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G.15.3 Area, One-way, and Floor Loads

Often a floor is subjected to a uniform pressure. It could require a lot of work to calculate the equivalent member load for individual members in that floor. However, with the AREA, ONEWAY or FLOOR LOAD facilities, you can specify the pressure (load per unit square area). The program will calculate the tributary area for these members and calculate the appropriate member loads. The Area Load and Oneway load are used for one way distribution and the Floor Load is used for two way distribution.

The following assumptions are made while transferring the area/floor load to member load:

  1. The member load is assumed to be a linearly varying load for which the start and the end values may be of different magnitude.

  2. Tributary area of a member with an area load is calculated based on half the spacing to the nearest approximately parallel members on both sides. If the spacing is more than or equal to the length of the member, the area load will be ignored. Oneway load does not have this limitation.

  3. These loading types should not be specified on members declared as MEMBER CABLE, MEMBER TRUSS, MEMBER TENSION, MEMBER COMPRESSION, or CURVED.

Note: Floor Loads and One-way Loads can be reduced when included in a load case defined as "Reducible" according to the UBC/IBC rules.

An example:

Example floor structure with area load specification of 0.1

Member 1 will have a linear load of 0.3 at one end and 0.2 at the other end. Members 2 and 4 will have a uniform load of 0.5 over the full length. Member 3 will have a linear load of 0.45 and 0.55 at respective ends. Member 5 will have a uniform load of 0.25. The rest of the members, 6 through 13, will have no contributory area load since the nearest parallel members are more than each of the member lengths apart. However, the reactions from the members to the girder will be considered.

Only member loads are generated from the Area, Oneway, and Floor load input. Thus, load types specific to plates, solids or surface are not generated. That is because, the basic assumption is that, a floor load or area load is used in situations where the basic entity (plate, solid or surface) which acts as the medium for application of that load, is not part of the structural model.

The Oneway load is intended to be used in areas with relatively large aspect rations between adjacent sides. It should not be used on members with square tributary areas unless the TOWARDS option is used, which then specifies to which of the two equal directions the load should be applied. Otherwise, the Floor Load type should be used.

Note: Failure to specify a TOWARD side on a Oneway load applied to a square tributary area will likely result in lost load or unintended load path changes.