Steel + Long Reinf (n=n) vs. Steel + Long Reinf (n=3n) section property

Please email me your model to Vinay.mysore@Bentley.com

Please follow FHWA (and AISC) design example which explains why would the properties for Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) be different.

Also there is explanation in the FHWA example which says :

“Although not required by specification, for stress calculations involving the application of long-term loads to the composite section
in regions of negative flexure in this example, the area of the longitudinal reinforcement is conservatively adjusted for the effects
of concrete creep by dividing the area by 3 . The concrete is assumed to transfer the force from the longitudinal deck steel to the rest
 of the cross-section and concrete creep acts to reduce that force over time”

Reference : FHWA design example

thanks

Based on the search results, it seems that the MOI in the negative bending region of a composite steel girder run is different for the Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) properties. The reason for this difference may depend on the specific design criteria and assumptions being used. However, here are some possible explanations based on the search results:

1. The negative bending region may be affected by the presence of shear connectors. According to the FHWA, stresses due to loads applied to the composite section for service and fatigue limit states may be computed using the composite section assuming the concrete slab to be fully effective for both positive and negative flexure. This suggests that the presence of shear connectors may affect the MOI in the negative bending region[3].

2. The design criteria may require different assumptions for the Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) properties. For example, the AASHTO LRFD design criteria assume that the girder is composite in the positive bending moment region and noncomposite in the negative bending moment region during construction. This may result in different MOI values for the Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) properties[5].

3. The design criteria may require different assumptions for the steel member and the reinforcing steel. According to the IRC:22-2015 standard specifications and code of practice for road bridges, negative moments over internal supports should be checked against section strength assuming steel girder acting integrally with concrete (considering uncracked)[2]. This suggests that the design criteria may require different assumptions for the steel member and the reinforcing steel, which may result in different MOI values for the Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) properties.

It is important to note that the specific explanation will depend on the specific requirements and resources being used. Therefore, it may be helpful to review the design criteria and other resources to ensure that you understand the assumptions being made and how they may affect the MOI values for the Steel+Long Reinf (n=n) and Steel + Long Reinf (n=3n) properties.  MyLabCorp.com