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Shearwalls generates wind C&C loads on building faces of each block according to the parameters entered in the Site Information dialog and the mean roof height of the block. These loads are used for the design of sheathing to resist bending and shear forces, and nail withdrawal.
The design methodologies used are ASCE Chapter 30, Part 1, 30.4 for buildings with mean roof height less than 60 feet, and ASCE Chapter 30 Part 3, 30.6 for buildings with mean roof height greater than 60 feet. These methods are applied on a block-by-block basis, so it is possible to have two methods applied to the same building.
Wall Loads Only
No component and cladding loads are generated on roofs, as Shearwalls does not design roof sheathing. For this reason, the two load magnitudes generated for C&C loads, for interior and edge zones, are sufficient, and there is no need to add a third for roof corners.
Tributary Area
For the computation of C&C loads for the purpose of designing sheathing and for nail withdrawal calculations, a tributary area A is chosen as less than 10 feet (3m). This is because the area tributary to each fastener does not exceed 2 square feet (.2 m), and the area tributary to each line of fasteners on a sheathing panel does not exceed 8 square feet (1sq m.). For our purposes, this distinction is not important, as in each case the coefficients derived from A are constant for A < 10 sq. ft..
Leeward vs Windward Loads.
Shearwalls calculates and displays both leeward (suction) and windward loads. However, for all surfaces in the ASCE C&C method, the leeward criterion is greater than the windward one for tributary widths < 10 feet, therefore the suction C&C loads govern for both nail withdrawal design and plywood bending strength, and it is the positive internal pressures that are considered for C&C loads.
Different Blocks along Same Building Face
Because different wall lines can have different reference heights h because they are on different blocks, but be on the same face of the building in terms of N, S, E, W directions, the program assigns separate C&C loads to each wall line
It is possible that one wall line can span two or more blocks, so loads from more than one building block are applied along different points of that line. The program uses the highest of these C&C loads to design walls along that line.
Note that the only difference between C&C loads are due to the exposure and gust factors, which in turn depend on the mean roof height. Typically there will be only a few percentage points difference between these factors on blocks with varying roof heights. Taking the worst of these along a line is a minor conservative approximation.