Reinforced Concrete: Bending Moment & Axial Force Analysis
Bending Moment & Bearing Capacity: Introduction
Most reinforced concrete structures can be subdivided into beams and slabs subjected primarily to bending, and columns subjected to axial compression accompanied in most cases by bending. Typical examples of bending members are slabs and beams. The beam reactions cause axial forces in the columns. The slab is assumed to transfer loads in one direction and hence is called a one-way slab. If there were no beams, the slab would carry the load in two directions. Such a slab is referred to as a two-way slab.
Two different types of problems arise in the study of reinforced concrete:
- Check (Checking): Given a cross section, compressive concrete strength, reinforcement with its location and yield strength, to compute the resisting bending moment.
- Design (Proportioning): Given action effects, to select suitable cross sections, including dimensions, concrete strength, reinforcement, and so on. Although both types of problems utilise the same fundamental principles, the procedure followed is different in each case.
Checking is easier since all the decisions concerning reinforcement location, cross-section dimensions, material characteristics, etc., are given and it is necessary to apply the calculations principles and determine the resisting bending moment. Proportioning, on the other hand, involves the choice of beam sizes, material strength, and reinforcement to design a cross section and the structural system that can resist the actions which will be imposed on it.
A moment that causes compression on the top surface of a zone for the on the bottom surface is called a positive moment. The compression positive and the negative moment are shown shaded in Fig. 4.2. Bending moment diagrams are usually plotted on the tension side of the member.
Combination of Axial Force & Bending Moment
A column is a vertical structural member transmitting axial compressive loads, with or without moments. A column is a member for which the section depth does not exceed 4 times its width and the height is at least 3 times the section depth. Otherwise it should be considered as a wall. Columns and walls support vertical loads from the floors and the roof and transmit these loads to the foundations. A column resists loads primarily by means of internal axial forces N and moments M (Fig 6.1). Usually, shear forces can be neglected, but columns in seismic regions require special detailing to resist shear forces from earthquakes.
