ST0301-CS040722Freese and Nichols
Simon W. Freese, P.E.
Marvin C. Nichols, P.E.
I900-1990
I896-1969
MEMORANDUM
TO:
FROM:
SUBJECT:
DATE:
Suzan Taylor
Tricia Hatley, P.E.
CPL03243 Bethel Road Bridge Reconstruction and Sidewalk Options
July 22, 2004
This memorandum is in response to the City's request to investigate two items related to
the Bethel Road bridge: 1) the possibility of raising the existing bridge deck by adding a
concrete overlay, and 2) options for adding new sidewalks to the existing bridge.
Background
In lieu of replacing the existing bridge, the City decided to keep the bridge in place since
the top of the roadway is not overtopped by the 100-year base flood elevation. The deck
ranges from 1.19' to 4.87' above. However, the western portion of the existing roadway
deck is a maximum of 0.81' below the freeboard limit specified in the City's drainage
criteria. The City asked FNI to investigate the possibility of raising the existing bridge
deck above the required freeboard elevation by adding a concrete overlay to the existing
deck.
The City also asked FNI to investigate two options for adding a new sidewalk to each
side of the bridge regardless of whether the new concrete overlay could be added to the
bridge. We investigated t~vo options:
1. Cantilever the new sidewalk from the existing bridge bent.
2. Support the new sidewalk with separate beams, columns and drilled piers.
Findings
1. New Concrete Overlay
The proposed new concrete overlay will impose an additional load for which the
original bridge was most likely not designed. This additional load will be
transferred to the foundation soils through superstructure members, columns and
Frecse And Nichols, Inc. · 1701 N. Market St., Suite 500 LB51 · Dallas, Texas 75202
Telephone: 214-920-2500 · Fax: 214-920-2565
drilled shafts. All existing structural members including the deck, pan joists, bents,
abutments, columns and drilled shafts as well as the foundation soils must be
capable of carrying this additional load without being overstressed.
Since the original construction drawings for the bridge are not available, we would
need to do extensive non-destructive testing and structural analysis to determine
the strength of the main structural members. However, we cannot investigate the
existing abutments and drilled shafts because they are buried underground. We
also cam~ot determine the soil strength capacity without knowing the drilled shaft
penetration depths. There are no economically feasible ways to verify the existing
drilled shaft penetration depths and use that to determine the foundation soil
strength capacity.
Since we cannot directly investigate the foundation strengths, we have estimated
the capacity based on standard drawings for bridges of this type. The current
TxDOT standard drawing entitled "BCG-28" (Bents for Concrete Slab and Girder
Spans) illustrate a similar bridge. This standard drawing shows a bridge that has a
pan-joist deck 28 feet wide with about a 37 degree skew and 40 feet spacing
between bents. This standard drawing indicates that the design load for each
drilled shaft to be 180 kips. The bent shown on this standard drawing, however,
contains three columns, while the bents on the Bethel Road bridge only have two.
This means that design load on the Bethel Road bridge will most likely be higher
than 180 kips. For the purposes of this investigation, the original design load was
conservatively assumed to be 180 kips.
A nine-inch thick uniform concrete overlay will impose approximately 67 kips to
each drilled shaft. By comparing this load with the likely original design load, the
new concrete overlay load is about 25% of the design load. It is doubtful that the
existing bridge was designed for an additional 25% capacity. Therefore, we do
not recommend placing any additional concrete overlay on top of the existing
deck.
2. Sidewalk Options
Below are two options for adding new sidewalks to the existing bridge, together
with the pros and cons of each.
Option A - Cantilever new sidewalk from existing bridge bents
This option involves extending the existing concrete bent cap with cantilevers to
support the new pan joist girders, sidewalk and side rail, as shown in the attached
sketch (see attachment 1 - Option A). The bent cap would be deepened and
reinforced with carbon fiber reinforced laminates. The exterior column would also
be strengthened with carbon fiber reinforced laminates.
Pros:
· No new drilled shafts or columns would be needed, thus avoiding setting up a
drilling rig in the water and all the associated environmental hazards.
· Aesthetically pleasing approach.
Freesc And Nichols, [nc · 1701 N. Market St, Suite 500 LB51 · Dallas, Texas 75202
I-elephone: 214-920-2500 · Fax: 214-920-2565
Cons:
· The biggest negative is that we cannot verify the load-carrying capacity of the
drilled shaft foundation, and therefore risk overloading the fotmdation with this
approach.
· Extensive and expensive non-destructive investigation of the bent cap and
column reinforcing would be required.
· Since the existing bent cap would be extended, cast-in-place pan joists to match
the existing superstructure construction must also be used. This will be more
expensive than using precast concrete beams.
· Construction would be complicated (especially dowelling into existing
concrete) and therefore expensive.
· May require closing the entire bridge or half of the bridge at two stages during
construction for strengthening the existing bent caps.
Option B - Support the new sidewalk with separate beams, columns and
drilled shafts
This option involves constructing essentially a new parallel bridge, with new
columns, beams and drilled shafts, as shown on the attached sketch (see attachment
2 - Option B). The new columns would line up with the existing.
Pros:
· No non-destructive investigations or structural analysis required for the existing
bridge.
· Since the new separate bent cap does not have to match the existing bent cap
elevation, precast Type C concrete beams could be used instead of the more
expensive cast-in-place pan joists.
· Entire bridge or at least one lane of the existing bridge can remain open during
construction.
· New sidewalk loads will not impact the existing bridge. No strengthening of
the existing bridge is required.
· Simpler construction.
Cons:
· Drilling operations in the water below would be necessary.
· Precast beams would be about 8-inches deeper than the existing pan joists.
This difference in beam depth does not cause an increase in the existing 100-
year base flood elevation.
· This approach would not be as aesthetically pleasing (although it would not be
unattractive).
We recommend Option B, mainly since it is simpler (and therefore less expensive)
and since the existing drilled shaft foundation most likely cannot be strengthened to
receive the additional loads. See attachment 3 for the Opinion of Probable Construction
Costs - Support the New Sidewalk with Separate Beams, Columns and Drilled Shafts.
Freese And Nichols, Inc. · 1701 N. Market St,, Suite 500 LB51 · Dallas, Texas 75202
Telephone: 214-920-2500 · Fax: 214-920-2565
Our additional fee for evaluating these options is $8,262 which was included in the
change order. This fee does not include any non-destructive testing that would be
necessary should the City select Option A above for the new sidewalk addition.
In April of this year we submitted cost estimates for four bridge options. Since that time
quantities have been refined and cost estimates were recalculated for Option 1 (to close
the road) of the previous cost estimates. Based upon current information we have
concluded the new cost estimates for reconstruction of the Bethel Road bridge to be
$591,105. This would require the road be closed during the duration of the bridge
construction phase. Access would be allowed up to the bridge; however all traffic would
need to be detoured during construction. See attachment 4 - Opinion of Probable
Construction Cost for Bridge Reconstruction (Closed Road Option).
In conclusion, we do not recommend adding an additional concrete overlay to the bridge
and we propose to design the new sidewalks with a new independent support system.
Based upon our recommendation of having the existing bridge in place and adding
sidewalks to both sides of the bridge, the bridge modification will be $341,100.
I look forward to discussing these recommendations with you. Should you have any
questions please do not hesitate to contact me at 214.217.2217.
Freese And Nichols, lnc · 1701 N. Market St., Suite 500 LB51 · Dallas, Texas 75202
Telephone: 214-920-2500 · Fax: 214-920~2565
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