divide by the weight of the module by the size of the module. (size may be an area or a length depending on the nature of the module and the way the load will be used).