bridge erection techniques and their influence on permanent des

8/2/2019 Bridge Erection Techniques and Their Influence on Permanent Des

1/9

Bridge Erection Techniques and their Influence on the Permanent Designs Er. V. G. Abhyankar

National workshop at COEP, Pune on Innovation in Bridge Engineering 15-16th

Oct2011Under (Late) Shri S. B. Joshi Memorial Activity

Page - 1 of 9

Bridge Erection Techniques and their influence

on Permanent Designs

Paper by Er. Vivek G. Abhyankar C.Eng.

Mumbai

Abstract :

Bridge engineering has significantly grown-up in past 50 years because of the increasing

infrastructure demands and the developments in other peripheral branches of engineering. Many

new complex bridges are coming up in reality, now. But this has resulted into a few problem areas

also. Unlike in past, the development that is taking place today, in the design of bridges, is not going

hand-in-hand with the Construction (Execution) technology (especially in developing countries like

India). In fact, in most of the modern bridges it is observed that unless the Construction methods are

excelled, there is no point in adopting any modern theory / software in the bridge design. Theincreasing infrastructure demands necessitate faster, safer, economical, yet superior quality of

erection of bridge superstructure. In certain cases, the modern bridge erection technology imposes

significantly larger loads than the design loads. On the other hand, the construction methodology

has to confirm the design requirements.

In the present paper the author has attempted to present the linkage between the Bridge

erection technology and the Permanent design of bridges with certain live case studies.

Key Words :

Enabling works, Permanent Works, Bridge Super structure, Arch Bridge, Segmental Construction, Launching Girder,

Floating Crane, Cantilever form traveller (CFT), Moving scaffolding system (MSS), Construction stage analysis,

Construction loading, stability, Curved Girders, HSE, Santiago Calatrava, Firth of Forth Bridge, Steel Arch,Cantilever Bridge, Secondary Stresses, Cable Crane (Cable way), JV

Introduction :

Bridges are the backbones of any Transpiration / Infrastructure Project (Railway / Road).

Bridge Engineering has a history of thousands of years, and finds its origin in Indian Mythology

and spiritual scripts like Ramayana / Mahabharata. In those days bridges made of hip of stone blocks/ bunds / timber logs were the only options possible. The old bridges were expected to merely act as

a beam spanning across the gaps along the road / railway alignments. But as the growth of

civilisation (society / cultural development) started taking place, the demand on all the branches of

engineering (industry) increased by exponentially. The increased demand called for more and more

research that started taking place. From study of engineering growth in last hundred years, we may

observe that the development of Civil / Structural engineering is closely associated with the

developments that took place in the peripheral branches like Material Science, Mathematics,

Communication, Electronics, Computing technology (computer science), IT, and the Newly

emerging branches of engineering.

As all the branches of engineering started growing, it was observed that the growth was rapid, butnot parallel! This fact resulted into a serious problem or a gap between imagination (Conceptual


2/9




Page - 2 of 9

planning / Design) and the ground-reality (i.e. actual bridge erected at sites). The end results of this

gap (rather a knowledge gap) and the solution to bridge this gap is discussed in the further sections.

Bridge Projects in Olden days :

In olden days the computing technology / simulation software etc. were not available. Engineers hadto struggle hard to prove the adequacy of the concept, before starting the design and construction.

Fig.1 shows, photograph of Firth of Forth Bridge, Scotland, which is a steel cantilever bridge. It

was built in the 1880s by Allan Stewart and Sir Benjamin Baker. But both of them had to struggle

hard to demonstrate by a Conceptual Model (as shown in Fig.1 below) to the sceptical Victorians

that a cantilever bridge would be safe. Fig. 1 also shows the actual bridge after construction.

Fig.1 Firth of Forth Bridge, Scotland (A Cantilever Bridge)

Designed by Allen Stewart and Sir Baker

Similar to above bridge Fig.2 shows another interesting configuration of a Suspension cable bridge

done out of India. In this structure the main pylons look like a posture of a standing person, recliningback slightly, with both the arms folded, kept on waist.

Fig.2 Another innovative bridge which had abutments like a racking human body

Introduction to New Types of Bridges : A Challenges for Designers :

Unlike 50 / 60 years back, the computing technology, materials sciences have lead to converting

even the complex shaped bridges in the reality. Fig.3 shows two a beautiful String Cable stayed

bridge designed by Famous Architect and Structural Engineer Santiago Calatrava. During

conceptual planning and the design of such modern bridges, often various challenges are faced. A

few of these challenges are as listed ahead.


3/9




Page - 3 of 9

The various challenges that are faced are as listed below :-

Complex Loading temperature, wind, erection, blast etc. and their combinations

Complex Structural Geometry difficult load transfer mechanism

Refined understanding about the Structural Behaviour failure theories, overall performance

based design, reliability analysis as compared to Limit state vs. Working stresses Changing Codes / Standards requirements QAQC, Crack width, exposure, non-linearity etc.

Advances in associated fields like :-

Material Science (Concrete-steel-etc.)

Electronic devices (Strain Gauges), monitoring

Surveying Instruments

Increasing Project demands (Time, Cost, Safety, Quality, Aesthetics)

New research towards Bridge execution / Testing

HSE (health safety and environment) Requirements

Construction Stage Analysis

Structural Health monitoring systems

Life expectancy . . . etc.

Fig.3 Two beautiful String Cable stayed bridge

The Architect / Structural Designer / Planner or Execution engineer can not overcome all these

problems by working in isolation. A complete Team Work is essential to solve these problems and

to converge to an effective solution. But, from the failures of many ambitious bridge projects, it can

be observed the there is clear absence of the team work (except a few cases). Typical failure of

bridge may happen in case the designer dont know how the contractor is going to construct that

structure, and on the other hand, even if the contractor do not know how that bridge is designed

(boundary conditions, assumptions, standard design practices etc.). The typical reasons of failures

are (a) Insufficient Formwork, (b) Inadequate Launching Girder, (c) Improper execution, (d)

Unexpected forces (wind / Blast) (e) misunderstanding between Designer-planner and the site

personnel etc.

Fig.11.a shows a typical failure of Bridge staging at Vietnam during construction stage itself. In

certain cases, wrong sequence of pre-stressing exerts a huge / concentrated or even eccentric force

for the supporting formwork / staging (for which usually the formwork may not have been

designed), leading to collapse. In such cases after the collapse of structure occurs no one can exactly

trace out where the things went wrong, i.e. whether the structure was designed wrongly or executed

wrongly. Unless thorough investigations are made, one can not conclude in such cases. This explains

that the work of designer, planner and the execution (site) engineer shall be complementary to each


4/9




Page - 4 of 9

other and not the contradictory. Thus each one of them must know the works of others (their

counter-parts) to a sufficient extent.

Introduction to various Bridge Erection Techniques : A Challenges for Execution

From the above discussion it is clear that the complexity in Bridge Engineering is increasing, whichcan not be handled alone by the bridge designer or site engineer. The Designer must know the

proposed method, which the contractor wants to employ for the construction site of the said bridge;

and on the other hand the execution person and planner must know briefly, how the designer has

designed the structure, with its Limiting conditions / Boundary Conditions / Assumptions. In certain

cases, the execution of design, as it is, become difficult due to site specific conditions (probably,

which could not have been addressed by the Structural Engineer at design stage). In such cases the

modification in design becomes necessary. If the contactor knows the design basis, he can develop a

better proposal indicating desired changes in design at a particular spot in the project. (On the other

hand even, if the designer knows the constraints at site, in which the site engineer is supposed to

work, he can develop better design, right from construction stage).

A few of the popular methods adopted for construction of bridge super structure are as listed ahead :-

Erection using Traditional Methods

Using Formwork

With land based hydraulic cranes (single / double cranes for (i) span-by-span

construction or even (ii) random span construction

Erection using Modern Methods

Floating Cranes / Barges / Jack-up platforms

Using Bridge Launching Girders

Cable Cranes

CFT (Cantilever Form Traveler) / MSS (Moving Scaffolding System)

Tower Carnes

Bridge erection using a Floating Crane :

In case of long span bridges, Launching

Girder (LG) is commonly used by the

contractors (ref. Fig.6 and Fig.7, which

shows a typical Truss type Bridge

Erection Launcher, used at Bang-Na-Trade Highway Project, Bangkok). But

if the bridge is passing over a large water

body it is convenient to use floating

cranes kept on the Barges / vessels /

jack-up platforms. Fig.4 shows such

typical floating crane (and Fig.5 shows

bridge erection using a single land based

crane in segmental bridge). But during

use of such methodology there are points

to ponder as listed ahead.

Fig.4 Showing A typical floating Bridge erection Crane


5/9




Page - 5 of 9

Fig.5 Bridge erection with single

Points to Ponder :

1) Duration of Project

2) Capacity of Floating Cart

3) Availability and Characteristics of water way4) Facts of actual Site :

Casting Yard location

Wind Speed

Tidal variations

5) Cost / Budget

6) Cooperation from Designers / Design capacity

Bridge erection using a Launching Girder :

From Fig.7, which is showing Bridge Erection Launcher,

used in Bangkok, it may be seen that the Permanent structurehas good Aesthetic look, which is not only utilized by the

execution engineer to place the LG, but also the impact of

LG on the permanent structure. Also, from Fig.8, one may

observe the necessity of stability of erection system during

girder erection. Faulty system may lead to incorrect bridge geometry or even accumulation of

secondary stresses in the girder body /distortion. Apart from these, there could be accidents leading

to damage to structure or human life (as shown in Fig. 10, 11).

Fig.6 Showing Schematic Diagram of Bridge launcher

Apart from the regular bridge erection techniques like Formwork / Staging, Launching Girders and

advanced technique of Floating Cranes, A suspended erection crane / cableway is also used now

days as shown in Fig. 9, for a steel Arch Segmental Bridge in China. In such cases the Permanent

structure has to be designed keeping in mind the construction / erection and dismantling

methodology. The construction scheme / methodology imposes large loads on the permanent

structure, which are often greater than the Design loads even (i.e. critical combination of self wt,

Live load, wind / earthquake etc.).

Bridge erection using a suspended Cable Crane (cableway) :

This is unique and challenging technique of bridge construction. Fig.9 a photograph of steel

segmental arch bridge construction, in China, using cable crane (or alternatively called as a cable

way). In Indian on Chenab river bridge project similar technique is planned to be used. Cableway is

also- being used on the Concrete Arch Bridge at world famous Hoover dam. In this case the designand construction must go hand-in-hand to reach to successful conclusion. Issues like tension in the


6/9




Page - 6 of 9

cable, wind velocity for the stability of ropes under self wt and lifted wt condition (i.e. operating and

idling conditions), safety from falling objects at night, anything being hit to the cable etc. are the

critical issues in such method. These issues must be addressed prior to the start of design (at

conceptual stage itself).

Fig.7 Bridge Erection Launcher, used at Bang-Na-Trade Highway Project, Bangkok

Fig.8 Bridge Erection Launcher, used at Bang-Na-Trade Highway Project, Bangkok


7/9




Page - 7 of 9

Fig.9 Steel Arch Bridge Erection Using a Cable Crane

Fig.10 Collapse of a Construction Worker During Bridge Erection

Due to Sudden Cutting of a Wire Rope

(a) Bridge Formwork Failure (b) Launching Girder Failure

Fig.11 Photographs of Bridge Failures during erection stage


8/9




Page - 8 of 9

As mentioned earlier there are many innovative methods like CFT / MSS / erection using tower

crane. They can be used only if there is close interaction between the designs detailer contractor

is established.

Conclusions

From the discussion till now it may be clear that the permanent design and erection method

(enabling works) have very close relation to each other. Following are some of the projects where

the permanents designs and enabling works have affected each other :-

Permanent Design affecting the Enabling Works

a) Allahabad-Nainy Cable Stayed bridge project :

This project was done by Joint venture (JV) of HCC and Hyundai. In this project almost 20m

wide bridge deck was supported with 1400mm wide and 3500mm deep two Pre-stressed

rectangular girders (which were monolithically cast with the deck slab). Each span was 60mlong. There were 4 modules to be cast. The longest module has nine continuous pres-stressed

spans. Every 60m span was cast along with the 1/5th

(i.e. 12m) of next span i.e. at theoretical

point of contra flexure.

During the application of pre-stress as the 60m portion of the girder used to get hoggedup,

the remaining 12m portion used to get dipped-down, because of the continuity in the

construction, This resulted into enormous increase in the load on the formwork / staging in

the 12m portion (which was temporarily acting as cantilever, till the remaining 48m was cast

in next pour). This effect was predetermined and the Formwork and Staging was designed

accordingly. Also the pre-camber was pre-determined and set in the staging during erection

work.

b) Koaldam hydro electric project, (Himachal Pradesh) :-

In this project the De-silting chamber was to be construed. The RCC I Girders, were to be

cast on the top of De-silting chamber at height of 50m above the ground (where wind speed

is beyond imagination). Even the tower carne available was having a limited load lifting

capacity (5Mt at 20m radius). Hence the centering steel trusses (to cast the girder) were

spliced in three parts (to reduce self wt below 5Mt), and these Steel-centering-trusses were

supported on the piers. While dismantling they were suspended from the set girder during

lowering operation. The holes for suspension wire were left-out after discussion with thedesigner.

Enabling works affecting the Permanent Design

a) Longest Railway Bridge at Cochin for RVNL :-

In this project Rail Vikas Nigam Limited (RVNL) had initially proposed a 600Mt single box

girder per span. But as the SBC of soil at casting yard was very less, the contractor proposed

Two-Pre-stressed concrete I girders at each span. This Modification of Bridge Super

Structure considering the Limitations of Soil Bearing Capacity in casting yard, Time Cycle

and the possible erection methodology (NRS Launching girder) was readily approved by theclient. The project was completed ahead of the schedule, within the budgeted cost. This


9/9




example describes how the enabling works forced to modify the permanent structure, yet

leading to ultimate success of the project.

b) DMRC project :

In this project the LG threw more reaction (Shear force and bending moments, both) on theGirder, which was more than the Design value. This was pre-estimated at design stage itself

by the close interaction between client, consultant and the contactor. Even at one place as the

load on pier due to LG was more than the design load at later stage the design had to be

modified.

Lets hope that all these projects, and the discussion above is eye-openers to all the stakeholders

(Client, Designer, PMC, Planner, QAQC, Site Engineer) involved in the execution of bridge

superstructure. If practiced sincerely, this will be definitely lead to better infrastructure for our

society.

Acknowledgement & References :

The author is thankful to the Afcons Infrastructure Limited and entire site team for making all the

relevant data available for this paper. The thanks go to organiser of the National Workshop, COEP.

Alumni, also for considering this paper in this bulletin. All the references mentioned here are purely

from academic interest and there is no purpose to promote anything, directly or indirectly or to

accuse anyone.

In addition to the present Paper following papers may be referred for more information :-

a) Nad-al-Sheba Race Course Development Project : Construction of three Bridges authorV. G. Abhyankar published as Cover Story in quarterly journal of Indian Society of

Structural Engineers (ISSE), Mumbai, Vol-13/2, April-May-June2011 issue.

b) Construction Longest Railway Bridge Project at Cochin author V. G. Abhyankar published as Cover Story in quarterly journal of Indian Society of Structural Engineers

(ISSE), Mumbai, Volume 11-3, Jul-Aug-Sep

c) Various posts / articles published by the Author on Structural Engineers Form (Sefi) andLinked-In.

d) Other Various websites / projects, mentioned below the respective photo / locations.

About Author:

Er. Vivek G. Abhyankar C.Eng, Sr. Manger (Design), AFCONS

Infrastructure limited, has more than thirteen years of experience in planning

and design, detailing of various enabling and permanent works in Reinforced

concrete and Steel. The Author himself was deputed at the Project site

described in the present papers to handle all the technical matters. The author

has written many papers on Civil / Structural engineering aspects and also has

proof checked a book on earthquake engineering.

(email [email protected])

* * *

bridge erection techniques and their influence on permanent des

Documents