Bridge Rehabilitation with Structural Lightweight Concrete

Bridge
Rehabilitation
Permits
Higher
Live
Loads
By Dennis w. Stolldorf, PE(l) and Thomas A. Holm, PE(2)
Abstract
The elevated section of the Whitehurst Freeway was upgraded to an HS20
loading criteria during the rehabilitation of the Washington, DC corridor system
.structure
with only limited modifications to the steel framing superstructure.
Improved load carrying capacity was obtained becauseof the significant dead load reduction brought
about by using structural lightweight aggregateconcrete to replace the ordinary concrete and asphalt
overlay in the original deck slab.
The original elevated freeway structure was designed for H20 live load according to the AASHTO
1941 specifications. With the new lighter replacement concrete deck, a minimum of the structural steel
framing required strengthening and little interruption at the street level below was required to upgrade
the substructure to an HS20 live load criteria. Structural steel rehabilitation details and physical properties of the concrete are reported.
Figure 1. Whitehurst Freeway in background
Figure 2. Whitehurst Freeway, Washington, DC
Introduction
The Whitehurst Freeway is an elevated
roadway bound by the Georgetown section
of Washington, DC adjacent to and paralleling the Potomac River (Fig.l,2,3). The
Whitehurst Freeway is a heavily traveled
artery essential to the flow of traffic
between downtown Washington, DC , the
Key Bridge to Virginia and the canal roads
leading to the Maryland suburbs.
Figure 3. Whitehurst Freeway Elevated Structure
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The Washington, DC Department of Public Works and the Federal Highway
Administration decided that in order to eliminate the past practice of having large
trucks exit into the Georgetown streets, the new replacement elevated structure
should be upgraded to H20-S 16 truck loading (Fig.5). The Whitehurst Freeway elevated structure consists of longitudinally reinforced slabs with stringers placed transversely. The stringers are supported on longitudinal girders spacedat a maximum of
5.95 m (19 ft. 6 in.) on center. The longitudinal girders frame into cross-girders
which are supported at their ends on steel columns. The span between columns
varies, but is normally 18 m (59 ft.) center to center. The span for longitudinal girders is from 12.2 to 22.9 m (40 to 75 ft.) with the majority in the range of 13.7 to
16.8 m (45 to 55 ft.).
AsphaltConcrete
The longitudinal girdOverlay
ers are attached
to the
Bare Deck.
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Initial studies were
directed at increasing
the rated load capacity
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using regular weight
concrete while
25 mm (1") - ]a:
0 {3
strengthening the
-L
-*psf
existing structure.
psf
Rehab Deck
kg/m2
Original
Deck
kg/m2
77
A new 203 mm (8 in)
94
203 mm LTWT Concrete Slab = 376
190 mm Concrete Slab = 459
ordinary concrete deck
25
I).WT = 581 -376 = 205 kgm2
50 mm Asphalt = 122
119
(119- 77 = 42 psf)
Total
581
would have weighed
less than the original
Figure 6. Original and Rehabilitated Decks for Whitehurst Freeway
191 mm (7.5 in) concrete plus 50 mm (2
in) overlay. However, difficulties arose in applying the increase in live loading to the existing longitudinal and cross-girders. A 40% increase in live load occurs in a 15.3 m (50 ft) span when upgrading
from H20 to HS20.
The only practical way to allow rehabilitation of the existing structure was to use structurallightweight concrete. The use of lightweight concrete allowed the new deck to be placed with a net savings
of 205 kg/m2 (42 psf), more than one-third of the original deck weight. Fig. 6 shows a comparison of
slab weights between the original and rehabilitated structure. This reduction in self weight greatly off Q
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set the increase in loading due to both additional bridge width as well as the increased live load requirements. The transverse stringers remained adequateexcept on the river side where the widening necessitated greater cantilever dimensions. Of 180 existing longitudinal girders, only 42 required strengthening. Overload of the existing structural members was low enough to allow the strengthening repair to
be accomplished by adding flange plates bolted to the existing wide flange beams. The strengthening
was completed while the deck was removed. The plates were attached using high strength bolts (Fig
7). The existing girders were analyzed in their partially unloaded condition with a live load restriction
of H10 on the half of the structure remaining in service. Plates were added to the bottom of the top and
bottom flange to eliminate the need for disturbing the transverse stringers. New plates were added to
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about ESCS or Structural Lightweight Concrete:
Contact your local lightweight aggregate supplier.
Expanded
Shale,
Clay
and Slate
Institute
2225 Murray Ho11aday Road. Suite 102
Salt Lake City, Utah 84117
801-272-7070. Fax 801-272-3377
www.escsi.org .e-mail: [email protected]
Printed
in U.S.A.