Section 4.1
Overview
Last Revised: 05/14/2025
Reinforced concrete structures are often cast such that joints are continuous. Unlike steel and timber structures where the structures are often assembled from discrete elements where simple connections are inexpensive, cast in place concrete structures typically have elements that are continuous across multiple joints. Pre-cast concrete elements can be assembled into structures with simple joints.
Because of the continuity across joints and supports in cast-in-place concrete structures these structures are generally indeterminate and require the use of indeterminate structural analysis techniques to determine deflections and internal forces. There are a number of sophisticated structural analysis software packages on the market that can be used to determine the required strength values to be used for the design of all elements of the structure. For certain less complex continuous structures (such as continuous beams), there are several procedures taught in structural analysis courses that can be used by hand and/or easily programmed into a spreadsheet or mathcad.
In structures made from components with equal strength throughout the span, it is only necessary to know the maximum internal forces to size the member. In reinforced concrete members, the strength varies along each member depending on cross sectional dimensions and the size and placement of both flexural and shear reinforcement. Consequently, it is necessary to know the value of internal forces for all load combinations at every point in the structure.
The bottom line is that the design engineer must be able determine the range of internal axial force, shear, and moment at every point in every member of the structure. Once the range of forces are known, then members can be proportioned and reinforced to have sufficient strength to carry the forces.
Structural analysis, while necessary, is not the focus of this course. It is assumed that those studying this material have a had a course in structural analysis and are familiar with procedures for computing the internal forces in structural members.
What this section is intended to do is to highlight the need to be able to compute the 'envelope' (i.e. maximum and minimum values of internal forces) values at every point within the structure.
A Beginner's Guide to Structural Mechanics/Analysis chapter on Continuous Beam Analysis goes into some detail on techniques for computing moment envelopes for beams. The same techniques can be extended to entire structures.