cracked section or uncracked section? 5

[Engmember]

for slab on steel plate girder continuous bridge, when doing the live load analysis, for negative moment zone, the cracked section or uncracked section properties shall be used to analysis the force? AASHTO C6.10.1.5 said:

"Field tests of composite continuous bridges have shown that there is considerable composite action in negative bending regions (Baldwin et al., 1978; Roeder and Eltvik, 1985; Yen et al., 1995). Therefore, the stiffness of the full composite section is to be used over the entire bridge length for the analysis of composite flexural members."

according to this, looks like uncracked section properties shall be used for the negative moment zone. but british code or the euro code said the cracked section shall be used for the negative moment zone when doing the loading analysis.

any input will be much appreciated!

Gerry

 

[rscassar]

If the concrete is subject to tension as it would over the supports I would be inclined to use the cracked section properties.

 

[rceconsulting]

Previous versions of AASHTO dictated that only the steel section be utilized in the negative moment zone, along with additional negative moment deck reinforcement. Concrete properties are ignored. Again this is previous version of AASHTO. I am not fully up to date on LRFD code.

 

[ishvaaag]

In my view the correct interpretation is that composite action is to be used anywhere, but this not meaning with the uncracked section properties, but as compatibility of deformation demands, i.e., cracked if cracked, and uncracked if not, this both for serviceability checks and limit states checks. This way you get full coherence with the science of construction, that would be frofeited if not.

 

[VoyageofDiscovery]

What are you willing to stamp?

If you can prove that after the analysis, the section stress will not crack the concete, then use uncracked section properties if you must.

However if in actuality it cracks from plastic shrinkage and the like during construction, then your midspans will start to take that load, so you might as well prepare for that eventuality.

HTH

VoD

 

[Engmember]

many thanks to all of you!

if my understanding is right, when doing the load analysis for the girders to get the moment and shear, the full composite section shall be used (means stiffness along the girder shall include the concrete, reinforcement in concrete and the steel)!

however, when doing the section capacity design and steel stress check, only the steel section and the reinforment in the slab shall be considered!

 

[larsacious]

Correct. Make sure you provide sufficient studs for composite action.

 

[VoyageofDiscovery]

If the logic of my second sentence is true, yes.

If not, then use cracked inertial properties where the stresses from the first analysis dictate.

Typically I have run the analysis with cracked over the piers as I am the designer and choose to do it this way. This is the designer's input which used to be allowed for in the code. The assumption should be in line with the way the bridge is designed.

HTH

VoD

 

[Engmember]

like the AASHTO explained in C5.2.2

Tests indicate that in the elastic range of structural behavior, cracking of concrete seems to have little effect on the global behavior of bridge structures. This effect can, therefore, be safely neglected by modeling the concrete as uncracked for the purposes of structural analysis (King et al., 1975; Yen et al., 1995).

that is why the uncracked section properties should be used when doing the global analysis.