2383

composite structure with a 4 mm thickness of the steel pipe wall allowed for corrosion. The diameter of a pile is 813 mm and thickness is 16 mm. Reinforcing bars are placed in the uppersection of the piles.

The cross section of the deck is a traditional composite structure of two open girders. The girders are 3.2 m high I- beams (Fig. 33). There are crossbeams at 7 m centres. On the crossbeams there is a 1 m high secondary longitudinal girder. In the north part of the bridge, where the deck is 30 A wide, there are two secondary girders.

The cable crossbeams are inclined to the cable direction. Between the main girders, the cross section of the cable crossbeam is an open I-beam. The girders are stiffened with steel bars. Outside the main girders, the cable crossbeam is a composite box girder.

In the competition proposal, the pylon was a steel structure. In the general design, the costs of steel, composite and concrete pylon were compared. The concrete pylon proved to be the most economical.

The pylon is formed of two T-shaped hollow concrete towers which are joined together by two crossbeams. There is an elevator inside one tower and stairs inside the other. The horizontal geometry of the road has a curvature of R = 4000 m. This caused large horizontal forces on the towers. Therefore, the pylon was not placed centrally to the deck cross section, but with an eccentricity of 0.6 m. Due to this placement, the horizontal forces, and hence, the number of steel piles decreased considerably.

The bridge has 24 factory-fabricated parallel wire cables. The cables consist of 7 mm steel wires (253-403/cable) injected with grease inside a high density polyethylene pipe that is covered with white Tedlar tape. The pylon and cables are illuminated at night.

The fixed bearing of the bridge is at the southern abutment. It is a hinged structure, where horizontal forces are transferred large steel bars.

CONSTRUCTION

The piling and the concreting of the foundation slabs and columns took 14 months. The joint welding of the steel girder sections and the launching were completed in one year. The largest concrete works, such as towers and deck slabs, were placed in summer, because the temperature in winter can be as low as -25 °C.

At the beginning of the construction period, the design for the pile foundation was changed to a caisson structure. The caissons were built in a dammed trough on the north side of the bridge. The caissons were floated into position and the piling work was done through access holes in the caisson slab. Finally, the piles and the inner part of the caisson were cast in dry conditions. Except for the slab of supports T3 and T9, all casting work on the substructure was done in dry conditions. The casting of pile slabs succeeded well and there were no faults typical of underwater concrete casting.

The steel girders of the superstructure were launched from both abutments

and joined by welding between supports T5 and T6. Three auxiliary piers had to be built for the cable:stayed spans during launching. The top of the girder was lifted on a pontoon when the spans exceeded 70 m. The launching bearings were vertically adjustable because the girder was very stiff. The longest beam sections were 42 m long and the maximum weight was almost 80 t.

The bridge was originally designed using steel grade Fe 355 E, but the steel contractor and steel manufacturer proposed to change the steel grade of the support sections of the girder to a new thermomechanically manufactured S 420 ML steel. Using this material, the contractor slightly reduced the weight of the girder and benefited from the superior welding properties of the new steel grade. The total amount of steel in the superstructure is about 50501.

The towers of the pylon were cast using a climbing formwork, with the height of one casting section being 4.15 m. The longitudinal reinforcing bars were jointed with screw sleeves.

The crossbeams between the towers and the cable anchor areas were pre: stressed. The surface of the concrete tower had to meet stringent quality requirements. A textile covering was placed on the formwork surface to enhance the strength of the concrete surface and to reduce air blisters. A green concrete paint was applied to the surface of the pylon.

Construction of the pylon was complicated because it has three inclined walls. Therefore, the scaffolding had to be adjusted at every stage. The

reinforcing of the cable anchor area was difficult because of restricted space. The temporary support structure of the upper crossbeam was also difficult to install and there were problems to stabilise it. Nevertheless, the pylon was completed on schedule and all casting work was completed before winter.

The deck slab of the superstructure was cast using two movable scaffolding units. The longest casting section was 24 m. The slab was prestressed transversely in two equal stages.

The stay cables were installed from the bottom upwards. Cable stressing was accomplished using four jacks at the level of the deck slab. The cables were stressed in pairs. The total weight of stay cables is 214 t. They were tested against fatigue and ultimate load, the test cable being the largest cable of the bridge. In the fatigue test, where a 5 m long test Cable was loaded 2 million times with a stress variation of 193 MPa, only 9 wires broke. After this test the same cable was stressed up to the breaking load, 24.6 MN.

Client:

Mikkeli Road District

Design:

Suunnittelukortes AEK Ltd, with Leonhardt, Andra and Partner

Main contractor: WT Corporation Ltd

Steel contractor: PPTH Steel Ltd

Construction duration: January 1991 to November 1993

The Pitan Bridge, Taiwan

Kwong M. Cheng				constraints and concerns that guided
Pres.,	OPAC Consulting		Engineers,	the design of the bridge, it had to:
San Francisco, CA, USA				- meet all freeway design and perfor-
Introduction				mance requirements of the Northern
				Taiwan Second Freeway
				-	be	aesthetically	pleasing,
To alleviate traffic congestion near				constructable, and economical
Taipei, in 1985 the Taiwan Area Na-				- require minimum maintenance.
tional Freeway Bureau planned the				The following factors were considered
construction of the 108 km long North-				in the selection of the bridge type:
ern Taiwan Second Freeway. The ma-				-	Structural systems issues such as
jority of the Second Freeway's align-				bridge piers, span lengths and
ments are on grade, on embankments,				arrangement, foundations, vertical and
in tunnels, or on viaduct structures. At				horizontal clearances, capacity to resist
several	locations,	however,	obstacles	horizontal forces and seismic actions,
were identified that required special				etc.
bridges. The Pitan Bridge spans one of				-	Aesthetic issues such as the overall
these areas.				appearance of the bridge, its com-
The Pitan Bridge, actually two nearly				patibility with the surrounding land-
parallel bridge structures, is located				scape and with existing built facilities
about 15 km south of Taipei. It				- Construction issues such as the con-
connects typical low-level viaducts to				struction method, equipment, con-
a tunnel. Both bridge units have a total				struction time and the capabilities of
length of approximately 800 m, a 750				local contractors
m radius curvature and a slope of				- Cost issues such as the cost of relat-
approximately 1%. The Pitan Bridge				ed	works,	of maintenance	and the
has become a landmark in this subur-				cost of the bridge itself.
ban area of Taipei.				Four bridge types consisting of varia-
SELECTION OF THE BRIDGE				tions on girder-and-arch designs were
				identified as potentially meeting the
TYPE				above design considerations.
				The selected bridge type consists of a
The design of the Pitan Bridge was				post-tensioned cast-in-place concrete
based primarily on the 1983 AASHTO				box girder bridge deck supported on
Standard Specification for			Highway	delta-shaped reinforced concrete box
Bridges, the 1987 AASHTO Design -				piers, which are joined together at the
and Construction		Specifications for		centre of the main span to form an
Segmental Bridges (No. 2 Draft), and				arch. This scheme adapts well to the
local design codes. Among the general				constraints of the site and provides a

stiff, strong bridge with a clean, effi-	structural layouts. They each consist of
cient, and very modern appearance.	nine spans totalling 781.5 m, with a
BASIC LAYOUT	160 m main span. The northbound
	bridge has an additional simple span of
	21.7 m (Fig. 35).

The two almost parallel bridge structures have nearly identical

Fig. 35

Expansion joints have been kept to a					The preliminary design made use of
minimum, located only at the extreme					simple and rational analytical tools that
ends of the structure. The four main					have proven reliable for many
bridge piers (3, 4, 5 and 6) are fixed to					structures. The following were consid-
the girder and footings with monolith-					ered at this stage of the design in order
ic, moment-resisting connections. The					to determine the dimensions of the
smaller piers and the abutment are					main structural elements of the bridge:
fixed to their footings only and support					the cross section, the longitudinal sys-
the girder through bearings that are					tem, the pier shafts, footings and piles.
fixed	transversely,		having	some	SUPERSTRUCTURE DESIGN
limited freedom-to slide longitudinally.
This combination of few expansion
joints and high pier fixity was evaluat-					Detailed design and analysis made use
ed for serviceability under the required					of more refined analytical tools in or-
seismic excitation, as well under the					der to verify the preliminary design.
influence of creep and shrinkage in the					The following tasks were performed:
concrete, and temperature change in					- development of structural models for
the	structure.	The	piers	and	service-load analysis
foundations provide enough flexibility					- evaluation of design values for mo-
so that the time-dependent strains and					ment and shear
temperature strains of the concrete,					- transverse and three-dimensional
accumulated over the entire girder					considerations
length between expansion joints, cause					- pier moments, shears, and axial
acceptably small bending stresses in					forces
the piers and axial stresses in the gird-					- thermal effects.
er.					The bridge superstructure consists of a
					single cell box girder (Fig. 36). The di-

mensions of the box girder were de-	The web longitudinal prestressing is
signed to comply with the strength re-	dimensioned to provide adequate mo-
quirement as well as provide space to	ment capacity and stress control at
house the necessary prestressing ducts.	mid-span and over the pier to resist su-
The main span girder depth varies	perimposed dead load, live load, creep,
from 8.882 m at the face of the delta-	and temperature moments. Each of
shaped pier to 3.5 m at mid-span, and	these tendons extends over the full
is prestressed longitudinally with can-	length of a span, and is anchored in the
tilever tendons in the top slab and con-	diaphragm walls over the pier.
tinuity tendons in the webs. All pre-	The deck transverse prestressing is di-
stressing is grouted in ducts embedded	mensioned to resist transverse bending
in the slabs and webs of the girder.	from dead load and live load. The gird-
	er cross-section reinforcement includes
	a grid of mild steel bars in each direc-
	tion on each face of each structural el-
	ement.
	The delta-shaped piers are designed to
	reduce the length of the main span,
	thus allowing the relatively slender
	box girder to meet the strength and
	stiffness demands on this bridge. They
	are designed as wall-type elements
	with post-tensioning to provide a
Fig. 36	residual compressive stress		under
The top deck longitudinal prestressing	combined axial force and bending for
is dimensioned to resist all structural	all service load conditions. The
dead loads during cantilevering, as	schematic prestressing layout of the
well as construction loads and the	main span is shown in Fig. 37.
weight of the travelling formwork. All	SEISMIC	ANALYSIS	AND
anchorages are located in the well-
confined and reinforced intersection	DESIGN
fillets at the tops of the webs.

Fig. 37

Initial, seismic analyses were perseismic design criteria. These analyses formed in accordance with the local provided controlling conditions for de-

sign of the piers and foundations.	and no plastic hinges develop below
A more refined analysis using earth-	the ground surface.
quake response spectrum developed	Foundations consist of 2.0 m diameter
for the vicinity of the site, was used	drilled, cast-in-place, reinforced con-
later to verify structural dimensions	crete piles, supporting reinforced con-
and reinforcement requirements.	crete footing blocks. The piles, up to
Substructure Design	14 m in length, act as friction piles
	with some limited end-bearing support.
Each pier consists of a four-walled cel-	Analysis and Modelling
lular reinforced concrete shaft, heavily
reinforced to form a ductile cellular el-
ement. All fixed piers participate in	Constructability will largely determine
providing the required capacity to re-	the success of a project. Therefore, a
sist seismic and horizontal forces. For	thirty-eight step stage-by-stage analy-
transverse seismic loads, the piers be-	sis of the bridge was performed to vali-
have as cantilevers, with significant	date the time-dependent behaviour of
moments developed at the bottom	the construction sequence. The analy-
only. For resisting longitudinal seismic	sis included:
excitation, the main piers behave as	-	development of structural computer
members in a rigid frame. The creep,	models for construction procedure
shrinkage, and temperature deforma-	-	evaluation of stresses and deflections
tions of the superstructure are ade-	at critical construction stages
quately accommodated in the mono-	- evaluation of stresses and deflections
lithic piers by the flexibility of the	over the service life of the bridge.
foundations and the pier shafts.	The bridge was modelled using a com-
The basic cross section of the main	mercially available general-purpose
piers continues through the depth of	structural analysis program which is
the box girder cross section to provide	capable of static and dynamic analysis
diaphragms capable of developing the	of finite element models for a wide
plastic moment of the pier into the	range of structure types.
girder. These diaphragms also provide	All important structural features of the
a convenient location for anchorage of	bridge were modelled, including the
the longitudinal prestressing tendons in	piers, the box girder, and the delta-
the superstructure.	shaped piers. The girder was modelled
The abutment is designed as a conven-	as a line of frame elements represent-
tional cantilever wall which supports	ing the entire cross section, oriented
the bridge girder on sliding bearings.	along the centroidal axis of the proto-
The footing blocks supporting the piers	type girder. The piers were modelled
were designed for the static dead and	similarly, including semi-rigid ele-
live impact loads and the seismic loads	ments in the pier-girder intersection.
such that the movements at the base of	The intersection zone between the
the piers are within allowable limits	delta-shaped piers and the girder was

modelled by a separate finite element	distribution between the diaphragms
model in order to capture the stress	and the webs in this area.
ROOSEVELT LAKE BRIDGE, GILA COUNTY, ARIZONA
Maurice D. Miller, Civil Eng.	portant, the deciding factor for the new
HTNB Corp., Kansas City, MO	Roosevelt Lake Bridge was flood
Crossing Roosevelt Lake	protection. When the US Bureau of
	Reclamation raised the dam by 23.5 m
	to provide additional protection to
Located in Gila County, a rugged area	Phoenix and the Valley of the Sun, it
of east-central Arizona, Roosevelt	removed State Highway 188 from the
Lake is a hydroelectric reservoir used	dam.
for flood control, irrigation and recre-	Bridge Type Selection
ation. It is midway between the cities
of Globe, which is the county seat, and
Payson, in a ranching and recreational	Located about 305 m upstream of the
area. State Highways 88 and 188 con-	dam, the new Roosevelt Lake Bridge is
nect the two major cities. From Globe,	just below the confluence of the Salt
Route 88 winds northwest through	River and Tonto Creek. Here, the Salt
desert canyons and along the south-	River just begins the erosion of its
west shore of Roosevelt Lake to a ma-	canyon through the Mazatzal Moun-
sonry dam, where it connects with	tains. The steep-walled features of the
Route 188 and continues west and	canyon were ideal for the construction
north to Payson.	of the world's highest masonry dam,
The one-lane road traversing the dam	creating a lake 300 m wide and 60 m
across Roosevelt Lake had not been	deep at the proposed crossing.
improved since construction of the -	The Arizona Department of Trans-
dam in 1911. Travel across the dam	portation based its selection of the best
was slow and treacherous, with abrupt	type of bridge for the crossing on eco-
90° blind corners. When two cars met	nomics. Because of the steep valley
on the dam, one had to yield the right-	walls and the deep valley floor, under-
of-way and back up, a gesture that was	water foundations were determined to
not always made willingly by the local	be too costly. The two-lane bridge
ranchers.	would have to span the lake with foot-
Because of the narrow and crooked	ings on each shore. Two engineering
route over the dam, vehicles over 9 m	firms prepared complete construction
in length were prohibited. Commercial	plans for two alternatives: a steel box-
traffic between Globe and Payson was	rib-through-arch and a concrete stayed
forced to detour through Phoenix,	girder bridge.
adding hundreds of miles to the route.	The box-rib-through-arch was selected
Although traffic congestion was im-	for the steel alternative based on an in-

depth study of three steel bridge types:	box section varies from 2.44 m at the
trussed-through-arch, box-rib-through-	crown, where compressive forces are
arch, and cable-stayed girder. The	the smallest, to 4.27 m at the base,
twisting alignment required for the	where compressive forces are the
mountainous terrain made it difficult to	greatest. The shape of the arch rib was
get a tangent alignment long enough to	also established to minimize the dead
accommodate minimum-length back	load bending stress in the arch. The re-
spans for the cable-stayed and trussed	sultant shape is defined by both second
arch bridges.	order and fourth order curves, making
The arch span was the most economi-	the entire arch appear smooth and
cal. It adapted readily to the curved	continuous.
alignment of the approach structures	The steel arch rib section is made up of
and had superior aerodynamic proper-	two 1.2 m-wide flanges with two web
ties. The welded steel arch was the	plates that vary from 2.44 m at the
overwhelming favorite of most	crown to 4.27 m at the base plate. The
bidders. It was bid at 30% less than the	webs of the box section are stiffened
concrete alternative.	by two T-sections welded to each web.
Arch Rib Design	The stiffeners are continuous and par-
	ticipate fully in carrying the various
	loads. Sixteen longitudinal fillet welds,
Existing terrain and future lake levels	over 13.4 m long, were required to
were important factors in establishing	connect the flanges and the stiffeners
the arch rib geometry. The arch spring-	to the webs.
line is at Elevation 649.5 m, well	Stability against global buckling and
below the 200-year flood elevation of	transmission of wind loads for the arch
663 m. To keep the steel arch above	rib, which is 15.2 m center-to-center,
water, the lower 158 m of the arch rib	was achieved through the use of a K-
is made of concrete. The resulting	bracing system fabricated from struc-
composite steel-concrete structure	tural tubing and welded box sections.
consists of the following sections: 21.6	The selection of the K-bracing was
m of concrete rib, 323 m of steel rib,	based on both first-cost and life cycle
and 21.6 m of concrete rib, for a total	costs. The K-bracing required 901 less
arch rib length of 366.4 m and an arch	steel and reduced wind deflections to
span of 329 m.	half as much as those caused by a
The steel and concrete portions were	comparable Vierendeel system. The
made continuous by anchoring a base	closed tubular sections were selected
plate at each of the steel-concrete con-	to minimized corrosion due to
nections. This required 32 MN of pre-	entrapment of water and debris. To to-
stress force at each base plate.	tally close the ends of the tubular
The geometry of the entire arch rib op-	shapes, horseshoe-shaped plates were
timizes the use of material through the	welded to the ends of the tubes.
length of the rib. The depth of the steel

At the portals, the K-bracing was terminated with a three-dimensional X- shaped connection. The multi-jointed, three-dimensional intersection of the bracing was achieved by welding plates into an octagon and welding the ends of the tubes to the sides of the octagon. More than 760 m of fillet welds were used to make up the bracing.

Another part of the bracing system is a 2.1 m square welded box strut between arch ribs directly below the roadway at each end of the bridge. Access to the interior of the ribs is through lockable doors in these boxes. Electrical distribution centers are also inside the struts. The floor system for the 11.5 m wide roadway consists of longitudinal stringers, supported on welded I-girder floor beams. Floor beams are spaced at 15.2 m centers and are hung from the arch ribs with eight wire ropes, four at each end of a floor beam. Welded steel girders 2.0 m deep are used as exterior stringers and also function as stiffening girders for aerodynamic ''stability. About 2.1 m of fillet welds were required to fabricate the floor system. The overall stiffness of the system - arch ribs, bracing, and floor - was verified by wind tunnel tests.

Erection of the bridge was accomplished

using temporary towers resting on the arch footings and a system of bridge strand

backstays and forestays. Hydraulic jacks in

the stays allowed alignment adjustments to be

made during erection.

The erection progressed by lifting and bolting successive 15.2 m long welded steel rib sections into place with a barge-mounted crane floating on the lake. As adjacent rib sections were placed, the welded tubular K-bracing was installed to maintain alignment and rigidity of the ribs. Shop fabrication accuracy was maintained so well that there were no mismatches during erection.

Owner:

Arizona Dept of Transportation

Engineers:

HNTB Corp., Kansas City, MO

Main contractor:

Edward Kraemer & Sons, Inc., Plain, WI

Service date: 1990

ERECTION

	BRIDGE SCOUR FAILURES	Introduction
	D.V. Mallick M.M.Tawil
	Technical Advisors	Failure of a bridge is viewed seriously
	National Consulting Bureau, Tripoli,	by the public since it involves not only
	Libya	traffic disruption and the loss of tax
		payer's money but can also result in