Lecture No.1 diseño de puentes

8/10/2019 Lecture No.1 diseño de puentes

http://slidepdf.com/reader/full/lecture-no1-diseno-de-puentes 1/115

CE 5154 Introduction to Bridge Engineering

Lecture No. 1 -- Historical Overview and Introducti



Golden Gate Bridge, USA

Firth of Forth Bridge, Scotland Sunshine skyway Bridge, USA



Introduction

• MOVIE



LECTURE -1

• Bridge Definition

• Bridge type

• Aesthetics in bridge design • Factors considered in deciding bridge types

• Bridge components

• Bridge specification

• Role of Bridge Engineer

• Exposure to AASHTO code (1996) and PCPHB code (1967)



What is a BRIDGE?

•Bridge is a structure which covers a gap

•Generally bridges carry a road or railway across a natural or

artificial obstacle such as, a river, canal or another railway or

another road

•Bridge is a structure corresponding to the heaviest

responsibility in carrying a free flow of transport and is the most

significant component of a transportation system in case of

communication over spacings/gaps for whatever reason such asaquatic obstacles, valleys and gorges etc.



Bridge is the KEY ELEMENT

in a Transportation System



It Controls the Capacity of the System

If the width of a bridge is insufficient to carry the

number of lanes required to handle the traffic volume,

the bridge will be a constriction to the flow of traffic.

If the strength of a bridge is deficient and unable to

carry heavy trucks, load limits will be posted and

truck traffic will be rerouted.

The bridge controls both the volume and weight ofthe traffic carried by the transportation system.



Highest Cost per Mile of the System

Bridges are expensive. The typical cost per mile of a

bridge is many times that of the approach roads to the

bridge.`

Since, bridge is the key element in a transportation

system, balance must be achieved between handling future

traffic volume and loads and the cost of heavier and wider

bridge structure.



If the Bridge Fails, the System Fails

The importance of a Bridge can be visualized by considering the

comparison between the two main components of a highway system

i.e. a road and bridge itself.

EXAMPLE: Suppose in a road there occurs deterioration andultimately a crack, thus making a sort of inconvenience but it wont

result in stopping of the flow of traffic as traffic can pass or

otherwise a bypass can be provided. The traffic no doubt will pass

with a slower speed but in case of a bridge its flow is completelystopped incase of the failure of the bridge, that is the reason its often

called “If the bridge fails the structure fails” as the function of the

structure could no longer be served at all.



Tacoman arrows



Classification of Bridges

Steel Concrete WoodHybrid Stone/Brick

Pedestrian Highway Railroad

Short Medium Long

Slab Girder Truss ArchSuspension Cable-Stayed

Material

Usage

Span

StructuralForm

Structural Arrangement



Discussion on Classification According To

STRUCTURAL FORM

Distinctive Features of Girder Bridge

Distinctive Features of Arch Bridge

Distinctive Features of Truss Bridge

Distinctive Features of Suspension Bridge Distinctive Features of Cable-Stayed Bridges



Distinctive Features of Girder Bridges

•Widely constructed

•Usually used for Short and Medium spans

•Carry load in Shear and Flexural bending

•Efficient distribution of material is not possible

•Stability concerns limits the stresses and associated economy

•Economical and long lasting solution for vast majority of bridges

•Decks and girder usually act together to support the entire load inhighway bridges




•Arch action reduces bending moments ( that is Tensile Stresses )

•Economical as compared to equivalent straight simply supportedGirder or Truss bridge

•Suitable site is a Valley with arch foundations on a DRY ROCKSLOPES

•Conventional curved arch rib has high Fabrication and Erectioncosts

•Erection easiest for Cantilever Arch and most difficult for Tied

Arch•Arch is predominantly a Compression member. Buckling must beworked to the detail so as to avoid reductions in allowable stresses.



•Classic arch form tends to favor Concrete as a constructionmaterial

•Conventional arch has two moment resistant components :The deck and the Arch Rib.

• Near the crown of the arch and the region where SpandrelColumns are short, undesirable B.M. can occur. By using Pinended columns it can be avoided

•Space beneath the arch is less and hence danger for collision

with the Rib, specially on a highway

•Curved shaped is always very pleasing and arch is the most

successful and beautiful structure




Stone Arch Bridge Movie




•The primary member forces are axial loads•The open web system permits the use of a greater overalldepth than for an equivalent solid web girder, hence reduceddeflections and rigid structure

•Both these factors lead to Economy in material and areduced dead weight

•These advantages are achieved at the expense of increasedfabrication and maintenance costs

•Other bridge types have rendered the truss bridge types less

likely to be used due to its high maintenance and fabricationcosts.

•The truss is instead being used widely as the stiffeningstructure for the suspension bridges due to its acceptableaerodynamic behavior since the wind gusts can pass through

the truss as is not with the case in girder, arch bridges.




•It’s a light weight structure it can be assembled member bymember using lifting equipment of small capacity.

•Rarely aesthetically pleasing complexity of member

intersections if viewed from oblique direction

•In large span structures poor aesthetic appearance of the truss

bridge is compensated with the large scale of the structure. For

moderate spans its best to provide a simple and regular

structure



Distinctive Features of Suspension Bridge

•Major element is a flexible cable, shaped and supported in such away that it transfers the loads to the towers and anchorage

•This cable is commonly constructed from High Strength wires,either spun in situ or formed from component, spirally formed wire

ropes. In either case allowable stresses are high of the order of 600MPA

•The deck is hung from the cable by Hangers constructed of highstrength ropes in tension

•As in the long spans the Self-weight of the structures becomessignificant, so the use of high strength steel in tension, primarily incables and secondarily in hangers leads to an economical structure.

•The economy of the cable must be balanced against the cost of theassociated anchorage and towers. The anchorage cost may be high

where foundation material is poor



Distinctive Features of Suspension Bridge

•The main cable is stiffened either by a pair of stiffening trusses or by a system of girders at deck level.

•This stiffening system serves to (a) control aerodynamicmovements and (b) limit local angle changes in the deck. It may beunnecessary in cases where the dead load is great.

•The complete structure can be erected without intermediatestaging from the ground

•The main structure is elegant and neatly expresses its function.

•It is the only alternative for spans over 600m, and it is generallyregarded as competitive for spans down to 300m. However, shorterspans have also been built, including some very attractive

pedestrian bridges

•The height of the main towers can be a disadvantage in some

areas; for example, within the approach road for an AIRPORT



Distinctive Features of Cable-stayed Bridge

•The use of high strength cables in tension leads to economy in

material, weight, and cost..•As compared with the stiffened suspension bridge, the cables arestraight rather than curved. As a result, the stiffness is greater

•The cables are anchored to the deck and cause compressive forcesin the deck. For economical design, the deck must participate in

carrying these forces•All individual cables are shorter than full length of thesuperstructure. They are normally constructed of individual wireropes, supplied complete with end fittings, prestretched and notspun.

•There is a great freedom of choice in selecting the structuralarrangement

•Less efficient under Dead Load but more efficient in support LiveLoad. It is economical over 100-350m, some designer wouldextend the upper bound as high as 800m



Distinctive Features of Cable-stayed Bridge

•Aerodynamic stability has not been found to be a problem instructures erected to date

•When the cables are arranged in the single plane, at the longitudinalcenter line of the deck, the appearance of the structure is simplifiedand avoids cable intersections when the bridge is viewed obliquely




SPAN

Small Span Bridges (up to 15m)

Medium Span Bridges (up to 50m)

Large Span Bridges (50-150m)

Extra Large ( Long ) Span Bridges (over 150m)



Small Span Bridges (up to 15m)

Culvert Bridge

Slab Bridges

T-Beam Bridge

Wood Beam Bridge

Pre-cast Concrete Box Beam Bridge

Pre-cast Concrete I-Beam Bridge

Rolled Steel Beam Bridge



Medium Span Bridges (up to 50m)

Pre-cast Concrete Box Beam & Pre-cast Concrete I-Beam

Composite Rolled Steel Beam Bridge

Composite Steel Plate Girder Bridge

Cast-in-place RCC Box Girder Bridge

Cast-in-place Post-Tensioned Concrete Box Girder

Composite Steel Box Girder



BOX GIRDER



Large Span Bridges (50 to 150m)

Composite Steel Plate Girder Bridge

Cast-in-place Post-Tensioned concrete Box Girder

Post-Tensioned Concrete Segmental Construction

Concrete Arch and Steel Arch



Extra Large (Long) Span Bridges

(Over 150m)

Cable Stayed Bridge

Suspension Bridge




Structural Arrangement

•Main Structure Below the Deck Line

•Main Structure Above the Deck Line

•Main Structure coincides with the Deck Line

The classification of the bridge types can also be according to

the location of the main structure elements relative to the

surface on which the user travels, as follows:



Main Structure Below the Deck Line

Arch Bridge

Masonry Arch

Concrete Arch

Inclined Leg Frame Arch

Rigid Frame Arch

Truss-Arch Bridge Steel Truss-Arch

Steel Deck Truss



Main Structure Above the Deck Line

Suspension Bridges

Cable Stayed Bridges

Through-Truss Bridge

M i St t C i id ith th



Main Structure Coincides with theDeck Line

Girder Bridge

Slab (solid and voided)

T-Beam (cast-in-place)

I-beam (pre-cast or pre-stressed

Wide-flange beam (composite & non-

composite

Concrete Box (cast-in-place, segmental

& pre-stressed

Steel Plate Girder (straight & haunched)

Steel box (Orthotropic deck)



FACTORS CONSIDERED IN DECIDING

BRIDGE TYPE

•Geometric Conditions of the Site

•Subsurface Conditions of the Site •Functional Requirements

•Aesthetics

•Economics and Ease of Maintenance

•Construction and Erection Consideration

•Legal Considerations

In general all the factors are related to economy, safety and

aesthetics.



Geometric Conditions of the Site

•The type of bridge selected will always depend on the horizontal

and vertical alignment of the highway route and on the clearances

above and below the roadway

•For Example: if the roadway is on a curve, continuous box girders

and slabs are a good choice because they have a pleasing

appearance, can readily be built on a curve, and have a relatively

high torsion resistance

•Relatively high bridges with larger spans over navigable

waterways will require a different bridge type than one with

medium spans crossing a flood plain

•The site geometry will also dictate how traffic can be handled

during construction, which is an important safety issue and must be

considered early in the planning stage



Subsurface conditions of the soil

•The foundation soils at a site will determine whether abutments and

piers can be founded on spread footings, driven piles, or drilled shafts

•If the subsurface investigation indicates that creep settlement is going

to be a problem, the bridge type selected must be one that can

accommodate differential settlement over time

•Drainage conditions on the surface and below ground must be

understood because they influence the magnitude of earth pressures,

movement of embankments, and stability of cuts or fills

•For Example: An inclined leg frame bridge requires strongfoundation material that can resist both horizontal and vertical thrust. If

it is not present, then another bridge type is more appropriate.



•The potential for seismic activity at a site should also be a

part of the subsurface investigation. If seismicity is high,

the substructure details will change, affecting the

superstructure loads as well

•All of these conditions influence the choice of

substructure components which in turn influence the choice

of superstructure

Subsurface conditions of the soil



Functional Requirements

•Bridge must function to carry present and future volumes of traffic.

•Decisions must be made on the number of lanes of traffic,inclusion of sidewalks and/or bike paths, whether width of the

bridge deck should include medians, drainage of the surface waters,

snow removal, and future wearing surface.

•For Example: In the case of stream and flood plain crossings, the bridge must continue to function during periods of high water and

not impose a severe constriction or obstruction to the flow of water

or debris.

•Satisfaction of these functional requirements will recommend some bridge types over others.

•For Example: if future widening and replacement of bridge decks

is a concern, multiple girder bridge types are preferred over

concrete segmental box girders.



Aesthetics

•It should be the goal of every bridge designer to obtain a positive aesthetic response to the bridge type selected

•There are no equations, no computer programs or design

specifications that can make our bridge beautiful.

•It is more an awareness of beauty on our part so that we can

sense when we are in the presence of something good.

•Aesthetics must be a part of the bridge design program from

the beginning. It can’t be added on at the end to make the

bridge look nice. At that time it is too late. From the beginning,

the engineer must consider aesthetics in the selection of spans,

depths of girders, piers, abutments, and the relationship.



Economic and ease of maintenance

•The initial cost and maintenance cost over the life of the bridge

govern when comparing the economics of different bridge types.

•A general rule is that the bridge with the minimum number of spans,

fewest deck joints, and widest spacing of girders will be the most

economical.

•For Example: (1) By reducing the number of spans in a bridge

layout by one span, the construction cost of one pier is eliminated.

(2) Deck joints are a high maintenance cost item, so minimizing their

number will reduce the life cycle cost of the bridge. (3) When usingthe empirical design of bridge decks in the AASHTO (1994) LRFD

Specifications, the same reinforcement is used for deck spans up to

4.1m. Therefore, there is little cost increase in the deck for wider

spacing for girders and fewer girders means less cost although at the

“expense” of deeper sections.

i f i



Economic and ease of maintenance

•Generally, concrete structures require less maintenance than steelstructure. The cost and hazard of maintenance painting of steel

structures should be considered in type selection studies.

•One effective way to reduce the overall project cost is to allow

contractors to propose an alternative design or designs.

C i d E i C id i



Construction and Erection Considerations

•The length of the time required to construct a bridge is

important and will vary with the bridge type.

•Generally, larger the prefabricated or pre-cast members shorter

the construction time. However, the larger the members, the

more difficult they are to transport and lift into place.

•The availability of skilled labor and specified materials will

also influence the choice of a particular bridge type.

•For Example: if there are no pre-cast plants for pre-stressed

girders within easy transport but there is a steel fabrication plantnearby that could make the steel structure more economical.

•The only way to determine which bridge type is more

economical is to bid alternative designs.

L l C id i



Legal Considerations

•Regulations are beyond the control of an engineer, but they arereal and must be considered.

Examples of certain regulations are as follows:

•Permits Over Navigable Waterways

• National Environmental policy Act

•Department of Transportation Act

• National historic preservation Act

•Clean Air Act

• Noise Control Act

L l C id ti




•Fish and Wildlife Coordination Act

•The Endangered Species Act

•Water Bank Act

•Wild and Scenic Rivers Act

•In addition to the environmental laws and acts defining

national policies, local and regional politics are also of

concern

L l C id ti




•Fish and Wildlife Coordination Act

•The Endangered Species Act

•Water Bank Act

•Wild and Scenic Rivers Act

•In addition to the environmental laws and acts defining

national policies, local and regional politics are also of

concern



Discussion on Bridge Components

•Common bridge components

•Components of a Girder bridge (Beam Bridge)

•Components of a Suspension Bridge



General Bridge Components

Bridge Bearings: These are supports on a bridge pier, which carry the

weight of the bridge and control the movements at the bridge supports,

including the temperature expansion and contraction. They may be metal

rockers, rollers or slides or merely rubber or laminated rubber ( Rubber with

steel plates glued into it).

Bridge Dampers & Isolators: Bridge dampers are devices that absorb energy

generated by earthquake waves and lateral load

Bridge Pier: A wide column or short wall of masonry or plain or reinforced

concrete for carrying loads as a support for a bridge, but in any case it is

founded on firm ground below the river mud



General Bridge Components

Bridge Cap: The highest part of a bridge pier on which the bridge bearings or rollers are seated. It may be of stone, brick

or plain or reinforced concrete.

Bridge Deck: The load bearing floor of a bridge which

carries and spreads the loads to the main beams. It is either ofreinforced concrete., pre-stressed concrete, welded steel etc.

Abutment: A support of an arch or bridge etc which may

carry a horizontal force as well as weight.

Expansion Joints : These are provided to accommodate thetranslations due to possible shrinkage and expansions due to

temperature changes.

C f Gi d b id (B B id )



Components of a Girder bridge (Beam Bridge)

Components of a Suspension Bridge



Components of a Suspension Bridge

• Anchor Block: Just looking at the figure we can compare it as a dead man

having no function of its own other than its weight.• Suspension girder: It is a girder built into a suspension bridge to distribute the

loads uniformly among the suspenders and thus to reduce the local deflections

under concentrated loads.

• Suspenders: a vertical hanger in a suspension bridge by which the road is

carried on the cables• Tower: Towers transfers compression forces to the foundation through piers.

• Saddles: A steel block over the towers of a suspension bridge which acts as a

bearing surface for the cable passing over it.

• Cables: Members that take tensile forces and transmit it through saddles to

towers and rest of the forces to anchorage block.

Anchor Block Movie



Anchor Block Movie



BRIDGE SPECIFICATIONS • Meaning of bridge specifications.• Need of bridge specifications.

History

Development

Lack of specification and usage of proper codes and safety

factors -------reason of failure of a structure (bridge)

Use and check of safety factors case study of wasserwork bridge

for the check of present working capacity.

Assignment: Main reason of failure for some bridge/bridges



BRIDGE SPECIFICATION• Basically the word specification stands in general for a

collection of work description upon which there is a

mutual agreement of the most experienced group of

people based upon their practical and theoreticalknowledge

• Bridge specification:

Applying the above mentioned definition, context to

bridge makes it self explanatory.



HISTORY AND NEED OF

BRIDGE SPECIFICATIONS• Early bridge were design built type contract.

• No proper specifications so contract went to lowest bidder

• Statistics of built bridges in 1870’s show 40 bridges failed per year.

• Engineers thought about a mutual ground of practice that is both economicaland general along with restricting the bidding companies to follow a course ofwork there by improving the quality of structures and forcing them tocompromise on quality which was a very common practice in case of absenceof any code or specification.



Development• First practical step was taken after the collapse of a locomotive bridge on 29th

September 1876 across Ashtabula Creek at Ashtabula.

• 1914 American Association of State Highway Officials (AASHO) was formed

• 1921 committee on Bridges and Allied Structures was organized..

• The first edition of standard specifications for Highway Bridges and IncidentalStructures was published in 1931 by AASHO.

• In 1963 AASHO became AASHTO (American Association for State Highway andTransportation Officials)

• In the beginning the design philosophy utilized in the standard specification wasworking stress design (allowable stress design). In the 1970s variation in theuncertainties of loads were considered and load factor design was introduced as analternative method.

• In 1986 the subcommittee on Bridges and structures initiated study of the load and

resistance factor design (LRFD) .• The subcommittee authorized a comprehensive rewrite of the entire standard

specification to accompany the conversion to LRFD. The result is the first edition ofthe AASHTO (1994) LRFD Bridge Design Specification.

CASE STUDY TO VISULAIZE THE



CASE STUDY TO VISULAIZE THE

IMPORTANCE OF BRIDGE

SPECIFICATIONS

Location:

Waserwork strasse, Zurich Switzerland, slab bridge modeled in CUBUSsoftware then later on modeled in SAP 2000.

Problem:

A 70 year old slab bridge (sort of cause way) was asked to be checked for thecurrent code of practice in turn checking the safety factors.

Solution:

The bridge was analyzed for the current loading situations according to thecurrent codes of practice and the results were compared with the results of theolder bridge analysis.

Result:

The safety factors were found in accordance with the older analysis and designof bridge on which it was being built.



ROLE OF A BRIDGE ENGINEER

The role of an engineer can be broadly classifiedin two major working environments.

• Consultancy Environment

• Contractor Environment



Contractor Environment• On site decision making keeping in mind factors such as cultural

& environmental factors etc

• Quality assurance to the consultants there by working up to theneeds of clients

• Be economical to the contracting firm along with not making acompromise on quality.

• Proper time management and scheduling of works without unduedelays.

• Beneficial use of labors at various important locations of bridge.



CASE STUDY• LOCATION:• Arachtos, Greece.

• Arachtos bridge pier design for construction phase modeled in SAP 2000.

• Problem------Counter acting the forces just introduced for construction phasedue to heavy machinery to be used.

• Solution------Attaching with a cable or some other appropriate element with thegirder end so as to take part of loads.

• Result------calculation of the percentage of loads taken by the cable element.

• Acrachtos bridge pier design for construction phase modeled in SAP 2000 afterthe introduction of cable attached to the box girder.



Definition Aesthetics and Beauty



Definition Aesthetics and Beauty

•Aesthetics is the study of qualities of beauty of an object andof their perception through our senses.

•Even if this particular aesthetic air be the last quality we seen

in a bridge, its influence nonetheless exists and has an

influence on our thoughts and actions. ( Santayana )

Qualities of Aesthetic Design



Qualities of Aesthetic Design“ There are not HARD & FAST rules or formulas for aesthetics of bridge

design. It finally gets down to the responsibility of each designer on each

project to make personal choices that will lead to a more beautiful

structure “

•Function

•Proportion

•Harmony

•Order & Rhythm

•Contrast & Texture

•Light and shadow

Function



Function•For a bridge design to be successful, it must always safely perform its

function.

•For example, a bridge is designed that fulfills every requirements of

aesthetic consideration and other requirements such as economy,

constructability etc. but is somehow unable to perform the function for

which it was designed, then however beautiful it is, it won’t be

appealing.

•The very first notion of beauty in a bridge is that it performs its

function efficiently and people using it are satisfied.

•Moreover, the IMPORTANCE of function also enhances theBEAUTY or AESTHETICS of the BRIDGE.

•For Example: A bridge across straits of Bosporus at Istanbul. This

bridge replaces a slow ferry boat trip, but it also serves the function of

connecting two continents (Asia and Europe).

Proportion



Proportion

•Good proportions are fundamental to achieving an aesthetically

pleasing bridge structure

•It is generally agreed that when a bridge is placed across a relatively

shallow valley, the most pleasing appearance occurs when there are

an odd number of spans with span lengths that decrease going up theside of the valley.

•The bridge over a deep valley again should have an odd number of

spans, but should be of equal length. And slender girders and the tall,

tapered piers can add to the aesthetic pleasure

Proportion



•Another consideration is the proportion between piers and girders.From strength viewpoint, the piers can be relatively thin compared

to the girders. However, when a bridge has a low profile, the visual

impression can be improved by having strong piers supporting

slender girders.

•Slender girders can be achieved if the superstructure is made

continuous. Infact, the superstructure continuity is the most

important aesthetic consideration

•The proportions of a bridge change when viewed from an obliqueangle.

Proportion

Harmony



o y

•Harmony means getting along well with others. The parts of the

structure must be in agreement with each other and the whole structure

must be in agreement with its surroundings.

Harmony between the elements of a bridge:

•It depends on the proportions between the span lengths and depth of

girders, height and size of piers, and negative spaces and solid masses.

Harmony between the whole structure and its surroundings

•The scale and size of a bridge structure should be relative to its

environment.•For Example, a long bridge crossing a wide valley can be large

because the landscape is large. But when a bridge is placed in an urban

setting, the size must be reduced.

Order and Rhythm



Order and Rhythm

•Repeating similar spans too many times can become boring andmonotonous

•It can also become aggravating to be driving down the interstate

and seeing the same standard over crossing mile after mile. The

first one or two look just fine, but after a while a feeling offrustration takes over the pleasing affect of however the beautiful

the construction.

Contrast and Texture



•There is a place for contrast, as well as harmony in bridge

aesthetics.

•All bridges do not have to blend in with their surroundings. “

when a bridge is built in the middle of the country, it should

blend in with the country side, but very often, because of its

proportions and dynamism, the bridge stands out and dominatesthe landscape”

•The dominance seems to be specially true in case of Cable-

stayed and suspension bridges.

•There can also be contrast between the elements of a bridge to

emphasize the slenderness of the girders and the strength of the

piers and abutments.





•Texture can also be used to soften the hard appearance ofconcrete and make certain elements less dominant.

•Large bridges seen from a distance must develop contrast

through their form and mass, but bridges with smaller spans

seen up close can effectively use texture.


Light and Shadow



Light and Shadow

•Designer must be aware of how the shadows occur on thestructure throughout the day

•If the bridge is running north and south the shadows will be quite

different than if it is running east to west.

•For Example: When sunlight is parallel to the face of a girder or

wall, small imperfections in workmanship can cast deep shadows.

Construction joints in concrete may appear to be discontinuous

and hidden welded stiffeners may no longer be hidden.

•One of the most effective ways to make a bridge girder appear

slender is to put it partially or completely in shadow.

Light and Shadow



•Creating shadow becomes especially important with the useof solid concrete safety barriers that make the girders look

deeper than they actually are.

•Shadows can be accomplished by cantilevering the deck

beyond the exterior girder.

•The effect of shadow on a box girder is further improved by

sloping the side of the girder inward.

Light and Shadow



End of show

• Construction & history of Brooklyn Bridge

• Construction & history of Golden Gate Bridge



GIRDER BRIDGE



GIRDER BRIDGE

GIRDER BRIDGE



GIRDER BRIDGE



Truss Bridge



Truss Bridge

Truss Bridge



Truss Bridge



ARCH BRIDGE



ARCH BRIDGE

ARCH BRIDGE



ARCH BRIDGE



Suspension Bridge

Lecture No.1 diseño de puentes

Documents

Transcript of Lecture No.1 diseño de puentes