Patterned Loading and AS3600

[asixth]

Hi guys,

There is something that I have been having trouble with since I graduated from university and that is the application of patterned live load.

According to AS3600 Section 7, when analyzing your floor system based on the idealised frame approach, it is required that 75% of the design live load is to be applied to alternate spans. However, when designing your floor system by a linear static analysis, 100% of the design live load needs to be applied to alternate spans.

My interpretation is that I am designing my slab by the idealised frame method so I can apply the former of the two situations above, but to analyze my frame, don't I need to perform a linear static analysis?

The software package I am using sets a default value for patterned loading to 100%, which can significantly increase the amount of flexural steel that I require over my supports.

Basically my two questions are:

1. Does anyway consider 100% of the design load applied to alternate spans and

2. When is the analysis considered a linear static analysis.

 

[sdz]

Applying load to two adjacent spans will give the maximum moment over the support in between.
Applying load to alternate spans will give the maximum mid-span moments.
The combination is intended to give you simplified patterns that will result in a safe design.
Using 100% for both patterns will be conservative but remember that moment redistribution is allowed if ku<0.4 and preferably ku<0.2.

A linear static analysis uses small deflection theory and does not consider any change in moments resulting from deflection of the structure.

Do you have a senior engineer you can talk to about this?

 

[rapt]

asixth,

I asked this very question of the code committee several years ago because 7.1, 7.5 and 7.5 just did not make sense to me. Prof Bob Warner was put in charge of sorting it out.

Basically, 7.5 and 7.6 both apply to the same thing for vertical load analysis but have different pattern factors. 7.5 actually refers to 7.6 for vertical load analysis as an option but excludes the 7.6 pattern factors.

Anyway, after several discussions with Bob Warner, he decided he did not know which was supposed to mean what and it was decided to rewrite all of chapter 7 to get rid of the inconsistencies.

In the rewrite, which hopefully will be released early next year (still not holding breath but it is getting closer), the pattern factor is 1.0 for all cases. This is consistent with ACI, BS8110 and Eurocode 2.

But there are conditions where pattern load analysis is not required for strength and crack control design in the new code, basically if LL < .75 DL. For deflections, pattern load is always required in the new code.

 

[asixth]

sdz,

I do have senior engineers in my office but they weren't convincing with there responses.

The general consensus was that 75% was used to allow for the fact that the maximum positive and negative moment can never co-exist when patterned loading is taken into consideration.

rapt,

Thanks for your response, I can find plently of reference material which states "patterned loading should be taken into consideration", but I was still uncertain on how this should be applied in my design. It is a great benefit having people such as yourself participating in this forum.