Design and Construction of the Ayuntamiento 2000 Bridge

Design and Construction of the
Ayuntamiento 2000 Bridge,
Cuernavaca, Mexico
Ing. Luis Felipe Cruz [.esbros
Universidad Nacional Autónoma de
Mexico (UNAM)
Grupo Previ, SA. de C.V.
Santiago de Querétaro, Mexico
Ing. René Carranza Aubry
General Director
Servicios y Elementos Presforzados,
S.A. de CV. (SEPSA)
Mexico City, Mexico
Dr. Eduardo Reinoso Angulo
Instituto de lngenierIa
Universidad Nacional Autónoma de
Mexico (UNAM)
ERN Ingenieros Consultores S.C.
Mexico City, Mexico
58
The Ayuntamiento 2000 Bridge in Cuernavaca,
Morelos, Mexico, is a six-span, 200 m (660 ft) long,
four-lane structure that provides a fast and efficient
throughway between Cuernavaca proper and the
city’s west side. The bridge superstructure
comprises nine prestressed concrete box beams for
each of the six spans, precast concrete segmental
bridge piers [the tallest of which is 42 m (138 ft)],
and cast-in-place concrete for the foundation,
foundation-to-column connections, bridge slab,
sidewalks, and abutments. The City of Cuernavaca
is located on several hills and is divided by eastwest running gorges and a river very challenging
site conditions for infrastructure construction. Labor
and equipment concerns, cost and speed of
construction mandates, and difficult topography
were successfully addressed with the precast,
prestressed concrete design and construction. The
bridge was complete and open to traffic in less than
5 months, at a cost of US$2.5 million.
—
uernavaca is known as the “City of the Eternal
Spring” and is the capital of the small Mexican state
of Morelos. An historic city dating back to the
Aztecs, Cuernavaca is located about 90 km (60 miles) from
Mexico City, and stretches over several hills, bordered by a
series of gorges that run from east to west. Most of the eastwest roadways and highways, built parallel to the gorges,
are wide and straight, but the north-south throughways in
most cases are narrow, winding, and steep, creating very
slow driving conditions.
For many years, there had been a need to build a bridge
and more direct road system to provide a fast and efficient
means of traversing the city center and the west side. The
Ayuntamiento 2000 Bridge is an integral part of a major
C
PCI JOURNAL
(!STOTAL LENGTH-161.SZm (530 ft)
20270,87(I)
30.27 ,o (100 II)
35.00
,,,
(115
f
25.270,83111
25.54 0,84 (II
1
25.270,181(11
EXPANSION JOINT
EIEV.
1529.00,
cs—s
CS—S
ELEV.= (522.700,
C3—E
04-I
:7.830,
ELEV,
SLOPE
=
7.5%
Fig. 1. Profile elevation of the Ayuntamiento 2000 Bridge, showing six spans.
transportation project to fulfill that
need (see Fig. 1).
This six-span structure was built in
just four and a half months at a cost of
US$2.5 million, using prestressed con
crete box girders and segmented pre
cast concrete bridge piers. To the au
thors’ knowledge, the Ayuntamiento
Bridge is the first bridge in Mexico
using precast columns higher than 40
m (130 ft). Typically, only about 60
percent of short-span bridges in Mex
ico use precast concrete systems, with
less use in long-span bridges (15 per
cent); however, less than 1 percent
have been built with a total (columns
and beams) precast system. This paper
describes the design and construction
details, the project challenges, and the
benefits of the Ayuntamiento precast,
prestressed concrete bridge system.
BRIDGE DESCRIPTION
The Ayuntamiento 2000 Bridge is a
straight vehicular bridge 200 m (660
ft) long and 18 m (59 ft) in total width,
including four vehicular lanes and two
pedestrian sidewalks. The bridge is di
vided into six spans with lengths that
vary from 20 to 35 m (66 to 115 ft).
Because it crosses a relatively deep
gorge, the height of the bridge piers
varies from 12 to 42 m (39 to 138 ft).
Each span comprises nine precast, pre
stressed concrete box girders, each
1.35 m (4.4 ft) deep.
Over the precast box girders is a 150
July-August 2003
—
—
-.-
Fig. 2. Looking north, an aerial view of the south face of the Ayuntamiento 2000 Bridge.
mm (6.0 in.) thick continuous cast-inplace (CIP) concrete topping, designed
as a compression-tension element and
as a wearing surface. The elimination
of joints in the continuous topping
makes driving very smooth, dramati
cally reduces the amount of required
bridge maintenance, and provides
durability to the bridge superstructure.
The north end of the span is 15 m
(49 ft) higher than the south end, re
sulting in a longitudinal incline of 7.5
percent (see Fig. 1). The longitudinal
grade, along with the standard 2.0 per
cent transverse grade, allows rainwater
to drain quickly and efficiently toward
the southwest corner of the bridge (see
Fig. 2).
Two end abutments and five interior
bent piers compose the bridge sub
structure. A strip foundation, four pier
columns, intermediate strength beams,
and caps form each interior bent pier.
The anchorage end abutment of rein
forced concrete (Pier 1), the expansion
abutment of unreinforced concrete
(Pier 7), and the interior precast con
crete piers (Piers 2 to 6) range in
height from 12 to 42 m (39 to 138 ft).
Each one of these piers was formed, in
the transverse direction, by four hol
low rectangular segmented precast
columns. Each precast column has
cross-sectional dimensions of 2.0 x 1.5
m (6.6 x 4.9 ft) with each column box
having a thickness of 150, 200, or 300
59
BRIDGE WIDTH, OUT TO OUT
ROADWAY WIDTH
,
720
,,
=
1800
ROADWAY WIDTH
=
720
,,
BRIDGE CENTERLINE
00
1Lb
I
river that was enclosed under the
bridge site with a permanent precast
concrete culvert with a hydraulic area
2 (81 sq ft). A working plat
of 7.5 m
form for the cranes and heavy equip
ment was built over this culvert. This
river culvert also helps to protect the
foundation of the bridge at Piers 3 and
4 from river current erosion and scour
damage.
MATERIALS AND
DESIGN LOADS
,
.
‘
—f
i
L
-4-
:
..
,.
—
NzHi[ 1
/__
-------
I
‘‘
‘°
250
‘50
250
‘50
250
150
175
Fig. 3. Cross-section elevation showing total width of bridge, vehicular lanes,
sidewalks, and a typical bent (dimensions in cm).
mm (6.0, 8.0, or 12.0 in.). The precast
column segments were connected with
high performance concrete using pro
cedures developed by Servicios y Ele
mentos Presforzados (SEPSA), one of
the oldest precast/prestressed concrete
producers in Mexico.
To resist lateral forces due to seis
mic loading, each pier works as a can
tilevered beam in the longitudinal di
rection, with the maximum moment at
its base, and the pier acts as a two- or
three-story reinforced concrete frame
in the transverse direction. Fig. 3
shows the cross-section elevation of
the bridge at a typical pier.
The construction process and the
60
weight and dimensions of the ele
ments were governed by the steep to
pography and labor and equipment ac
cessibility at the project site, as well as
consideration for locating and operat
ing the erection crane, which was
rated for 115 tons (104 Mg). Due to
the relatively high soil resistance and
light weight of the precast structure,
the foundation design was entirely
above ground, with a strip footing
under each pier and abutment (see
Figs. 1 and 3). All precast element
connections, construction procedures,
and foundation specifications were
those of SEPSA.
At the bottom of the canyon runs a
Concrete with a design compressive
strength of 35 MPa (5000 psi) was
used for the precast concrete columns,
transverse post-tensioned beams, and
longitudinal prestressed superstructure
box beams. CIP concrete with a com
pressive strength of 25 MPa (3600 psi)
was used for the foundations, founda
tion-to-column connections, bridge
deck topping slab, sidewalks, and end
abutments. For the connections be
tween columns, 40 MPa (5800 psi)
concrete and 85 MPa (12,300 psi)
epoxy grout were used. All nonpre
stressed reinforcement was 420 MPa
(Grade 60) steel, and all prestressing
steel was 1950 MPa (270 ksi) low-re
laxation strand.
Live load carrying capacity of the
bridge was based on the AASHTO
HS2O truck rating procedures, revised
with a T3-S2-R4, a lorry much heavier
than the HS2O design truck (similar to
an AASHTO type 3-2 trailer truck).
The T3-S2-R4 lorry is specified in the
load capacity rating procedures of
“The Manual of the Federal Ministry
of Communication and Transporta
tion.”
For the seismic design, specifica
tions from the “Manual for Civil
Works of the Federal Commission for
Electricity” were followed. This man
ual categorizes the Cuernavaca envi
rons as a medium- to low-intensity
earthquake region, Region B (the
lower seismic plate is Region D). A
Region B zoning prescribes the seis
mic coefficient of C = 0.14, already
reduced by structural overstrength.
The bridge was built over compact,
stable soil with no adverse impact to
the underlying soils (sand-clay strata
with large boulders). The contractor
did encounter an old sand mine under
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two foundations; these sand pockets
were filled with expansive concrete
(bentonite type material). The Ayun
tamiento Bridge is considered a very
important regional infrastructure link,
so the seismic coefficient was multi
plied by a factor of 1.5 to further en
sure structural integrity. The piles were
designed as ductile elements with a
ductility coefficient of 2 (maximum
ductility coefficient is 4 as the seismic
coefficient is already reduced).
CONSTRUCTION
HIGHLIGHTS
One of the principal challenges of
building bridges over deep canyons is
the construction of the supporting
piers. Typically, the weight and over
all dimensions of tall piers tend to be
very large, presenting serious equip
ment demands and labor concerns.
Traditional on-site construction tech
niques in these challenging topograph
ical locations are usually slow and ex
pensive.
To address the canyon challenges of
the Ayuntamiento project, SEPSA de
veloped a system of constructing tall
piers with hollow reinforced precast
concrete column segments. Relatively
small precast sections, with a maxi
mum depth of 15 m (49 ft) and 60 ton
(54 Mg) weight, were fabricated at the
precast yard, located about 20 km (12
miles) from the project site. These pre
cast column sections were then trans
ported easily without special trucks or
loading concerns, and erected by rela
tively small cranes.
Precast elements were transported
by truck, with single loads limited to
30 tons (27 Mg) and pieces not ex
ceeding 12 to 14 m (39 to 46 ft) in
length. Longer sections were trans
ported one at a time. Limits on the
number and length of precast compo
nents transported to the site were due
more to the narrow streets of down
town Cuernavaca than to road weight
restrictions (see Table 1).
The bridge substructure and super
structure are supported by CIP abut
ments and strip footing foundations.
End Pier 1 is a reinforced concrete
abutment that serves as a fixed an
chorage. Because of the site’s very
hard, stable soil conditions, lateral dis
July-August 2003
Table 1. Precast components for the Ayuntamiento 2000 Bridge.
:
Shaft
Hammerhead
width/height,
width/height,
Length,
Weight,
cia
cm
tons
Element
Number
cm
Precast culvert
15
340/240
Column C2A
2
200/150
200/420
200/350
—
Column C2B
2
200/150
Column C3 lower
4
200/150
Column C3 E
4
200/250
Column C3A tipper
2
200/250
200/420
200/350
Column C38 tipper
2
200/250
Column C4 lower
4
200/250
Column C4A upper
2
200/250
Column C4B upper
—
122
7
712
22
712
24
1961
56
1392
43
1494
42
1494
40
2016
58
200/420
1494
42
2
200/25j53
1494
40
Column C5 lower
4
200/250
1150
26
Column C5A upper
2
200/250
200/420
1033
Column C5B tipper
2
200/250
200/360
r
1033
32
o
25
r2200’
200/420
1081
28
200/420
1081
26
280
2
Column C6A
Column C6B
2
Beam T-3
12
30/80
Box beam CA I
9
200/135
200/250
Box beam CA 2
9
200/135
Box beam CA 3
9
200/135
Box beam CA 4
9
200/135
L 9
Box beam CA 6
L
Note: 1 cm 0.39 in.; 1 ton 0.91 Mg.
Box beam CA 5
=
—
—
—
—
—
—
200/135
—
200/135
—
30
2055
3320
59
3058
47
2807
48
2556
42
2556
42
=
Table 2. Construction schedule.
Project milestones
Start date
Precast yard production
August I. 999
October 15, 1999
Approach roads
August 1. 1999
September 29. 1999
Precast sewer installation
August 21, 1999
Augtist 24, 1999
Precast column erection
August 26, 1999
October 2. 1999
Precast beam erection
September 1, 1999
November 7, 1999
Project complete and open to traffic
placements due to potential seismic
loading are relatively small. Strip
foundations are designed to receive
the first (base) segments of the precast
columns, with a rigid connection
formed between both elements.
The hollow columns have a rectan
gular section width of 1.5 to 2.0 m
(4.9 to 6.6 ft). In addition to resisting
all service dead, live, wind, and seis
mic loading, the thickness of the col
umn walls [150, 200, and 300 mm
(6.0, 8.0, and 12.0 in.)] was designed
so that the components would resist
all loading during fabrication, deliv
ery, and erection. Taking into account
all these loading conditions, the loca
tion of the pier strength beams was de
termined (see Fig. 3).
—
Completion date
December 15. 1999
The hollow precast column sections
provided nearly the same stiffness and
strength as a solid concrete section, but
with only 60 percent of the weight.
Lower structural weight results in a
smaller seismic impact and reduces
column and foundation dimensions.
Another design advantage of using
hollow precast columns was the avail
ability of more maneuvering space be
tween piers to properly construct the
column-to-colunm connections.
The physical properties of the pre
cast concrete elements that is, their
relatively small size and light weight
and the method of construction were
not only dictated by the steep topogra
phy and restricted accessibility at the
Cuernavaca site, but equipment capac
—
—
61
jr
-r
ity limitations of the contractor’s 115
ton (54 Mg) crane also governed the
structural system selection for this im
portant project.
The bridge owner, the Cuernavaca
City Municipal Government, required
the project to be complete and open to
traffic by the end of the year less
than five months from the start of con
struction (see Table 2). The owners
mandated a service life of 50 years, at
a cost not to exceed US$2.5 million.
—
-
Fig. 4. Base of the columns showing the structural steel to temporarily support the
columns and to protect the main reinforcement of the column. Small openings,
visible in the middle column, allow for passage of the reinforcing steel.
Fig. 5. Final stages
of the foundationto-column
connection. Note
the small window
opening to allow
drainage for the
CIP concrete
connection.
Fig. 6. First column (base) segment ready for erection showing the longitudinal main
reinforcing steel and drainage portal for the cast-in-place foundation concrete.
62
Column-to-Foundation
Connections
The first column base sections were
attached to the CIP slab foundation.
Each column has a structural steel
base designed to support the column
temporarily and to protect the longitu
dinal reinforcement that would be con
nected to the foundation reinforcing
steel (see Fig. 4).
The longitudinal column reinforcing
steel is either long enough to yield at
the column base or welded to the
structural steel. The upper reinforce
ment of the foundation passes through
small apertures formed in the columns
(see Fig. 4). Once the bottom sections
of the four columns that form each
pier are properly set in place, the rein
forced concrete foundation is then
formed and placed, resulting in a con
nection that reaches full design
strength (see Fig. 5). A window open
ing near the base of the column allows
for drainage of the concrete later
placed inside each column connection,
using admixtures that control shrink
age (see Fig. 5). These openings were
later sealed with concrete.
The advantages of this column-tofoundation procedure are as follows:
• The main column reinforcing steel
is fully connected to the foundation re
inforcement, which is a critical speci
fication in earthquake design to prop
erly transmit seismic stresses, shears,
and moments.
• The full height of the foundations
works to resist loads and is taken into
account in the foundation stability de
sign. This is very important since ver
tical loads are large.
• Shorter construction time is possi
ble, since columns are fabricated in
the precast plants at the same time as
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the bridge excavation and approaches
are built.
Pier Erection and
Column-to-Column Connections
After the first section of each pier
was connected to the foundation, all
other precast sections were positioned
and connected to the column and the
transverse beam. The practical maxi
mum limits of dimension and weight
for the precast columns (due to site
topography, accessibility, and equip
ment capacity) were determined to be
15 m (49 ft) for length and 60 tons
(54 Mg) for total weight (see Fig. 6).
Because the total height of the central
piers was 42 m (138 ft), it was neces
sary to form three-piece precast
columns.
Earthquake design dictated a seis
mic-resistant vertical column-to-col
umn connection for forming a single
42 m (138 ft) pier. Each pier, together
with its adjacent three piers, was con
nected together in the transverse direc
tion by the strip footing foundation,
transverse intermediate strength
beams, and the post-tensioned pier cap
(see Fig. 3).
Although there is no compulsory
building code for seismic connections
in Cuernavaca, common Mexican en
gineering and construction practice in
seismic regions is to avoid the connec
tion of more than 50 percent of the
longitudinal reinforcement in the same
transverse section in a column. The
column-to-column connection design,
patented by SEPSA, allows 50 percent
of the longitudinal reinforcement of
the top segment to pass through metal
lic ducts included in the bottom seg
ment, as specified and constructed by
SEPSA (see Fig. 7). The metallic
ducts with the reinforcement are filled
with epoxy grout. This connection will
work as a unit to resist shear, flexure,
and tension forces.
The remaining 50 percent of the
longitudinal reinforcement that is at
tached to the bottom segment will be
overlapped with the reinforcement of
the top segment, with the upper rein
forcing overlapping 2.0 m (6.6 ft)
more than that of the bottom overlaps.
When the column connections are
filled with CIP concrete, the longitudi
Fig. 7. Bottom section of columns ready to receive the upper sections.
Fig. 8. Upper
segment of columns
with the portal
opening visible.
The opening allows
the formation of the
CIP intermediate
transverse beams
and the projections
at the top of the
column to form the
post-tensioned
pier cap.
—
—
July-August 2003
Fig. 9. Last step of the pier construction process: The pier cap is formed by bending
the steel extending from the top column projections, using OP concrete and
post-tensioning to form the sides.
63
nal reinforcement is firmly embedded
in the concrete. This assembly proce
dure produces a rigid connection be
tween the precast column segments
and the transverse beam, thus ensuring
that the pier will maintain its structural
integrity in safely transmitting all ver
tical and lateral loads.
Pier Cap Beam
Fig. 10. Composite topping concrete runs the length of the continuous bridge and is
designed to act like a large horizontal beam.
Fig. 11. Nine
longitudinal
precast box girders
already in place at
the south end and
lower elevation of
the canyon.
:..,:
‘
The upper segment of the precast
columns has projections on both lateral
sides to make the cap, forming a ham
merhead column (see Fig. 8). These
projections have column steel that is
later cold bent in the field to form the
main cap reinforcement after the
columns have already been erected.
The gaps between the columns were
filled with a high performance CIP
concrete, leaving ducts for passage of
the prestressing steel. Once this con
crete has reached its required strength,
the tendons are post-tensioned and the
pier cap beam is formed. The pier cap
incorporates seismic devices (neo
prene bearing pads with laminated
steel) that allow displacement in both
directions.
At the end of the bridge, a stiff pier
was formed with the strip foundation,
columns, cap and intermediate beams.
The last step was placing CIP concrete
at the base and installing the elas
tomeric bearing pads, upon which the
longitudinal box girders rest.
Superstructure
Because of the very high slender
ness of the columns, the total trans
verse displacement of the bridge was
limited so that flexural moments due
to lateral forces, temperature changes,
and seismic loads were not exces
sive. Horizontal displacements were
restricted by the composite slab de
signed to act like a large horizontal
beam (see Fig. 10). The longitudinal
reinforcing steel in the composite slab
runs from Pier 1, where it is firmly at
tached, to the end of the bridge, where
it is connected to Abutment 7 with an
expansion joint.
For the erection of the box longitu
dinal box girders (see Figs. 11 to 13),
an auxiliary metallic temporary beam
(similar to a steel launching truss) was
used to guide, position, and support
—
—
Fig. 12. Completion of two spans covered by the longitudinal precast box girders.
64
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each concrete box beam. Lastly, the
compression slab, pedestrian side
walks, and all the additional finish
components were constructed (see
Figs. 14 and 15).
CONCLUSIONS
Today, it is more important than
ever to build structurally sound and
durable bridge structures that blend
harmoniously with the environment.
Precast concrete technology facilitates
the optimization of materials, efficient
production of quality structural com
ponents under controlled plant condi
tions, and minimizes environmental
damage. As a result, site work and en
vironmental effects are reduced to a
minimum, with little adverse effects
on nearby residents, businesses, or
natural resources.
The Ayuntamiento 2000 Bridge was
a success not only because of the final
structural and aesthetic qualities of the
bridge, but also because the total con
struction time was only four and a half
months. The project cost, approxi
mately $800/rn
2 ($74/sq ft), makes this
precast design a very economical solu
tion for medium-span bridges that
cross relatively deep topography. The
owners were most satisfied with the
completed project. In fact, The City of
Cuernavaca’s president distributed
photos of the completed Ayun
tamiento Bridge as part of his electoral
campaign, and he is today Governor of
Morelos State.
..
-
Fig. 13. Box girders span over post-tensioned pier cap.
Fig. 14. Final stage of construction of the Ayuntamiento 2000 Bridge.
CREDITS
Owner: H. Ayuntamiento de Cuer
navaca (Cuernavaca City Municipal
Government)
Public Works Ministry: Secretarfa de
Desarrollo Urbano Obras y Servi
cios Püblicos
Director of Public Works: Ing. Pedro
Leech Balcazar
General Contractor, Engineer of
Record, and Precast/Prestressed
Company: Ing. René Carranza
Aubry, SEPSA Chairman, and Ing.
Luis Felipe Cruz Lesbros, Research
and Special Projects Director,
SEPSA
Structural Analysis: Ing. Enrique Soto
Nachón
July-August 2003
Fig. 15. The Ayuntamiento 2000 Bridge in use with traffic and pedestrians.
65