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Dead load components are derived from user-input dead loads on wall lines at the same building level as the hold-down location. These dead loads are to be manually calculated from distribution of area loads from floors and roofs to line loads on walls and floors.
They arrive at the hold-down location by three different routes:
- The resisting moment due to dead loads over the sheathing segment adjacent to the hold-down location
- The dead load over top of the adjacent opening is distributed to the opening support at the hold-down location
- The dead load component from hold-down forces on the level above, if they are at the exact location as the force on the level below.
These contributions are added to create the dead load component at a hold-down location.
Dead load components are not calculated if there is no wind or seismic shear overturning component at a location, as there is no need for a hold-down at these locations.
Load Combination Factor
The calculation of the dead load component of tension hold-down forces uses the counteracting load combination 7 from ASCE 2.4.1 for wind design, and combination 10 from 2.4.5 for seismic design, both with a factor of 0.6 D. The calculation of compression forces at the other end of the segment uses load combinations 5 and 8, respectively, with a factor of 1.0 D.
Building Masses vs Dead Loads
IMPORTANT: The generation of building masses for seismic load generation does not affect the value of dead loads on shear lines for hold-down force calculations. Because Shearwalls does not at present have a gravity load distribution system to determine which elements support the building mass of the floors, it is necessary to input the dead loads directly on the wall lines as line loads resulting from the area and loads on roofs and floors.