SFO BAY BRIDGE SECOND CROSSING
ENGINEERING CONSIDERATIONS
By Ronald F. Middlebrook, S.E. and
Roumen V. Mladjov, S.E.
The new second
crossing of the San Francisco – Oakland Bay Bridge is proposed to be built as a
combination of a new West Crossing and a refurbished and reinforced structure salvaged
from the abandoned existing structures of the East Crossing.
The New (2nd) West Crossing is proposed to consist of three structural parts each a double-decker to match the existing 1936 structure with five traffic lanes in each direction or with four traffic lanes plus a shoulder in each direction. Optionally there is an opportunity to use a wider deck to accommodate an additional lane to extend to San Francisco the bicycle/pedestrian lane currently included with the new East Crossing construction only between Oakland and Yerba Buena Island (YBI). This would require new ground level bike lane transitions on YBI between the new West Crossing and the 2013 East Crossing.
1st Transition structure (San Francisco approach) with total length of 540 m (1,772 ft) on three or four continuous spans;
Suspention structure – with total length – 2,980 m (9,777 ft, or 1.85 mile) including:
Two side spans of 370 m (1,214 ft) Two interior spans of 740 m (2,428 ft)
One Central/main span of 760 m(2,493ft)
Optionally, a Cable-Stayed system using the same spans as above should be studied and compared with the suspension bridge for selecting the more efficient cable-supported system (in cost, steel quantity and construction time).
The reasoning behind
the above proposed spans is that the new West crossing structure should be in
harmony with the existing iconic bridge and should not, in any case, overshadow
it. While today’s suspension bridges are built with spans easily exceeding 1,000
m and even cable-stayed bridges have been built with spans greater than 1.000 m,
the proposed spans of 740 to 760 meters are in the optimum range of the most
efficient bridge structures for the two suggested systems. In addition these spans are very close to the
spans of the existing suspension bridge and should not inhibit navigation any
more than it does.
The proposed
alignment avoids a new tunnel on YBI. The east anchorage of this bridge would
be a new pier just of the southern tip of YBI, which would also serve as the
support of the west end of the 2nd Transition, described below.
Based on the average
steel used for similar structures in the last 10 -12 years (maximum spans 760
m, average span 600 m) the estimated steel needed for a new suspension
structure is in the range of 78,700 tons, and for a cable-stayed – 86,000 tons.
It is a known that the unit steel cost
for a cable-stayed bridge is lower than the unit cost for a suspension bridge.
2nd Transition structure, near Yerba Buena Island, estimated
total length 530 m (1,739 ft) on three or four continuous curved spans. This 2nd transition structure will
connect the new West and East crossing.
The total steel
needed for the Second West Crossing is estimated as listed below:
1st Transition structure (540 m)
8,800 tons
Suspension structure (2,980 m) 78,700 tons
Optional Cable-Stayed structure (2,980 m) (86,000
tons)
2nd Transition structure (530 m) 8,600 tons
TOTAL for 4,050 m Second West Crossing
with Suspension structure 96,100 tons
with Cable-Stayed structure 103,400 tons
The new West
crossing should be designed at the highest level of today’s achievements in
bridge engineering and construction using orthotropic decks, high strength
steel and concrete, composite steel-concrete elements, using the best
experience in long-span bridge engineering. The new structure should be
designed as highly efficient in cost, use of material and construction time and
should be used as an example for a new generation of bridges with emphasis on
efficiency and economy.
The existing Cantilever Truss section, reinforced
(for example) with “cable-stayed” type post tensioned system creating applied
upward forces at the main span tips of the cantilevers, plus any necessary
replacement or reinforcing of existing individual elements. The “cable-stayed”
reinforcing could be prestressed to about 80-85% of the dead load reactions at
the cantilever ends therefore relieving the existing cantilevered structure of
about 60% of the total loads. Total length 736 m (2,416 ft)
Deep truss system with five spans by 153.6 m (504 ft), total length
768 m (2,520 ft). One possible simple option
for improving performance is to interconnect the five spans to form a
continuous truss system reducing the demands on the steel members from 25 to
70%.
Double-deck truss system with 14 spans by 87.8 m (288 ft), total
length 1,229 m (4,032 ft). Similar to
the deep truss system, providing continuity between the ends of the trusses may
be the most efficient approach for strengthening this part of the existing
structure.
The ten spans (single deck only, the eastbound lanes) of the Oakland landing, total length 327 m (1,073 ft) retrofitted as necessary.
The ten spans (single deck only, the eastbound lanes) of the Oakland landing, total length 327 m (1,073 ft) retrofitted as necessary.
A replacement of the
existing old concrete deck of the abandoned structures with new, lighter,
orthotropic deck (reducing the self weight of the bridge) should be studied as
a valuable option for increasing the overall load capacity of the bridge and
allowing less heavy equipment for the proposed re-alignment of the existing
structures.
All of the spans of
the original East Crossing should be re-assembled on, or moved on to new
foundations and piers aligned to achieve a smooth traffic transition from the
new West Crossing to the realigned East Crossing.
The Oakland landing
and part of the 14 – 288 ft spans should remain as close as possible to the
existing alignment.
The total estimate
for the steel needed for the Second East Crossing is:
- New substructure – piers 6,500
tons
- New elements for refurbishing the existing spans
12,000 tons
- Remaining original (existing) structures 48,500
tons
- TOTAL for
the Second East Crossing 67,000 tons
(This means only 18,500 tons for new
structure)
The total steel quantities for the proposed Second Crossing (West
and East) are estimated as:
- 163,100
tons (including 115,000 tons new structures) for the suspension option, or
- 170,400
tons (including 122,000 tons new structures) for the cable-stayed option.
The primary advantages
of a SFO Bay Bridge 2nd Crossing as proposed are:
- Engaging
some of the best engineers and
builders through an open competition/design-build project approach;
- Effectively
increasing the traffic capacity by 80-85% of the capacity expected for the
newly renovated Bridge; (2013;
- A reserve link between San Francisco and Oakland; extremely important in case of a serious accident, or other damage (to the 2013 bridge);
- Preserving and reusing an iconic historic bridge structure considered as one of the highest achievements in engineering;
- Significantly
less new steel and structures compared with any system of bridge, because
of the reuse of most of the
existing structures of the original East Crossing;
- Greatly
reduced construction time;
- Significantly
lower cost than the 2013 East Crossing replacement;
- Significantly
more environmentally friendly than any entirely new structure (smaller
carbon footprint);
- The
experience gained from this project could serve as new approaches for
renovation of many old existing bridges in the country (24% of the
existing bridges in the country, or 144,000 bridges are listed as
deficient based on December 2011 data.
Note: All the steel quantities above are in
metric tons (one metric ton = 1.102 U.S. tons, or equal to 2,205 lbs).
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