FRAMING (FORCES AND STRESSES)

FORCES AND STRESSES

Remember your high-school physics and Newton’s laws, which are actually connected to rules God uses to govern the motion of planets. You learned that forces act on a straight line and that for any object at rest, the forces acting on it are met with equal but opposite forces. On objects at rest, or in equilibrium, “For every action there is an equal but opposite reaction”. We expect our buildings to be “at rest” and not collapsing!

Forces in buildings come from Loads that cause Internal Stress within individual structural members that are resisting the Loads. The usual forces (stresses) concerning us are:

1) Moment (bending),
2) Compression (pushing)
3) Shear (sliding or cutting),
4) Tension (pulling),

(There are other stresses but they are generally not of concern in buildings.)

Force is the amount of push, bend, pull, etc. applied to and resisted by a structural member. We usually measure force in pounds per square inch (psi) for compression, tension, and shear; and inch-pounds or foot-pounds for bending.

LOADS

Loads are from building materials, people, stuff, and nature; e.g. shingles, beams, folks, pianos, pews, and wind, snow, or earthquakes. We classify loads in several ways.

By duration:

* Dead Load (DL) is permanent or long-term, such as the structure itself and what it continuously holds up, e.g. the roof, ceiling, walls, etc. (Somewhat like original sin.)

* Live Load (LL) is short-term, intermittent and re-occurring such as snow, wind, or people. (Somewhat like actual sin.)

* Impact is very short term shock loading, such as when a piano is dropped. Not usually considered in buildings except in seismic design. (Unlike steel and concrete, wood has a remarkable capacity to absorb short-term loading.)

By distribution:

* Concentrated Loads act in narrowly defined areas, e.g. a safe, or a beam bearing on a pier. Measured in pounds.

* Distributed Loads are either actually spread evenly over a surface, e.g. snow, wind, or carpet; or assumed to be spread evenly over a surface, e.g. people. Measured in pounds per square foot (psf) of surface, or pounds per linear foot (plf) along a beam supporting the surface.

By how they act on structural members:

* Axial Loads act along and parallel with the long axis (dimension) of the member, and produce compression or tension forces.







* Transverse Loads act at 90 degrees (“normal” or perpendicular) to the member’s long axis, and produce shear and also bending, which is actually comprised of tension and compression stresses.






Loads operate in a straight line (obvious but often un-recognized); and forces resisting them also operate in a straight line. All loads are supported by continuous paths of resistance from the building foundation to the point of application. Be aware of paths of load resistance. Identify and examine load paths to clarify critical members and joints, and to locate questionable situations. (A column should not bear in the middle of a floor with no underlying pier, for example.)

Any individual force can be “resolved” (i.e. broken down) into two or more components acting along different force lines through the same point. For example, a brace leaning against a wall will have vertical and horizontal forces acting against its weight, at each end of the brace.

Building codes specify the applicable design loads for floors, roofs, wind, etc. for each geographic area. Some typical live floor loads are 100 psf for hallways, and 40 psf for classrooms. Roof live loads vary geographically and are determined by weather (wind and snow) and seismic forces. Roof dead loads are typically 15 psf to 20 psf. Lumber weighs between 3 and 4 pounds per board-foot, depending on species.