Transverse Deck Slab Stiffness... 2

[a7x1984]

This question is directed to my structural colleagues who have modeled a composite steel girder/concrete deck bridge in a finite element program,i.e. MDX, RISA, etc.

We all know this standard structure type has loads distributed to long. girders via simplified S/5.5, etc. for new design. Refined methods are permitted by AASHTO as long as they adequately represent the actual working conditions of the bridge.

I am checking the rating of another engineering firm. The LFD rating is for a 700 kip superload truck. The rating is obviously operating and reduced impact. It is also centerlining the structure to prevent the hammerhead-style pier cap overturning/failure.

The engineering firm utilized MDX (finite element concrete deck over line-girders) to rate the bridge. The main feature of this program is utilizing the transverse stiffness of a concrete plate to distribute LL+I across all girders in a proportion less than determined by S/7.0, etc. They determined a minimum 1.06 operating rating, which is quite close considering their stiffness assumption and that all traditional rating programs(LARS, Merlin Dash, BAR7) failed the rating consistently around 0.7 of unity. Our concern is the use of the full 7.5" concrete deck to transfer moments transversely across long. girders.

We all know concrete sections crack. If this 40 year-old bridge deck wasn't design as a Class U (un-cracked) section (obviously not), how could they justify using a transverse gross moment of inertia?


"Structural engineering is the art of modelling materials we do not wholly understand into shapes we cannot..."...ah...screw it, we don't know what the heck we are doing.

 

[VoyageofDiscovery]

Only if the deck can handle the transverse moments in such a manner.

VoD

 

[a7x1984]

VOD, I am not sure what you mean by "handle". They can handle the loading to meet strength and deflection requirements, but that doesn't imply they have the stiffness required to transfer moments over the girders in the SAME manner as a solid, un-cracked concrete section, correct? I was looking for some specific precedence or maybe even a 'hidden' AASHTO reference, tech report, or code reference; that is why I made reference to the building code criteria that requires using a net section for stiffness modeling.

"Structural engineering is the art of modelling materials we do not wholly understand into shapes we cannot..."...ah...screw it, we don't know what the heck we are doing.

 

[VoyageofDiscovery]

The stresses in the deck rebar should not result in cracked transverse deck section.

Check the past version of the Canadian Highway Bridge Design Code S6-00 for this, I did not see it in the current version. I remember a number for the formula had to be less than 50,000.

HTH

 

[crossframe]

Line-girder analyses like Merlin-Dash, and probably Bar7, (never used LARS) use the "S-over" live load distribution factors as you've noted. As you've also noted, grid and finite-element analyses, such as MDX, DESCUS, etc., are considered by AASHTO to be a "refined analysis" and they may produce lighter sections or higher ratings due to the increased lateral distribution of the load.

Some of this distirbution may be attributed to the deck, and some to the crossframes/diaphragms. Both cause adjacent girders to feel the effects of a deflecting girder. It's been a while since I've used MDX, but the crossframes may be contributing "more" than the deck to this distribution.

Lastly, all of these programs have their limitations. Search for "Descus vs MDX" in this forum, and you'll probably trust them even less!

 

[a7x1984]

Crossframe, excellent note on the diaphragm contributions. Hence the pseudo-name "Crossframe", I suppose. I am afraid to search for Descus V. MDX!

"Structural engineering is the art of modelling materials we do not wholly understand into shapes we cannot..."...ah...screw it, we don't know what the heck we are doing.

 

[wiktor]

For this super load truck I would suggest using grillage model and the dedicated truck wheel loads.
Model the grillage with the longitudinal girders composite w/ concrete deck, real cross members or cross frames - also composite, if that's the case, and finally a concrete deck modeled as transverse members or plates in between. The details of the model will depend on the software used, so these may vary. This should give you a realistic lateral load distribution to the girders.
I would suggest carrying separate analysis for long term (DL and SDL) and short term loads, as the properties of your composite members will be very different. (old AASHTO 10.38.4.1) If you would like to be extremely accurate, assume that steel was supporting wet concrete and resulting stresses are “locked” in the steel alone…
As typical commonly used software is incapable of accommodating such “advanced” analyses simply add your results in the spreadsheet – steel only, composite n=30, composite n=10).
Rating points will be BBFS (bottom, bottom flange, steel), TTFS (similar top) and TC (top concrete).
The rating shall be done for maximum moment and for maximum shear, so the truck will be in different positions.