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Transverse deck steel development length

Question
State MO
Description Text

I’d like to get the position of MwRSF on the criticality of meeting the development length requirement in the cantilever slab top mat transverse steel reinforcement beyond the face of a concrete barrier especially in light of recent changes to AASHTO’s LRFD development length requirement increases.

Many crash tests with barrier on cantilever slab show that this length may not be critical since there is no slab damage.

What would a failure of slab look like with a development length failure?

It seems there are other load paths in play in case of “slipped” bar and surely the strength of slab in shear (like a blockout failure) and the barrier would come into play?

And would the size of the bar also be a factor, for example, when a crash test successfully shows that a No. 4 straight bar works, and we are using No 5 or No 6 bars that requires larger development lengths. Do the larger bars work as evidenced by the successful crash study?

Can I get your thoughts? 

Keywords
  • Bridge Rails
Other Keywords Bridge Decks, Development length, reinforcement
Date April 6, 2017
Attachment Transverse Steel Top Mat.jpg


Response
Response

A lot of what you are saying has merit. Most of the bridge deck damage that we witness following a full-scale crash test of a continuous concrete bridge rail fits into one of 2 categories: 1) cracking and spalling of the outside edge of the deck behind/underneath the concrete bridge rail and 2) longitudinal bending cracks in the top surface of the deck located above the outermost girder.   I do not recall witnessing cracking through the deck thickness directly in front of the bridge rail that would indicate that the transverse steel in the deck had exceeded their anchorage strength (developed strength in tension).

There are multiple load paths in play during an impact event that distribute the lateral loads.  For example, the bending of the concrete rail itself takes much of the impact load and distributes it along the longitudinal length of the deck. The shear at the base of the rail that gets turned into a tensile load resisted by the transverse steel is only a portion of the impact load.  Additionally, it should be recognized that many bridge rail reinforcement patterns utilize vertical steel bars (stirrups) which are anchored to the deck with 90 degree hooks and extend in toward the center of the deck.  This reinforcement would also supply some resistance to the tensile failure that would occur from a lack of development length.

The size of the bars would not be factor if the same number of transverse steel bars were utilized. For example, let’s say that a concrete rail and deck were tested with the transverse steel being #4 bars @ 12”.  But, due to an extended cantilever distance, the #4 bars needed to be increased to #5 or #6 bars to prevent bending failures over the outside girder.  The lateral loads (tensile) that the #4 bars withstood during the test should not be a problem for the larger bar sizes as they would only be stressed to a lesser degree, thus not needing a full development length.  Important to note this only applies if the same number of bars (bar spacing) is used.  Increasing the bar size and spacing may give the same design strengths, but the development length may come into effect as more load is now applied to each individual bar.

Hope this helps.  Let me know if you have further questions.

Date April 6, 2017


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