Building Structures [ARC 2213]

Audrey Chan Chu Sien 0300257

Bernard Ling Ching Chiong 0301892

Chong Zohan 0302126

Chuah Phaik Lin 0302069

Chloe Wong Choy Hoong 0310230

Kiu Guan Ying 0309003

Fettuccine Truss Bridge Analysis Report

Table of Contents

Introduction

Precedent Study - Kettle River Bridge

Analysis

Strength of materials - Fettuccine

Truss Analysis - From initial to final design

Testing

Truss structure analysis

Reason of bridge failure

Suggestions to strengthen the structure

Conclusion

Appendix

References

Introduction

In this project, we were assigned to construct a precedent study on truss bridge and design

a fettuccine bridge of 600mm clear span. The bridge will need to hold a point load of 5kg.

Next, we should do a thorough analysis on the success and failure of our fettuccine truss

bridge.

This project aims to develop students’ understanding on force distribution in a truss and also

helps student to understand compression and tension forces in a construction.

This report compiles our understanding and analysis on truss bridges especially on the

fettuccine bridge that we have designed. We have also discovered it’s failing point and

suggested ways to improving our truss bridge structure.

Precedent Study

Kettle River Bridge

Figure 1 is a picture of the Kettle River Bridge, a steel cantilever deck arched Pratt truss bridge. The Kettle River Bridge currently serves as a two lane bridge which carries traffic in both the east and west bound directions. The bridge consists of a system of two steel trusses (north and south) supporting a bridge deck floor system composed of a concrete deck supported by steel floor beams and floor stringers. Each truss system follows a Pratt Truss design with an arched bottom chord and is composed of three truss segments.

Figure 1

Figure 2. Truss and Spans Layout

The Bridge, which spans a 400 foot wide section, consists of two parallel steel Pratt trusses. The Bridge acts as three separate spans. The 300 foot central truss has a mid span of 200 feet which rests upon two concrete piers on either side of the river as seen in Figure 2. Two 50 foot cantilevered portions are on each end of the truss. The other two sections of the bridge are the shorter 50 foot sections on either side of the central truss. The central truss is statically determinate with a pin type support at the west pier and an elastomeric pad at the east pier which is assumed as a roller type connection for analysis.

The west pier supports the truss system with a 17.8 centimeter (7 inch) diameter pin connec-tion (see Figure 3a), while the east pier support uses an elastomeric bearing pad which allows for horizontal movement but resists vertical movement, creating a roller support condition (see Figure 3b). Both the east and west abutments of the trusses are supported by rocker supports resting on top of concrete abutments allowing horizontal movement in the east to west direction (see Figure 3c).

The truss system is composed of various member sections. The top and bottom chord of each truss is primarily constructed of hot rolled channel sections as shown in Figure 4. Each channel member is 45.7 centimeter (18 inch) in depth with various cross sectional areas; the flanges of each double channel section are oriented outward with 39.1 centimeter (15.375 inch) spacing between the webs of both channel members. The top and bottom flange of both channel members are connected with a riveted double lattice and/or cover plates, (Fig-ure 4).

(a) (b) (c)

Figure 3. Bridge Support Conditions; (a) West Pier (b) East Pier (c) Abutment

Figure 4. Typical Double Channel Truss Chord Member and Dimensions

Figure 5 shows the truss web members with their typical cross section dimensions.

The main vertical shown in Figure 6, is the largest and longest truss member, measuring at 11.9 meters (39 feet) in length. The main vertical extends down from the top chord and con-nects to a pier foundation. The main vertical section is built up with three 35.6 centimeter (14 inch) deep wide flange sections. The webs of the two exterior wide flanges are riveted to the flanges of an interior wide flange. The main vertical also has riveted stay plates every 76.2 centimeters (2 foot 6 inches) on center along the entire length of the member on both sides.

Figure 5. Typical Truss Web Member and Dimensions

Figure 6. Main Vertical over Pier Support and Dimensions

Figure 7. Truss Segment Upper Chord Pin and Lower Chord Slotted Connection

All vertical and diagonal members are connected to the top and bottom chords with rivets to large gusset plates. The cantilevered end of the main truss segment connects to a sus-pended truss segment by way of a 10.2 centimeter (4 inch) diameter pin on the top chord. The bottom chord connection of the suspended truss segment is also connected using a 10.2 centimeter (4 inch) diameter pins on both ends. However, the connection holes on both sides of the bottom chord are slotted to allow movement. These slotted connections in the-ory do not allow axial forces to be transmitted through the member, thus making the bottom chord of panel 3 a false member. The false members were created to account for any differ-ential settlement between the abutments and the pier foundations. This false member condi-tion occurs at each location where the bottom chord of the suspended truss is connected to the cantilevered ends of the main truss. Figure 7 shows the suspended truss to cantilevered end connections.

Both trusses are braced along the bottom chord of each panel between truss systems using parallel angles with riveted lat-tice. Both trusses are also braced between each panel point’s vertical web members and along the top chord using angle cross bracing for lateral support. There is also an angle cross brace spanning between truss systems from the bottom chord to upper chord.  The top chord of both trusses is loaded at panel points by a composite floor system, see Figure 9a. The steel portion of the floor system consists of floor beams which span across each truss and connect to the top chord of the steel truss at panel points. Each floor beam is a 53.3 centimeter (21 inch) deep wide flange and are 10.4 meters (34 feet) in length, see Figure 9b. Each floor beam is positioned with a 2.1 meter (7 foot) cantilever extending past the center line of each top chord.

Spanning between each floor beam along the longitudinal length of the bridge are 8 floor stringers. All floor stringers are 40.6 centimeters (16 inches) in depth wide flange section, see Figure 9c.

Figure 9. (a) Bridge Floor System (b) Floor Beam (c) Floor Stringer

(a) (b) (c)

Figure 8. Cross Bracing Between North and South Truss

The theory of pure axial forces existing in a truss member is dependent upon each joint con-nection being strictly a pinned connection with freedom of rotation and no fixity. However, the truss members of the Kettle River Bridge are connected using large gusset plates and multi-ple rivets. A gusset plate connection with multiple rivets could create an end fixity condition and induce bending in truss members.

Each stringer is attached between floor beams using a tabbed connection. This connection is intended to create a pinned connection on both ends of the stringer, causing the stringer to act as a simply supported member. A pinned-pinned connection allows bending stresses along the length of a beam but not at the supports.

Knowledges are gained from conducting this case study on the Kettle River Bridge. Several principles are applied during the design of our fettuccine truss bridge.

Figure 10. Bottom Chord Gusset Connection

Figure 11. Floor Beam to Floor Stringer Tab Connection

Analysis

Strength of Material - Fettuccine

We combined our group to another group from the class to test out the strength of our

building material, fettuccine.

A lever is made as a machine to assist the testing of the materials. Different brands and

thickness of fettuccine strips are placed on the lever to test the tension and compression

ability.

Figure 12 shows the testing of tension strength. The fettuccine strip is clamped at one edge

of the lever and water bucket is hung on the opposite edge of the lever. The bucket is filled

with water constantly until the fettuccine breaks.

Figure 13 shows the testing of compression strength. The fettuccine strip is placed under-

neath the wood plank of the lever and the water bucket is then hung on the same edge of the

plank. As similar to how we tested the tension strength, water is poured into the bucket con-

stantly until the fettuccine strip breaks.

After conducting the testing on fettuccine, we conclude that fettuccine is capable in with-

standing tension force, while weak in withstanding compression force.

Figure 12 Figure 13

Truss Analysis - From Initial to Final Design

A typical Warren Truss was selected as our fettuccine bridge design.

The initial bridge was tested with a load of 5kg. The bottom truss deflected downwards when

more weight were added onto it. As shown in the figure below.

In order to prevent deflection, we were advised to design the bottom truss towards the

opposite direction of the deflection. Therefore, we designed a new bridge according to our

test result, while the case study came into place as our reference in the designing process.

The bottom truss was designed to curve upwards to withstand the tension of members, in

addition to prevent the bridge from failing due to deflections. Furthermore, the vertical

members were extended to a higher height in order to be strengthen to withstand vertical

forces.

Testing

Truss Structure Analysis

The design of truss bridge were analyzed and the tension and compression members were

identified. A diagram below was drawn as a construction guide for building the bridge.

As the strength of materials was tested at the beginning of this assignment, we came to a

conclusion that fettuccine is strong in tension but weak in compression. Therefore, the

compression members of the bridge were build with a thicker layers of fettuccine while the

tension members are thinner.

The bottom curve was designed to be supported by the edge of the table. It is intended to

transfer the load to the table instead of holding the load by the truss members.

The bridge was then brought for testing. It has successfully withstand a load of 5kg without

any structure failure.

The main reason of failure that fails the bridge was the supporting member that holds the

load directly.

The bridge was tested for a few more times. When a load of more than 5kg was added, the

member at the side started to break. It showed a weak point of the bridge but it was

overlooked by us.

Pictures above show the failure of the tested bridge before the submission.

Picture of member failure at the side of the bridge.

However, we consider the bridge design as a success as it was able to withstand a point

load of 5kg which fulfill the requirement of the assignment submission. A new fettuccine

bridge is then constructed for submission and it was improved by adding a thicker layer of

fettuccine as the middle supporting member to hold the weight.

Reason of Bridge Failure

On the bridge submission day, the bridge was tested once again in class. Surprisingly, it failed

after a load of 3kg, resulting an efficiency as below :

The main failure of the bridge happened on the members at the side.

A thorough analysis was done to investigate the failure of the bridge.

Weight of bridge : 204 gram

Load withstand : 3kg

Efficiency : 0.044

Reason 01 : Mis-interpretation of tension and compression member

A mistake was found after undergoing a tutorial with the lecturers immediately after the failure

of bridge. Members at the both sides of the bridge was mis-interpreted as a tension member

when we were constructing the bridge.

A thin layer of fettuccine was used as the members, resulting a weak ability to withstand

compression force, also as one of the reason for the failure of the bridge. Figure 1 and Figure

2 show the improved diagram on the analysis of forces acting on the members of bridge.

Figure 1 Initial diagram of our interpretation of forces acting on

each member of the truss bridge

Figure 2 Corrected diagram of forces acting on each member of

the truss bridge.

Reason 02 : Condition of supporting table at both sides of the truss bridge

Throughout the process of analyzing the failure of the truss bridge, we found out that the

edge of the supporting does play an important role. The pictures below shows the different

table that we used to test our prototype bridge and the one we used on the submission day.

The flat and even edge of the table as shown in Figure 3, supports the edge of curve

effectively, in resulting a success in the first test. However, the table is Figure 4 is uneven and

the wooden plank of the table was cantilevered out of the steel frame. It failed to support the

edge of the curve as when the bridge was placed on the edge, it does not have contact on

the surface of the table.

By failing to have a direct contact at the edge, the the vertical member turned out to be the

member which is supporting the deflection of the curve when load is added onto the bridge.

It is not strong enough to perform well in supporting as it was not designed in the manner.

The highlighted part in Figure 14 became the critical part of the bridge failure. It needs to be

strengthen by adding additional members to hold the structure.

Figure 14

Reason 03 : Inverse effect of supporting the curve on the the edge of table

Instead of the intention of making the curve to sit on the edge of the table as a supporting

element, it creates an inverse effect, by constraining the spaces for the curve to be deflected.

As shown in Figure 15, the end of curve is placed exactly on the edge of the table, which

didn’t seem to be a good idea to do so. Instead of supporting the curve, there are also an

opposite force acted on the curve. The forces opposing each other do not allow the curve to

deform, hence causing it to break.

Figure 15

Suggestions to strengthen the structure

Several methods are thought in order to improve the structure, to construct a more efficient

structure.

Suggestion 01 : Adding a supporting member at both ends of the truss bridge

As identified in the analysis, the critical point of the bridge members are at both sides of the

bridge.

By adding a supporting member in the middle, it helps to distribute the compression force to

two members instead of only one member. In addition, by having the weakness in

withstanding compression force, adding an extra member is a good way to increase the

efficiency of the individual member.

.

Besides strengthen the bridge by adding another diagonal bracing at both sides of the

bridge, a small triangular structure can be added to the bottom of the bridge, in order to

distribute load to the table. (As shown in Figure 16)

Figure 16

Suggestion 02 : Allows deformation of bottom curve

In order to counter reason 03 which was identified in the analysis of out bridge failure, we

humbly suggest that the curve of the bridge should be placed on the table instead of the

edge of the table.

By placing the curve on the table, it provides spaces for the curve to be deflected when the

structure is overloaded. When deflections are allowed to occur, the curve will not snap

immediately when too much load is applied on the structure.

Conclusion

Through this fettuccine truss bridge study, we have learnt to construct a fettuccine bridge to its material's full potential and learnt to analyze the members to decide which member is the critical member that needs to be strengthened. We have done a detailed structural analysis of the truss to understand the truss better. We have also learnt to count the forces acting upon each member to identify the tension and compression members.

Appendix

A total of 6 trusses are designed for further analysis as an individual task in this assignment. The following are the task distribution for the cases :

Case 1 : Audrey Chan Chu Sien

Case 2 : Bernard Ling Ching Chiong

Case 3 : Chong Zohan

Case 4 : Chuah Phaik Lin

Case 5 : Kiu Guan Ying

Case 6 : Chloe Wong Choy Hoong

The analysis and calculations of trusses are attached after this page.

References

Bridge contest. 2013. Analyze and Evaluate a truss. Retrieved:17 Oct 2013 from http://bridgecontest.usma.edu/pdfs/la3.pdf

Ching. F. (2008). Building Construction Illustrated. Canada: John Wiley & Sons, Inc,.

Mau, S. T. (2012). Introduction to Structural Analysis : Displacement and Force Methods. US: CRC Press.