Paper Bridge

FORUM

Design and Construction of the Longest Rope-StayedNewspaper Foot-Bridge

Anita Rao4th Year B.Tech, Civil Engineering Dept., IIT Madras, Chennai 600036,Tamil Nadu, India. E-mail: [email protected]

Introduction

Shaastra is the annual technical festival at the Indian Institute ofTechnology, Madras in which engineering students of all disci-plines from all over India participate to compete and collaboratein many technical events.

As a part of Shaastra 2002, the students of IIT-Madras decidedto construct something that had never been built before. The mainidea was to build a record-establishing structure, involving a largenumber of students. After some contemplation, it was decided thatthe project should be in the field of structural engineering, andutilizing an unconventional, largely bio-degradable material. Ahigh-rise tower, crane, a large geodesic dome, a swing, and abridge were some of the initial ideas. Although each idea wasdistinct, they all had in common the use of ecofriendly and un-conventional material. The idea of a paper footbridge was se-lected, because such a structure had never been built on such alarge scale. Moreover, the students were impressed by the veryidea of walking over a paper bridge.

The design, material testing, fabrication, and erection of thefootbridge was completed during Shaastra 2002. It was a greatlearning experience for all the students involved. This paperintends to share the excitement and fun of designing and con-structing a footbridge made of unconventional and ecofriendlymaterials. As far as we know, this is also the longest and tallestbridge to have been built of this kind, using such unconventionalmaterials.

Conceptual Design

Having decided that a bridge made entirely out of newspaper andcotton rope would be built, it was further decided to adopt acable-stayed system because of the elegance of the solution andthe efficiency in load transfer such a system offered. The aestheti-cally pleasing appearance of such a design also influenced thedecision.

All components of the rope-stayed system are mainly in ten-sion or compression �Fig. 1�. The load on the deck of the bridge istaken up as tension in the stays. These stays are pretensionedagainst a walkway deck-level rope. As these stays are supportedon top of the towers, the horizontal component of the tension inthe stays is self-equilibrated, and the vertical component of thistension is transmitted as a compressive force onto the tower.Therefore, all members are in tension or compression with littleinduced bending, except in the deck unit. The towers and deck
units were to be made essentially of newspapers. All members
112 / JOURNAL OF PROFESSIONAL ISSUES IN ENGINEERING EDUCATIO

taking tension were pretensioned, so that they are under tension atall times. If the ropes that are to take tension are not taut, thenlarge deflections will occur in the structure before they becomeeffective. This could also lead to geometric instability and struc-tural failure. The lateral stability of the entire system was ensuredby means of the horizontal curvature in the ropes supporting thedeck elements, as shown in Fig. 1 and an A-frame configurationof the towers.

The challenge was to develop concepts for all phases of con-struction that would involve as many people as possible in themaking of the bridge. Therefore, the design required repeatablebasic units that could be produced on a mass scale.

Material Characterization

Because both the newspapers and the manila ropes were uncon-ventional materials, various tests had to be performed to obtaintheir material properties, such as Young’s modulus, maximumtensile strength, creep, and relaxation behavior.

Newspaper Properties

Eighteen sheets of newspaper were glued together. These sheetswere then cut to obtain the shape shown in Fig. 2. The 11.5 by11 cm rectangle was stiffened at the ends with cardboard to avoidlocal failure at the end grip points during the test. The two endswere held in the jaws of a universal testing machine, and thetension was applied using load control.

The thickness of 18 sheets of newspaper was 1.24 mm andwas measured prior to the test to an accuracy of 0.02 mm by

Fig. 1. Cable-stayed bridge configuration

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means of a vernier caliper. The Young’s modulus of the paper wastaken to be 1,000 MPa, and the tensile strength obtained was11 MPa.

Rope Properties

The ultimate strength of the rope was found by suspendingweights in increments of 10 kg until failure. The various ropestested were ordinary jute rope, 6.5-mm-diameter manila rope, and8.5-mm-diameter manila rope. It was found that the failure loadsof each rope on the average were 30 kg for ordinary jute rope,150 kg for 6.5-mm-diameter manila rope, and 200 kg for8.5-mm-diameter manila rope. The Young’s modulus of the ropewas taken to be that of cotton fiber, or 300 N/mm2.

Relaxation is the phenomenon of the decrease in stress at aconstant strain over time. Pretension applied to the stays initiallyshould not be lost with time. Therefore, various ropes were testedfor their ability to carry pretension without relaxation. Each ropewas held taut between two fixed supports and pretensioned bymeans of pencils used as tourniquets. These pencils were twistedalong with the rope to achieve a uniform degree of tension in alltest specimens. After 24 h, the degree of relaxation was measuredby plucking each rope. It was found that the degree of relaxationin jute rope was higher compared to manila rope of the samediameter. Therefore, it was decided that all tension units would bemade of manila rope.

Preliminary Design

Tower

An arrangement of two legs supported at the ground and joined

Fig. 2. Mechanical properties of newspaper. �a� specimen; and �b�stress-strain curve

together at the top to form an A-frame was chosen for each of the

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two towers. In the longitudinal direction, the tops of the towerswere to be held in position by stay ropes. Thus, the towers had tobe designed to carry compressive force from the stay cables andtransfer it to the ground support.

The bridge was to carry a load of one man. The compressiveforce on the tower would be larger than the gravity load compo-nents of the self-weight of the stays, decks, and the man’s weightdue to pretensioning of members. It was decided to design eachleg of the tower A-frame to carry a compressive force of 200 kgat the preliminary stage. With a factor of safety of 1.5, the legshad to sustain a compressive force of 300 kg before failure.

Hollow Pipe Column Concept for the Tower

Initially, each leg of the tower was to be made up of a largediameter pipe section built up by gluing paper in a spiral shape.The preliminary dimensions of the pipe section were to be140 mm in diameter and 8 mm thick. These dimensions took intoaccount material failure, overall buckling failure of a 4-m-longmember, and local buckling failure as a cylindrical shell.

It was decided to fabricate and test a pipe made of newspapersheets that were about 750 by 550 mm. A steel pipe 3 m in lengthand 140 mm in diameter was used as a mandrel. Five studentsworking synchronously made the paper tube, one applying glueon sheets of newspaper, two winding the sheets onto the pipe in aspiral, one handing the pasted sheets to these two, and anotherrotating the pipe when required. Spiraling was done to avoid for-mation of critical joints. After one layer was completed, the sameprocess was repeated from the other end; this time the paper waswound in the opposite direction. This was done to avoid unravel-ing of paper under compression, which would occur if paper wereto be spiraled in the same direction. A total of 30 layers of sheetswere wound over the pipe to make the pipe wall 8 mm thick. Thesteel pipe, with sheets of paper glued on, was left overnight todry. The next day the steel pipe was removed and the hollowpaper tube was tested. The tube failed by buckling locally atpoints where glue application was insufficient.

The drawbacks of this method were that• It took a long time for the glue to harden.• A few patches on the sheets that were left unglued acted as

weak points, causing the member to fail at these points. Thesepatches rendered the member weak because the layers of paperwere not acting together as a single unit.

• It was highly uneconomical to use so much glue.• The team of five took nearly 6 h to complete one column. The

work was monotonous and could not involve more people.• The skill required was high, because the spiral formation and

gluing had to be uniform.Therefore, this approach was abandoned. One of the aims of thisevent was to involve as many students as possible. It was neces-sary to make the elements using newspapers from some basicrepetitive units so that they could be mass-produced as well asinvolving a number of students with minimal supervision andquality control requirements.

Triangular Lattice Tower Concept

A solid roll of paper �750 mm long� was made by rolling 45sheets of newspaper lengthwise. The diameter of this unit, whentightly rolled, was around 60 mm. This unit was tested in com-pression over a length of 700 mm, and its ultimate strength in
compression was 150 kg. The unit was also subjected to a two-
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point-load �at 40 mm apart� bending test over a span of 60 mm.The unit failed at a load of 25 kg. The unit had sufficient com-pressive strength but very low flexural strength. Moreover, theself-weight of the structure would increase tremendously if a solidmember was used for the entire height of the tower. The restrictedlength of each element �length of one newspaper� was also adrawback of this idea. In spite of these problems, it was decidedto adopt this solid member as the basic unit because of its ease ofmass production and having very few quality control require-ments. The lattice tower was made using this basic unit.

The units involved in the lattice column �Fig. 3� are thefollowing:• Main member—made of 45 sheets of rolled up newspaper hav-

ing an outer diameter of 60 mm;• Horizontal members—made of 25 sheets of rolled up newspa-

per having an outer diameter 35 mm, interconnecting the threecorner units horizontally in compression with a 6.5-mm-diameter cotton rope connecting the three corner legs in ten-sion; and

• Diagonals—were 6.5-mm-diameter manila ropes formingx-bracings that are pretensioned.The distance between corner legs of the tower was calculated

so that the overall buckling strength of the column was greaterthan 150 kg with a safety factor of 2.0. The main leg memberstransfer the compression by bearing on top of each other. Tensileforces were transferred by means of pretensioned cotton ropestying together two leg members at the joints, as shown in Fig.3�a�. Any lateral sliding that could occur between the members atthe two joints was prevented by providing a paper sleeve �con-sisting of 30 sheets of rolled-up newspaper� and taping them in

Fig. 3. Elements and joi

position.


Horizontal Struts

Horizontal struts were placed at the joints between consecutivelegs and connected to the legs by means of tape. Manila ropes thatwere tightly tied around the triangle formed by the compressionstruts held the legs together in tension �Fig. 3�. Pretensioned di-agonal x-bracings on each of the three faces between parallelcorner legs and horizontal ties helped to hold and make the cornerlegs act together by transferring shear at any section �Fig. 3�.

Fabrication of Columns

Making of one unit of a corner leg member involved two people.One would roll the sheets of newspaper tightly while the otherwould feed in five sheets at a time �Fig. 4�a��. The rolling wasdone on a flat surface. The newspapers used were of equal length�710 mm� to ensure uniformity. Forty-five sheets of rolled news-paper constituted the main corner leg members in the main panelof the lattice tower. After rolls were formed, they were passedthrough a quality check. Rolls were graded A, B, and C, etc.based on their compactness and uniformity. Grades C and belowwere rejected. Grade A rolls were used for the columns. The twoends of the Grade A rolls were sawed by means of an electricrotor saw to obtain a good contact surface for bearing and had auniform lengths of 700 mm �Fig. 4�b��.

Two units were attached to each other by means of a papersleeve. At a distance of 50 mm from the joint, holes were drilledon either side. Manila rope �6.5 mm in diameter� was passedthrough these holes and tied together �Fig. 3�a��. They were thenpretensioned using pencils as tourniquets. Seven units joined to-

the lattice paper column
nts in
gether in this way comprised one corner leg of the column. Once


three legs were ready, a group would start making the column.Two legs were placed side by side, and horizontal struts wereconnected at all the joints and the two ends by means of tape. Thethird leg was placed on top of the triangle formed by these strutsand taped in position to form a triangular shape. A 6.5-mm-thickmanila rope was used to tie all three legs together at the level ofthe horizontal struts �Fig. 3� to take tension. Holes were drilled atthe joints to enable tying of the diagonal ropes. Initially, the6.5 mm ropes were passed through the drilled holes in each panel

Fig. 4. �a� Rolling of corner units; �b� finishing ends of corner legunits; and �c� lattice column testing

on all of the three faces. Before and after every hole, knots were


tied on the rope to eliminate slippage. After the diagonals weretied, all the diagonal ropes were pretensioned. This was done inthe x-diagonals for all faces and the seven panels of the leg toensure that no skewing of the column occurred during pretension-ing. If the two diagonals forming the “X” in any face were not ofequal length, the amount of pretensioning was adjusted.

The first 5-m-long lattice column was tested to failure in thestructural engineering laboratory to gauge its behavior andstrength. For this column, the diagonals were made using ordinaryjute rope; no horizontal tension ties were provided at the joints,and the diagonal ropes were not pretensioned. As a result, thecolumn failed by buckling at a load of just 80 kg. Based on all theshortcomings observed in the first test, another column wasfabricated, as described earlier, with all the refinements and sub-jected to nondestructive loading. This second unit with rectifica-tions was tested to see if it could withstand the design load of150 kg without any distress. In addition to the proof-tested col-umn, three more columns were fabricated along the same lines toobtain two columns for each tower.

Fabrication of the Deck

Deck Unit

The deck surface had to be supported on the two curved preten-sioned ropes �Fig. 1�. It was found that the basic unit �45 sheetroll� had very low flexural strength and could carry only 25 kg.This could be improved either by increasing the diameter of theunit or by reducing its effective span by pretensioned rope �Fig.5�. Increasing the diameter would mean a tremendous increase inweight. Moreover, maintaining a standard unit 45-sheet roll forease of mass production was desirable. Therefore, an alternativeof reducing the effective span was chosen. Two equally spacedprops, stayed by ropes as shown in Fig. 5, were used to reduce thespan of the deck member. The units were made of 25 sheets ofrolled-up newspaper cut to 350 mm. The manila ropes were pre-tensioned. One deck unit, when tested, could easily withstand aload of 100 kg. To make the deck unit holes were drilled at thetwo ends of each prop after it was sawed to the required 350 mm.Two holes were also drilled at the ends of the basic unit. Theprops were then tied to the main member to form the two tri-angles shown in Fig. 5. These props were then stayed by a set oftwo 6.5 mm ropes running through drilled holes on the mainmember and the props �Fig. 5�. Knots were tied before and after

Fig. 5. Deck assembly

every hole to prevent slippage.


Finite-Element Analysis

Finite-element modeling and analysis of the bridge was carriedout to understand its behavior, to work out the appropriate incli-nation of the towers from the vertical, to calculate the pretensionin all the stay cables to avoid excessive deformation of the deckunder load, and to arrive at the length of all the ropes and coor-dinates of the attachment points before pretensioning.

MSC-NASTRAN was used for modeling and analysis. Beamelements were used to model the tower legs, and truss elementswere used to model all stay ropes and deck level ropes. Tempera-ture changes in the stay ropes were used to create desired preten-sion. Analysis was carried out under combined pretension, and adeck load of 75 kg was used to study forces on all the members.The rope pretensions were adjusted such that all of them stayed intension during the entire process of placing the 75 kg load on thedeck. The stays of the ropes were designed to ensure that therewas a safety factor of at least 2.0 against their tensile strength,and the tower had a safety factor of 3.0 against overall and indi-vidual member buckling strength.

Erecting the Bridge at the Site

Site Preparation

The place where the tower legs were to be supported was markedon the ground. To avoid bending moments at the supports, thecolumns were to be hinged at the base and not fixed. For thispurpose, casuarinas wood cylinders about 100 mm in diameterwere used as rollers. A semicircular cavity was dug to providegood seating for the rollers �Fig. 6� and to transfer the vertical andhorizontal thrust. Twenty-five mm steel rod anchors 2 m longwere driven into the ground at locations where the ropes from thetower and the deck were to be tied.

Base Plate

Eighteen-mm-thick plywood base plates were used to transfer the

Fig. 6. Base plate detail

forces from the three legs of the tower to the rollers �Fig. 6�.


These consisted of two equilateral triangular plywood plates ar-ranged one on top of the other. The bottom plate was tied to theroller using 6.5 mm cotton ropes through holes drilled in the tri-angular plate. On the top plate, three circular holes were cut toreceive the three corner legs of the column. Two diametricallyopposite holes were drilled adjacent to the circular cut holes to tiethe column legs to the plate. Spacer pieces were used below thecorner legs to transfer the force to the bottom plate and to serve asthe distance pieces between the two plates. Nuts and bolts wereused to hold the two plates in position. Thus, the base plate couldbe adjusted to hold the two columns of the tower in position at thedesired inclination to achieve the A-shape of the tower. Four suchbase plates were used to support the four legs of the two towers.Two legs of each tower were connected to the casuarina roller,which was perpendicular to the centerline of the bridge.

Cap Plate

Two 18-mm-thick equilateral triangular plywood pieces boltedtogether were used as the cap plate to transfer the rope forces tothe tower legs and to join the two legs together to form the “A”tower �Fig. 7�. The bottom of the two plates had three circularholes to receive the column legs and to transfer the leg forces tothe top plate. Two diametrically opposite holes were drilledaround each circle to enable the tying of the column corner legs tothe base plate using 6.5 mm ropes. A circular hole was cutthrough the center of both plates to pass the stay ropes. A 25 mmsteel rod was attached to the top plate by means of nuts andU-bolts. The stay ropes were tied to the rod and passed throughthe central hole in the cap plate, later to be tied to the deck rope.The two cap plates at the column top were tied together to formthe A-frame.

Stays and Deck-Level Ropes

The lengths of the stay ropes before pretensioning were obtainedfrom finite-element analysis. These lengths were marked on thestays with a leeway of 10 cm for the knot that would be formedon the steel rod at the top of the tower. One end of the rope wouldconnect to the deck-level rope between the towers; the other end

Fig. 7. Cap plate details

would connect to the deck-level rope outside the towers. Each


rope was tagged to identify it during erection. The lengths of eachrope were marked.

There were two pairs of ropes at deck level to pretension thestay cables against as well as to support the deck units on eachside. Each deck-level rope consisted of a pair of 8.5 mm diametermanila ropes. The stay rope attachment points and anchor pointsobtained from finite-element analysis were marked on the deck-level ropes. After marking, the two deck-level ropes were tiedtogether using 6.5 mm rope at every mark. This was to enabletying of the stays to the deck-level rope.

Each wooden roller support was kept on top of the two ply-wood base plates that support the three legs of the columns andare tied to them by using 6.5 mm ropes. They were then placed inthe semicircular trench created at the locations marked on theground. The stays were tied to the bar on the cap plate. The capplates were then fitted to the top ends of the columns and tied tothem. The cap plates of the two columns to form the A-framewere brought together and attached by tying ropes through theholes drilled on the abutting edges of the top plate.

The tower was then hoisted slowly and placed on the baseplate such that the three corner legs in each column fit its respec-tive base plate. The legs were tied to the base plate using the6.5 mm ropes passing through the top base plate. Spacer piecesbetween the top and bottom units of the base plate gave supportvertically from below the legs. Two stays from the top of eachcolumn were anchored temporarily to the ground to ensure stabil-ity of the towers during construction, and allowing adjustment ofthe inclination of the tower to the desired slope.

To enhance the stability of the tower, a horizontal strut wasattached to the two legs of the tower just below the deck level,and vertical x-bracing was provided between the two columns ofthe tower below the deck level �Fig. 8�

With the towers in place, the deck-level ropes were anchoredat their ends, and the stays were tied to their respective marks onthe deck-level rope. Horizontal ropes �acting as ties� between thedeck level ropes were then put in place. The exact lengths of theseties were obtained from finite-element analysis. These tie lengths

Fig. 8. Sectional elevation of the bridge


ensured a horizontal curvature in the deck-level ropes. The stayswere then attached to the premarked lengths, to achieve the cal-culated pretension and the desired vertical curvature in the deck-level ropes. Slight adjustments were made to achieve the desiredtension and curvature. The deck members were fabricated concur-rently. These units were placed on and tied to the deck-levelropes. Fig. 9 shows the constructed bridge.

Testing

After the paper rope bridge was ready and before anyone wasallowed to walk over it, it was tested to see if it could withstandthe load of one man. One student held on to the deck-level ropesfrom below, and pulled himself up, thus subjecting the bridge to acenter-point load of one person. The bridge could easily resist thisload without any visible distress. After this, many students hadthe pleasure of walking over the first cotton rope stayed pedes-trian bridge supported on towers made of newspapers.

Summary

The entire exercise illustrated many lessons learned to the stu-dents involved in the project:• The process of designing, testing the constituent materials,

fabricating, and erecting a unique structure made of unconven-tional material;

• The challenges in executing the project and the process ofcollaborative work to achieve the goal;

• The need to innovate, analyze, and make decisions to solvesmall and large problems; and

• The pleasure of seeing one’s effort take shape and the dreambecoming a reality.

Acknowledgments

We thank Professor V. Kalyanaraman, Dr. Devdas Menon, and Dr.Satish Kumar of the Civil Engineering Department for helping inthe design of the bridge and completion of the event, as well asAshish Gupta and Y. Aswani Kumar, 4th year B.Tech, Civil En-gineer, IIT M for modeling the structure using finite-elementanalysis.

Fig. 9. Student walking over the bridge


Paper Bridge

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