EXODERMIC™ BRIDGE DECK DESIGN

OVERVIEW

The original Exodermic™ deck design (see History), which dates back to the early 1980’s, evolved from filled grids, with horizontal shear connection between the reinforced concrete slab and the grid effected by “tertiary bars” to which short vertical studs (generally #4 rebar) were welded. The tertiary bars were welded to the grid during grid panel fabrication and before galvanizing.

In the early to mid 1990’s, it was realized that starting with a clean sheet of paper rather than adapting existing grid deck designs, a more efficient, more economical, and easier to install deck system would be possible. Thus, the “revised” Exodermic™ deck design was developed and then tested.

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.

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.

In the revised Exodermic™ design, out of plane bending of the distribution bars due to horizontal shear flow is virtually eliminated, and a single quadrant weld between the distribution bar and main bar is acceptable.

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, Shallow WT4x5, 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
Overall Deck Thickness: 6" to 8"
Weight*: Lightweight concrete: From 39 lbs to 59 lbs
Standard weight concrete: From 47 to 70 lbs
Spans** to: 12 ft. (HS-20); 11 ft. (HS-25), main bars
transverse to traffic.
10 ft. (HS-20); 8 ft. (HS-25), main bars
parallel to traffic.
Main bearing bar spacing: 8", 10", or 12"
Distribution bar: 1 1/2" 2 1/4" @ 6" c-c
Rebar as required.

Shallow WT4x5
Overall Deck Thickness: 5 1/2" to 7 1/2"
Weight*: Lightweight concrete: From 40 psf to 57 psf
Standard weight concrete: From 47 psf to 68 psf
Spans** to: 10.7 feet (HS-20); 10.7 feet (HS-25), main bars transverse to traffic
9.9 feet (HS-20); 8.2 feet (HS-25), main bars parallel to traffic
Main bearing bar spacing: 8", 10", 12"
Distribution bar: 1 1/2" x 1/4" @ 6" c-c
Rebar as required

WT5x6 Main Bars
Overall Deck Thickness: 7" to 9"
Weight*: Lightweight concrete: From 40 psf to 61 psf
Standard weight concrete: From 48 psf to 72 psf
Spans** to: 14.5 feet (HS-20); 14.5 feet (HS-25), main bars transverse to traffic
12.9 feet (HS-20); 11.6 feet (HS-25), main bars parallel to traffic
Main bearing bar spacing: 8", 10", 12"
Distribution bar: 1/2" x 1/4" @ 6" c-c
Rebar as required

WT6x7 Main Bars
Overall Deck Thickness: 8" to 10"
Weight*: Lightweight concrete: From 42 lbs to 63 lbs
Standard weight concrete: From 51 lbs to 74 lbs
Spans to: 18 ft. (HS-20); 16 ft. (HS-25), main bars
transverse to traffic.
16 ft. (HS-20); 13 ft. (HS-25), main bars
parallel to traffic.
Main bearing bar spacing: 8", 10", or 12"
Rebar as required.

*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).

** Allowable strength, deflection, or fatigue may control

DESIGN METHODOLOGY & CRITERIA

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Standard Specification

LRFD Specification