Calculating Moment Capacity

[JSF-Office]

Hello,

This might sound basic, but I’m new to bridge design and am currently working on verifying the capacity of a 3' by 6' deck cover using LBFoster’s proprietary 4 1/4" interlock with concrete overfill product. The attached brochure provides the section modulus for the composite section, but I would like to understand how to calculate the moment capacity based on the provided information.

Here’s my understanding so far:

For steel, the moment capacity is calculated as:

M=S×Fy

For a composite section, I believe the moment capacity is calculated as:

S effective=S steel + n × Sconcrete

Where n is the ratio of the modulus of elasticity of steel to concrete: n=29000 ksi / 4500 ksi=6.4n

Using 6" spacing per the provided table, I calculate Seffective S as:

S effective=4.88+(6.4×63.82)= 415.9in3

To apply the moment capacity equation, I’m assuming Fy=36 ksi so:

M=415.9 in3×36 ksi=14,971.7 k-in

Is this approach correct? is there other checks that AASHTO requires in using such systems, Any feedback or suggestions are appreciated!

Thank you.

 

[BridgeSmith]

S for a composite section is more accurately calculated as the summation of Io + Ad^2 for the component sections, in the same way as you'd calculate the section modulus for other built-up shapes, except for transforming the concrete to an equivalent area of steel by dividing the width of the section by "n".

 

[JSF-Office]

That makes perfect sense. Thank you, BridgeSmith.