SPECIFICATION FOR THE DESIGN OF - Transcon Steel

Commentary on the Prescriptive Method for One and Two Family Dwellings - 2004 35
A6.1 Vertical Load Path
Figures A6.1-1 and A6.1-2 illustrate vertically oriented loads created, respectively, by
gravity and wind uplift. It should be noted that the wind uplift load originates on the roof
from suction forces that act perpendicular to the exterior surface of the roof as well as from
internal pressure acting perpendicular to the interior surface of the roof-ceiling assembly in
an outward direction. In addition, overturning forces resulting from lateral wind or seismic
forces create vertical uplift loads (not shown in Figure A6.1-2). In fact, a separate analysis of
the lateral load path usually addresses overturning forces, necessitating separate overturning
connections for buildings located in high-hazard wind or seismic areas. It is feasible to
combine these vertical forces and design a simple load path to accommodate wind uplift and
overturning forces simultaneously.
A6.2 Lateral Load Path
The overall system that provides lateral resistance and stability to a building is known as
the lateral force resisting system (LFRS). In cold-formed steel-framed construction, the LFRS
includes shear walls and horizontal diaphragms. Shear walls are walls that are typically
braced or clad with structural sheathing panels to resist racking forces. Horizontal
diaphragms are floor and roof assemblies that are also usually clad with structural sheathing
panels. Though more complicated and difficult to visualize, the lateral forces imposed on a
building from wind or seismic action also follow a load path that distributes and transfers
shear and overturning forces from lateral loads. The lateral loads of primary interest are
those resulting from (1) the horizontal component of wind pressures on the building’s
exterior surface area and (2) the inertial response of a building’s mass and structural system
to seismic ground motions.
As seen in Figure A6.2-1, the lateral load path in steel-framed construction involves
entire structural assemblies (i.e., walls, floors, and roofs) and their interconnections. The
distribution of loads in Figure A6.2-1 three-dimensional load path depends on the relative
stiffness of the various components, connections, and assemblies that comprise the LFRS.