Gravity Load Distribution
Architects must consider not only what loading conditions might effect a structure, but also how those loads are transfered from their point of origin through the structure to the ground. Architectural design decisions concerning the structural framing system will dictate this flow of forces. The most direct path from the point of loading to the ground will create the most efficient structure. The continuity of this line of transfer is essential. However, this direct path is not always possible. As a matter of fact, due to the nature of live loads this path is constantly changing!
The arrows indicate the load path. >>>Under re-construction<<<
The load at the midspan of the joist of the frame is illustrated as a large arrow pointing down. This load is split into two parts, with a proportional part going to each end of the beam. This in turn becomes a point load at the midspan of the smaller beams. They transfer the load to the earth. The load on each column in this case is only one quarter of the initial load. This is one method of distributing a load.
The second drawing illustrates a wooden floor of thick planks that span between two beams. Each of the four squares of the flat surfaces is the tributary area of the beam below it. A tributary area of a structural element (such as a joist, beam, column, or wall) is the area that contributes to the loading of that specific element. In this case, the plans transfer their loads and pass them on to the two beams. These beams can only transfer their loads equally to the two columns which support them. Thus, the tributary area can be drawn by determining the supporting characteristics of the horizontally spanning members.
This is again illustrated below for a simple frame structure. The loads are gathered by each structural element and passed on to that element's supporting elements. These supports in turn pass their loads to the next supporting element until the original load has been transferred all the way to the earth.
The load distribution pattern and the explicit summation of the loads can have a direct effect upon the size of the elements. Load distribution often causes unequal loading of the vertical supporting members. This may or may not be indicated by the designed form of that element.
WIND LOAD DISTRIBUTION
The essence of wind load distribution through a building from one building component to another, and how the building resists this load can be investigated by assuming a uniformly distributed wind load acting on any one face of the building at any one time. Normally a wind load design requires a separate analysis of wind from two perpendicular directions, such as wind from the north or south and then from the east or west.
Consider a one story, flat roofed, rectangular, wood framed building without a parapet. The wall receiving the direct pressure from the wind distributes the top half of its horizontal wind load to the roof and the bottom half to the foundation; it acts as a vertical beam with the roof and foundation acting as simple horizontal supports. The part of the load going to the foundation is distributed from the foundation to the ground. The portion of the load going to the roof tends to cause the roof to move laterally; this lateral movement is resisted by the end walls. The movement of the end walls is prevented by their connection to the foundation.
Since winds of 80 to 110 mph will create pressure on the side walls of buildings in the range of 15 to 30 psf, design forces of the wind will be considered between 20 or 25 psf (static load) in this course.
