Truss analysis is studying the truss's behavior imposed with loads. It serves as an initial step before designing it - finding the suitable material, size, and shape of the members and connections.
What is A Full Analysis?
When studying a truss, we need to perform the following analysis:
- Assessing its stability
- Finding the reactions
- Solving for internal force and stresses
- And analyzing the deflections
The internal forces of a truss would entail solving for axial forces. Part of this includes correctly identifying if it is tension or compression.
Analysis Assumptions
Basic Truss Analysis
In elementary truss analysis, there are several assumptions to take note of:
- The loads only act at the joint connections,
- We neglect the weight of the members, and
- The joints are assumed to be frictionless pin connections which means these are allowed to rotate (do not carry moment).
Why Assume?
These assumptions make the truss members a two-force member - a body with forces equal in magnitude but opposite in direction and collinear at its ends. As a result, it makes truss analysis simple, which we will see soon.
However, it is most likely that these assumptions are dissimilar from real-life:
- Forces may act on the member itself.
- Members do have weight.
- Joints may be rigid or semi-rigid connections.
Ideally, if we want to copy the behavior of an actual truss, it is logical to consider these conditions. So, why assume these things? Won't our results be wrong since it's not an exact model of the problem?
Remember that analysis aims to obtain reliable results. In most truss analysis situations, the results of a simplified study with these assumptions and an examination with actual behavior are nearly identical. The difference is that the latter is more complex and time-consuming, which is unnecessary. To support these assumptions, here are several reasons why these are valid:
- Most real-life loads do transfer the load at the truss's joints. For truss bridge construction, girders supporting the truss act on its joints.
- The self-weight of members transfers to the joints; because these weights are small compared to the imposed load, we can neglect it.
- Multiple studies show that joint connections for trusses have small or zero moments (provided that the bar forces meet at a common point); hence we can neglect it and assume the joint is a hinge.
Summary
Truss analysis is studying the truss's behavior imposed with loads.
There are four critical aspects for complete analysis: (1) stability, (2) reaction analysis, (3) internal force and stress analysis, and (4) deflection analysis.
Elementary truss analysis has essential assumptions: (1) the loads only act at the joints, (2) the weight of the members is neglected, and (3) the joints are assumed to be pin connections which means these are allowed to rotate.
