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How Loads Flow Through a Building?

Multiple elements are used to transmit and resist external loads within a building. These elements define the mechanism of load transfer in a building known as the load path. The load path extends from the roof through each structural element to the foundation. An understanding of the critical importance of a complete load path is essential for everyone involved in building design and construction.

The load path can be identified by considering the elements in the building that contribute to resisting the load and by observing how they transmit the load to the next clement. Depending on the type of load to be transferred, there are two basic load paths:
  • gravity load path 
  • lateral load path
Both the gravity and lateral load paths utilize a combination of horizontal and vertical structural components, as explained below.

1. Gravity Load Path
Gravity load is the vertical load acting on a building structure, including dead load and live load due to occupancy or snow. Gravity load on the floor and roof slabs is transferred to the columns or walls, down to the foundations, and then to the supporting soil beneath. Figure 1 shows an isometric view of a concrete structure and a gravity load path.
Figure :1 An isometric view of a concrete structure showing a gravity load path.
Figure :1 An isometric view of a concrete structure showing a gravity load path.
The vertical gravity load acts on a slab (1), which transfers the load to the beams (2), which in turn transfer the load to the columns (3) and then down to the foundations (4). The gravity load path depends on the type of floor slab, that is, whether a slab is a one way or a two-way system. In the one-way system in Figure 2a, the effect of external loads is transferred primarily in one direction, shown with an arrow. The slab-beam and-girder floor is an example of a one-way system. The gravity load acting on this system is transferred from the slab (1) to the beams (2) and then to the girders (3). Finally, the girders transfer the load to the columns (4).

The load path in a two-way system is not as clearly defined. The slab transfers gravity load in two perpendicular directions; however, the amount carried in each direction depends on the ratio of span lengths in the two directions, the type of end supports, and other factors. For example, in the slab with beams system shown in Figure 2b, the load is transferred from the slab (l) to the beams aligned in the two directions (2) and then to the columns (3).
Figure :2 Gravity load path in a floor slab: a) one-way system; b) two-way system.
Figure :2 Gravity load path in a floor slab: a) one-way system; b) two-way system.
2. Lateral Load Path
The lateral load path is the way lateral loads (mainly due to wind and earthquakes) are transferred through a building. The primary elements of a lateral load path are as follows:,
  • vertical components: shear walls and frames;
  • horizontal components: roof, floors, and foundations.
Figure :3 Lateral Load Path
Figure :3 Lateral Load Path
Figure 3 shows a reinforced concrete structure and the elements constituting the lateral load path: roof and floor systems (I) transfer the load to the walls (2), which in turn transfer the load to the foundations (3). Roof and floor systems (also called diaphragms) take horizontal forces from the storeys at or above their level and transfer them to walls or frames in the storey immediately below.

Shear walls and frames are the primary lateral-load resisting elements; however, these members also carry gravity loads. Shear walls receive lateral forces from diaphragms and transmit them to the foundations. Foundations form the final link in the load path by collecting the lateral forces from all storeys and transmitting them to the ground.

Tributary Area

The tributary area is related to the load path, and is used to determine the loads that beams, girders, columns, and walls carry. The reader is expected to be familiar with the concept of tributary area from other design courses, as it also applies to design of timber and steel structures; however, a brief overview is presented in this section. The tributary area for a beam or a girder supporting a portion of the floor is the area enclosing the member and bounded by the lines located approximately halfway between the lines of support (columns or walls), as shown in Figure 4. For example, a tributary area for the reinforced concrete beam AB that is a part of the one-way floor system is shown hatched in Figure 4a. A typical column has a tributary area bounded by the lines located halfway from the line of support in both directions (shown hatched in Figure 4b). In the case of uniformly loaded floors, tributary areas are approximately bounded by the lines of zero shear, that is, the lines corresponding to zero shear forces in the slabs, beams, or girders supported by the element for which the tributary area is determined. Zero-shear locations are generally determined by the analysis. For buildings with a fairly regular column spacing, the zero-shear locations may be approximated to be halfway between the lines of support.
Figure :4 Tributary area for reinforced concrete members: a) beams; b) columns.
Figure :4 Tributary area for reinforced concrete members: a) beams; b) columns.


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