AS 5100.6 - 2017 - Composite beam section capacity derivation

[Jack_TECL]

Hi Eng tips community,

I am working through the design of a composite beam bridge using AS 5100.6 - 2017. I understand how I would derive the capacity of a 'non-compact' composite cross-section but I am having difficulty deriving the capacity of a 'compact' composite cross-section.

Clause 6.4.3 states that:

The nominal section moment capacity (Ms) of a compact cross-section in sagging or hogging bending shall be determined as follows:

Ms = Mp

Where Mp = nominal section moment capacity.

Mp shall be determined by simple plastic theory using the following assumptions:

a) Plane cross-sections of structural steel and of concrete each remain plane
b) The effective section of the composite beam replaces an effective width of the concrete compression flange, determined in accordance with clause 4.4.1.


I am having trouble with clause 6.4.3b). There are no references in the standard as to how to derive this "effective section". Clause 4.4.1 uses the standard methodology for determining the effective width of the concrete compression flange. I have found some references to the 'effective section' in AS 2327.1 but I am unsure as to how relevant this working will be.

NZS 3404 is the standard I am familiar with. This method also uses an effective concrete deck width to derive capacities.

Can any Aussies help a kiwi out?

Cheers,
Jack.

 

[bugbus]

I feel the same way about 6.4.3 b). Highly confusing and I think just poorly worded.

My understanding (could be wrong) is that, for the section capacity, you just need to consider the appropriate effective deck width to account for shear lag, calculated from Clause 4.4.1.

I would be happy to hear someone's better explanation though.

 

[Settingsun]

I don't think there's meant to be any trick. The 2004 version had the same wording and also a reference to an appendix for equations. Here's the appendix.

https://files.engineering.com/getfi...4-9b36-7f76ec3bd1db&file=CompactComposite.pdf

 

[Jack_TECL]

Thanks, Steveh49 and gusmurr.

@Steveh49, in the attached document, b is referred to as the "width of the concrete flange in the composite section". Do you believe that this means we simply take the tributary width or still use Cl 4.4.1 as gusmurr has suggested?

 

[SobreroPeet]

@Jack TECL, I would use the effective width as per CL 4.4.1 and not the trib width.

 

[dik]

The Canadian S16 steel code puts limits on the effective width of a composite T or L, with the tributary width being the outside limit.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Settingsun]

I think 6.2.5 is saying to use the 4.4.1 effective width. My guess about the convoluted wording of 6.4.3(b) is that it's saying to ignore the bit of the slab that's in tension (if applicable). In the global analysis, you would have included the full slab depth in the stiffness calculation for sagging regions.