In Leap Steel Including shear studs or not in the negative moment regions, changes the results of the moment diagram for composite load cases. Not including shear studs in the negative moment regions, decreases negative moment values and increases positive moment values. This is most likely happening due to Leap Steel is not utilizing composite section when shear studs are not included and that results with redistribution of moment between negative and positive moment regions as they have different stiffnesses.
Even though shear studs are not included in the negative moment region, composite section behavior can present. Also not including shear stud results with lower moment values for the negative moment.
An option might be added to utilize composite section properties in the negative moment region independent from having shear studs or not for users to pick for composite moment calculations.
| Civil Product Used | OpenBridge Designer |
Could you explain why you would like to use a composite section in the absence of any shear studs?
Currently, by tying the analytical stiffness of the beam directly to the presence of shear studs, LEAP Steel creates a significant discrepancy in stiffness between non-composite and composite sections. This difference in relative stiffness directly impacts the moment distribution for any loads applied in the final sequence, such as live load.
ASHTO LRFD Article C6.10.1.5 indicates that the field tests of composite continuous bridges have shown considerable composite action in negative bending regions and stiffness of the full composite section is to be used over the entire bridge length for the analysis of composite flexural members.
NSBA G13.1-2019 Guidelines for Steel Girder Bridge Analysis, Article 3.2.1.4, states the following:
"Upon completion of the appurtenances, the structure will be opened to traffic. Short-term composite section properties should be used for all live load analyses. For continuous girder bridges, longitudinal distribution of live load is a function of girder stiffness. In the negative moment regions, the composite section for calculation of girder capacity is typically assumed to include only the girder and the longitudinal reinforcing steel embedded in the deck. However, from a stiffness standpoint, Article 6.10.1.5 of the AASHTO LRFD Bridge Design Specifications (AASHTO, 2017) requires that the concrete deck be assumed effective over the entire span length in the analysis to determine force effects due to composite loads. The associated commentary discusses that field tests of composite continuous bridges have shown that there is considerable composite action in the negative moment regions, even if shear connectors are not provided in those regions. In other words, for the purposes of determining live load force effects, the composite section may consist of the girder and the deck throughout the length of the bridge."
The article further notes:
"In cases where shear connectors are not provided in the negative moment region, shear continuity between the girder and the deck should not be assumed for strength capacity calculations. In this situation, since the deck is assumed to be cracked in the negative moment region and since there is no shear connectivity between the girders and the deck, the common assumption is to neglect the deck in the calculation of the girder section properties."
To align with this, the Michigan Bridge Design Manual indicates the following:
"The composite moment of inertia shall be used throughout positive moment regions. This moment of inertia is to be used in negative moment regions to compute beam stiffness only."
Considering these requirements, when shear studs are omitted in the negative moment region within LEAP Steel, the distribution of live load effects is altered by the reduction in relative stiffness at the supports. By modeling the negative moment region as non-composite while the spans remain composite, the program assumes a more flexible condition at the piers than what is typically intended for global analysis per AASHTO C6.10.1.5.
This analytical assumption causes a redistribution of force effects where the spans attract more moment and the supports attract less. Consequently, this can lead to lower negative live load moments at the supports and higher positive moments in the spans compared to an analysis performed using a consistent composite stiffness for the entire bridge length.
Other steel bridge design software packages either default to using the composite section for the entire length for stiffness or provide a dedicated option to do so. My suggestion is to provide an option in LEAP Steel that allows the user to utilize composite stiffness for the entire span for analysis purposes, per the designer’s discretion and the requirements of the governing agencies.