Analysis of a structure is very critical as it deduces the forces for which members need to be designed. Hence all the loads acting on the structure need to be considered as per their respective load combination factors.

But does considering all the loads acting together on the structure give the most critical member forces? Here comes the concept of pattern or uneven loading.

Let’s take an example of a 2-span continuous beam subjected to dead & live load only. Consider two cases.

Case 01: Dead load over the entire beam and live load on only first span of the beam.
Case 02: Both dead load and live load over the entire beam on both the spans.
Which case would give us most critical member forces?

[1] Live load which is taken to be as uniform and static for simplicity of analysis is transit & dynamic in nature.
[2] At any given time, live load could be absent in any of these spans, which will more be critical case for hogging moment at middle support.
[3] Taking dead & live load together over all the spans will not give the most critical hogging moment at supports.

Considering all loads together with maximum load combination factor does not always translate to maximum member forces. It is common misconception/error which leads to underestimation for hogging moment at middle support.

This is brief post on pattern or uneven loading. Any suggestions or comments are welcomed.