Evaluation of Load Rating for Continuous Stringer Bridges

Jonathan Gerdes Author

08/04/2020 Added

33 Plays

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A bridge was designed and load tested to help re-assess the methods behind load-rating the stringers and a more realistic moment gradient factor.

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[00:00:00.580]Hi everyone, I am Jonathan Gerdes,
[00:00:02.478]an undergraduate
[00:00:03.303]student in the
[00:00:04.035]Department
[00:00:04.788]of Civil and Environmental Engineering.
[00:00:07.168]Today I will talk about the evaluation of
[00:00:09.619]load rating for continuous stringer
[00:00:11.749]bridges.
[00:00:13.065]Some of Louisiana’s bridges built in the
[00:00:15.385]1950's were designed with continuous
[00:00:17.645]stringers that were supported by floor
[00:00:19.915]beams. Stringers are the beams that
[00:00:22.155]support the deck, while the floor beams
[00:00:24.565]are the beams that transfer loads from
[00:00:26.539]the stringers to the rest of the bridge.
[00:00:28.509]All of Louisiana’s bridges must be rated
[00:00:30.879]with load and resistance factor rating as
[00:00:33.319]specified by the American Association of
[00:00:35.824]State Highway and Transportation Officials
[00:00:38.424]. Current load-rating procedures for
[00:00:40.660]bending use a moment gradient factor of
[00:00:42.616]one, which may underestimate the flexural
[00:00:45.006]strength of the stringers. This could
[00:00:47.146]produce low-enough results that would
[00:00:49.526]enforce restrictive load postings or
[00:00:51.568]closure of a bridge. This issue affects
[00:00:53.758]the main highway systems of Louisiana. In
[00:00:56.238]addition, the current rating could lead
[00:00:58.568]to unnecessary and expensive bridge repair
[00:01:01.406]or replacement. Due to this, there is an
[00:01:04.376]urgent need to re-assess the methods
[00:01:06.956]behind load-rating the stringers and a
[00:01:09.596]more realistic moment gradient factor.
[00:01:14.617]The objectives of this project were to
[00:01:16.787]evaluate the behavior of a representative
[00:01:19.137]bridge when loaded and determine the
[00:01:21.358]total load capacity of the floor system
[00:01:23.541]with continuous stringers and develop a
[00:01:26.121]more accurate moment gradient factor for
[00:01:28.791]Louisiana to use in the future. The
[00:01:31.581]summer objectives were to construct the
[00:01:33.678]bridge to be tested, affix strain gauges
[00:01:36.188]and other instruments to the bridge, and
[00:01:38.198]assist in the testing of the bridge.
[00:01:41.815]A 10-foot by 50-foot, three span, steel
[00:01:45.815]stringer bridge was designed, and tested
[00:01:48.753]Louisiana’s highway stringer floor beam
[00:01:51.031]bridges. Non-composite decking was desired
[00:01:53.721]for this application. A non-composite deck
[00:01:56.872]means that the concrete is not
[00:01:58.802]mechanically connected to the steel,
[00:02:00.522]meaning there is slip between the stringer
[00:02:02.540]and the concrete. Once stringers were
[00:02:06.540]fastened together, formwork for the
[00:02:08.600]concrete deck, which consisted of plywood
[00:02:10.880]supported by 2x4's that extended to the
[00:02:13.731]floor and anchored between the stringers,
[00:02:15.843]was placed. The formwork needed to be
[00:02:18.193]designed to support the wet concrete load
[00:02:20.393]without excessive deformation and so that
[00:02:22.943]it could be easily removed. Oil was
[00:02:26.203]sprayed on the plywood to help the
[00:02:27.834]concrete resist adhesion to the plywood.
[00:02:29.784]Rebar was tied and placed based on
[00:02:31.904]provided design drawings. Concrete was
[00:02:34.878]poured and cured for four weeks. Once the
[00:02:38.338]concrete finished drying the formwork was
[00:02:40.724]carefully removed so the concrete and
[00:02:43.084]previously placed sensors were not damaged
[00:02:45.216]. Once the concrete cured, additional
[00:02:48.495]sensors were placed onto the top and
[00:02:50.695]bottom deck to measure the decks response
[00:02:52.888]to load. A two-part resin designed to
[00:02:55.118]stretch with the expansion of the concrete
[00:02:57.373]decking was spread over the desired areas
[00:02:59.757]for the sensors to be adhered to. Once
[00:03:03.508]the resin cured, the surface was sanded
[00:03:05.638]smooth and level. Glue was used to attach
[00:03:08.718]the strain gauges to the resin. Figure 6
[00:03:10.990]shows a fully attached strain gauge.
[00:03:14.170]After initial tests, the midspan stringer
[00:03:17.090]microstrain graph was obtained . It was
[00:03:19.560]determined that the bottom of the
[00:03:21.190]stringer was in tension and the top of the
[00:03:23.210]stringer was in compression. The graph
[00:03:25.445]shows there was more strain in tension
[00:03:27.683]than compression. A load of 80,000 kips
[00:03:30.003]were applied, at this point the stringer
[00:03:33.089]achieved 1300 microstrain. This was the
[00:03:35.994]maximum strain desired as failure would
[00:03:38.434]occur if exceeded. In Conclusion, the
[00:03:42.434]decking was successfully placed, the
[00:03:45.684]strain gauges were placed on the decking
[00:03:47.784]for assessing the bridges response to load
[00:03:49.784], the midspan stringer was graphed showing
[00:03:52.234]the maximum microstrain, and testing
[00:03:54.884]continued. Three additional tests will be
[00:03:57.974]completed to examine the bridges response
[00:04:00.274]to load at different locations. Data
[00:04:02.604]analysis to help determine a realistic
[00:04:04.864]moment gradient factor is being completed.
[00:04:08.864]Thank you.

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Evaluation of Load Rating for Continuous Stringer Bridges

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