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EXODERMIC™ BRIDGE DECK DESIGN
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REVISED DESIGN This design has been used on all new Exodermic™ deck projects since 1997, and supercedes the “original” design, as it has significantly better performance characteristics, is easier to erect, and is significantly less expensive. . With the revised Exodermic™ design, the tops of the main bearing bars of the grid are embedded directly in the slab, and 3/4" holes on 2" centers at the tops of the main bars act as shear connectors. Horizontal shear flow is direct from the concrete slab to the main bars of the grid. |
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In the original Exodermic™
design, horizontal shear flow from the slab to the main bearing
bars of the grid was indirect. That is, horizontal shear flowed
to the tertiary bars, then through the welds connecting the tertiary
bars to the distribution bars, then through the distribution bars,
and finally, through welds to the main bearing bars. Therefore,
the welds in the older style of Exodermic™ deck were particularly
critical. Furthermore,
testing at West Virginia University in the early 1990’s indicated that
two welds were advisable at each tertiary bar to distribution bar intersection
to avoid fatigue cracking at the slot in the distribution bar into which the
tertiary bar fits. See Fabrication
Details for Original Design. DESIGN CHOICES The designer has a number of choices to make in choosing an Exodermic™ deck configuration: main bar size and spacing, rebar size and spacing, and concrete thickness. A number of Exodermic™ decks have used a 4 1/2" concrete component in order to provide a standard 2 1/2" of cover over rebar, and 1" of bottom cover. Achieving desired deck thickness and weight may require reducing the concrete thickness. Exodermic™ decks have been constructed with concrete component thicknesses of 3" to 5". Service history dates to 1984, when an Exodermic™ deck was used on the longest bridge on the Garden State Parkway (NJ). Lightweight concrete can be specified where weight is particularly critical. While any steel grid can be used in constructing an Exodermic™ deck, use of industry standard grid configurations is advised where possible to avoid costs associated with new tooling. The standard types are referred to by the size of structural T employed as the main bearing bar: WT4x5, WT4x5 with dropped distribution bar, WT5x6 or WT6x7. Please check with D. S. Brown for availability of alternate main bar sizes. Section moduli and other properties of standard and non-standard Exodermic™ deck configurations are available from D. S. Brown. Choice of main bearing bar type is generally determined by desired deck thickness and span. Depending on span, the WT4x5 grid should provide the lightest option, minimizing the amount of full depth concrete over supports and the full depth transverse connection between panels. With deeper main bars, main bar spacing can be increased, potentially reducing the deck cost. The designer is encouraged to contact D. S. Brown for assistance in choosing a deck configuration.
WT4x5 Main Bars
WT4x5 Main Bars with dropped distribution bar (for
reduced deck thickness)
WT5x6 Main Bars
WT6x7 Main Bars *Note on weights: Weights shown
include grid, typical rebar, and concrete. Add weight for full
depth haunches over stringers and for shear keys or full depth
connections between panels. Concrete weight without rebar is 145
lbs/cu.ft. (normal weight) (10.38.1.3) or 115 lbs/cu.ft. (light
weight). DESIGN METHODOLOGY & CRITERIA Click on either link below to show corresponding PDF files containing engineering information not easily shown in HTML format. Files can be read with Adobe Acrobat Reader, 4.0 or newer.
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