Live Loads, L or Lr
- Live Loads, L:
- Short duration forces which change in location and magnitude.
- Include people and furniture.
- Based upon intended use of building = occupancy.
- Tabulated in UBC Table 16-A:
- floor uniform loads are a function of use/occupancy.
- for examples, the uniform floor load for:
- residential = 40 psf
- offices = 50 psf
- light storage = 125 psf
- Roof Live Loads, Lr:
- UBC recognizes that roofs carry lower loads than floors, since roofs are generally not occupied.
- UBC specified roof loads account for miscellaneous loads like roofing, equipment, servicing.
- Obtain minimum unit roof loads from 1994 or 1997 UBC Table 16-C:
- These live loads are assumed to act vertically over the area projected onto a horizontal plane.
- They are a function of roof slope and tributary load area with smaller unit loads for steeper members and large tributary
areas:
- Flat roofs: higher probability that high unit live loads could occur.
- Tributary area of member under consideration:
- "If a member has a small t.a., it is likely that a fairly high unit live load could be imposed over that entire small surface area... It is less likely that a large t.a. will be uniformly loaded by the same high unit load considered for a small t.a. member." (Design of Wood Structures by D. Breyer)
- Procedure: (see Breyer, Example 2.3)
- Calculate roof slope (generally the same as the member slope)
- Calculate t.a. of member.
- Choose Method 1 or Method 2 to obtain unit load.
- Method 1 is straightforward, but incremental.
- Method 2 provides a continuous range of loads, but requires the checking of two equations which can be found in '94 UBC 1606 or '97 UBC 1607.5.
- Apply the unit load to the horizontal plane. See upcoming example under snow loads for an explanation of horizontal plane.
