Bridge Builder® - Pre-Engineering Software
Contents
1. Allowable Force The allowable force is the maximum force that the individual member is capable of carrying by virtue of its size and length The larger the member size the more force the member can carry Of course bigger members weigh more When comparing this force to the actual force use the allowable compression force if the actual force is negative Use the allowable tension force if the actual force is positive no negative sign Efficiency Ratio This number is calculated by dividing the actual force disregarding its sign by the corresponding allowable force and then multiplying this quantity by one hundred to change it to a percentage The efficiency ratio represents the degree to which you are effectively using the strength of the member For example if the ratio is 50 the member is stressed to half of its potential strength Efficiency ratios near 100 represent excellent use of a member s available strength The efficiency ratio will be replaced by the word FAIL if the actual force is greater than the allowable force Note Some bridge geometries have members that do not appear to be serving any purpose because they carry no load actual force 0 These members may be adding stability to the structure by preventing other members from buckling These members are called Zero Force Members 10 2 Printer Output PRINTER OUTPUT Printer Output The Bridge Builder results can be output to a2. Animate once again from the Mode menu This time the bridge does not fail and the truck makes it across the bridge Can you build a lighter bridge 2 12 Important Points to Remember IMPORTANT POINTS TO REMEMBER Your bridge should only touch the left and right abutments at one point the abutment corner points These are the points where the hinge and roller have been provided to connect your bridge to the abutments Also note that a flat and level roadbed should be provided for the truck to drive on The maximum number of joints per bridge is 60 and no more than 30 joints can lie along the roadbed and the maximum number of members per bridge is 150 No member can exceed 130 feet in length Members must be connected at their end points See the illustrations below CORRECT CONNECTION Joints must be located at every intersection of the members and only at the ends of members 3 1 Important Points to Remember INCORRECT CONNECTION A member cannot intersect another member unless the intersection occurs at the end of the affected members Note If you draw a new member and end it on an existing member Bridge Builder will automatically make a correct connection of the resulting three members How Engineers Think About Forces How Engineers Think About Forces HOW ENGINEERS THINK ABOUT FORCES I m pushing on this desk M Engineers represent forces with arrows The arrow must show b
3. But there is another way You can Reflect a previously drawn member Select Reflect from the Members menu Now position the mouse pointer over the member just drawn Click the left mouse button Wow You just drew another member of the bridge Most of the bridges that you draw will be symmetrical about the center of the span The Reflect option allows you to take advantage of this fact Let s continue building the bridge Select the Build option from the Mode menu 2 5 uick Start o Bridge Builderv40 O CHA File Joints Members Display Mode Options Special Help p GRID COORDINATES DELTA X DELTA LENGTH ANGLE BRIDGE BRIDGE Sita Yo E TY ift ift ift degrees Wt kips NAME oso 100 0 0 0 Q 111 5 UNTITLED Draw the additional members as shown above The top of the bridge is at Y 60 To do this draw the members to the left side of the center span and Reflect the members required to complete the bridge Note leave out one diagonal as shown The Reflect option will work from left to right and also from right to left So if you want to draw the initial member right of the center span and reflect them left you can The reflect member option will not work on a member that crosses the center line of the span 2 6 uick Start File Joints Members Display Mode Options Special Help Save As File Name Directories op example brg
4. Mode menu Move To move an existing joint select this option then drag the joint to a new location You may drag a joint over the top of another Bridge Builder will automatically erase one joint and reconnect the members to the remaining joint To exit this option select the Build option Show Use the Show option to check the location X Y coordinate of a joint Select the option then click the left mouse button with the mouse cursor over the joint To scroll through the joints click away from the joints Exit this option by selecting the Build option under the Mode menu 9 3 Menu Options Members Delete Change Size Show Sizes Default Size Number Members Reflect Member Show Stats Delete Use this option to delete members First select the Delete option then click the left mouse button with the cursor arrow over the member When a member is deleted the remaining members are renumbered Exit this mode by selecting the Build option under the Mode menu Change Size To change the size of a member select this option from the pull down menu and the new size from the associated pop up menu Position the mouse cursor over the member you wish to change then click the left mouse button The new size of the member will be printed at the center of the member to confirm the change You may continue changing members to this new size by pointing and clicking Exit this option by ei
5. c Obbwin4 1 bb4 library a Obbwin4 amp 1bb4 gt library O Read Only Save File as Type Drives Bridge Files BRG c GRID COORDINATES DELTA X DELTA LENGTH ANGLE BRIDGE BRIDGE Shits Mtl ft ft ift degrees We kips NAME 000 100 0 0 0 0 111 5 UNTITLED Let s save the bridge before analyzing it Select Save As from the File menu Double click the Library directory and enter EXAMPLE for the name of the bridge Click OR We are now ready to analyze the bridge Pull down the Mode menu and select Animate This will test the bridge by rolling a 40 ton truck along the roadbed Turn the page to see what happens 2 7 uick Start O o Bridge Builder CH File Joints Members Display Mode Options Special Help GRID COORDINATES DELTA X DELTA Y LENGTH ANGLE BRIDGE BRIDGE x ife EIEE ift ift ift degrees We kips 125 100 0 0 0 0 111 5 The truck did not make it across In fact it didn t even make it to the bridge Why First look at the geometry of the bridge to make sure that it is composed of triangular shapes This bridge is not composed of all triangles Therefore it is unstable This means that the bridge can t even support its own weight That s why the bridge fell before the truck even reached the bridge 2 8 uick Start Bridge Builder v4 0 File Joints Members Display Mode Options
6. Bridge Builder A Program for Exploring Structural Engineering Build and Test Bridge Designs on Your Computer Pre Engineering Software Corporation Pre Engineering Software Corporation presents BRIDGE BUILDER Windows version A Program For Exploring Introductory Concepts in Engineering Written by Robert A Wolf HI P E Edited by Mary Anne Wolf P E Cover by graphics etc amp Bret Guidry Illustrations by Bret Guidry System Minimum Requirements IBM Compatible 80386 4 megs RAM 16 color VGA Microsoft Compatible Mouse Microsoft Windows 3 1 Legal Information Copyright 1987 2009 Pre Engineering Software Corporation All rights reserved License Agreement This software is licensed to the user for use on one computer In the case of a multiple license purchase refer to the license certificate for the number of computers upon which the program may be installed This software may not be duplicated or transmitted in any form by parties who have not received written authorization from the owner Pre Engineering Software Corporation This software is licensed as is The owner does not warrant this software or its user s manual to be totally error free The owner does warrant that the software will perform substantially in accordance with the user s manual and that the disk media is free from any material defect that would impair the performance of the software for a period of 90 days aft
7. GE e e ES a S aT ft ft 2 degrees Wt kips NAME 100 ooo 100 0 100 0 0 UNTITLED Move the cursor arrow to point to the top right corner of the left abutment Make sure that the X amp Y coordinates read X 0 Y 0 then press and hold down the left mouse button Now without releasing the left mouse button move the mouse to the right until the cursor arrow points at the coordinate X 100 Y 0 Now release the mouse button That s it You have just drawn the first member of the bridge uick Start S Bridge Builder CH File Joints Members Display Mode Options Special Help p GRID COORDINATES DELTA X DELTA Y LENGTH ANGLE BRIDGE BRIDGE x 62 EEE ift ft ift degrees We kips NAME 075 15 0 0 0 0 9 868 UNTITLED In general this is the way to add members to the bridge Here once again are the steps involved 1 Verify that Build is selected under the Mode menu item 2 Move the mouse arrow to the position where you want to start the member 3 Press and hold down the left mouse button 4 Move the mouse arrow to the position where you want to end the member 5 Release the mouse button uick Start S Bridge Builder PHA File Joints Members Display Mode Options Special Help p GRID COORDINATES DELTA X DELTA Y LENGTH ANGLE BRIDGE BRIDGE L lfe T TEGi ift ft ift degrees We kips NAME 000 100 0 0 0 0 19 74 UNTITLED
8. S Tension forces try to pull things apart 6 1 Compression Forces COMPRESSION FORCES Compression forces also try to warp or buckle things Failure Modes FAILURE MODES Pulling Buckling Crushing Apart If a bridge s members pull apart buckle or crush due to overload the bridge will collapse The bigger the size of a member the less the chance of overload In other words the heavier the member the stronger the member Individual member failure is not the only reason a bridge can fail The bridge might fail because it is unstable 8 1 Failure Modes There are two possible types of geometric instability in a truss bridge First a truss bridge will fall down if it is not made of triangles These bridges are called internally unstable Second a bridge will fall down if it is not supported by both abutments These bridges are called externally unstable Menu Options MENU OPTIONS File New Open Save Save As Print Print Setup Exit New Use this option to erase the current bridge and start a new one If the current bridge has been changed you will be given a chance to save it Open This option performs the same function as the New option listed above In addition this option brings up a dialog box for selecting an existing bridge from the library Save Use this option to save a bridge that has previously been saved with the Save As op
9. Special Help MEMBER SIZE LENGTH ACTUAL FORCE ALLOWABLE FORCE EFFICIENCY No No tE ikips ten iten k icomp k ratio 3 Now look at the results screen and check the individual member results The results screen is used to determine what to do to strengthen the bridge Notice in this case the actual force in the members is unavailable and reported as UNSTABLE The actual force is the force that was produced in the member as a result of the truck on the bridge In this case the bridge could not take any load These results indicate a geometric instability problem Let s add the missing diagonal 2 9 uick Start Bridge Builder v4 0 zl File Joints Members Display Mode Options Special Help CD GRID COORDINATES DELTA X DELTA LENGTH ANGLE BRIDGE BRIDGE Tita oY ft ift ift ift degrees Wt kips NAME 1 05 100 UNSTABLE 626 4 105 4 N A 250 030 0 0 D 0 123 EXAMPLE Draw the diagonal as shown above Let s save this version of the bridge before analyzing it Pull down the File menu and select Save Now let s analyze it Pull down the Mode menu and select Animate This time what happened The truck attempted to cross the bridge but did not make it all the way across Let s take a closer look at the results to find out what we can do to get it to work 2 10 uick Start S BridgeBuildery4 0 O CH File Joints Mem
10. Use Sound Make DXF Use this option to send a drawing of your bridge to another CAD or drawing program DXF stands for Drawing eXchange File Make VRML Want to see a 3 D model of your bridge This option creates a wrl file that can be displayed by many VRML enhanced Internet browsers or viewers You can view these files from your Internet page or from your own hard drive Copy to Clipboard This option copies the current bridge drawing to the Window s clipboard You can then use the paste command from other applications to insert the drawing of the bridge into notes or reports Use Sound If your system has an MCI compatible sound card you may select this option to toggle the sound option on and off 9 10 Menu Options Help Getting Started Menu Options About Getting Started This is where to look if you don t know what to do next This option will give you a brief description of how to design and test a bridge using Bridge Builder Menu Options You used the menu to access this help file This file contains information about the menu options and their use About The purpose of this notice is to inform you that Bridge Builder is a copyrighted software package and its use is subject to federal and international laws and treaties It is unlawful to copy any part of this software disk media or written manual without the written permission of the owner Note To clear a pull down menu click the left mouse button wit
11. bers Display Mode Options Special Help MEMBER SIZE LENGTH ACTUAL FORCE ALLOWABLE FORCE EFFICIENCY No No ft ikips ten iten k icomp k ratio 3 4 05 116 6 116 62 626 4 77 5 FAIL _ The failed member s are in red Click on the left diagonal as shown above This is member number 4 and it is a size 5 Refer to the Member Library chapter for a list of member sizes and their meaning The actual force in the member is 116 6 kips The actual force is the force in the member caused by the weight on the bridge in this case the truck The negative sign indicates that the member is in compression The next category is allowable force This member is capable of safely supporting an allowable tension force of 626 4 kips and an allowable compression force of 77 5 kips 2 11 uick Start o ridge Builder CHA File Joints Members Display Mode Options Special Help 06 06 06 06 l Ste GRID COORDINATES DELTA X DELTA LENGTH ANGLE BRIDGE BRIDGE e Cita Ye E TY ift ift ift degrees Wt kips NAME 005 100 a 0 0 Q 132 5 EXAMPLE What can we do to get this bridge to work Bigger members that is a member with a larger cross section can carry more load Let s increase the size of the failing members Select 6 from the Change Size popup menu Next click on the diagonal members shown above to change their size Save this bridge Now select
12. er the purchase date This software is not intended for professional use Do not design or construct any structure based on information obtained from the use of this software The purchaser agrees to defend save and hold harmless the owner of this software and its employees officers and re sellers against any suit or claim actual or alleged arising out of the use misuse or inability to use this software The owner its employees officers and re sellers shall not be liable for any incidental or consequential damages or any damages in excess of the original purchase price Some states do not allow the exclusion or limitation of implied warranty therefore portions of the above may not apply to you ii Legal Information Acknowledgment of Trademarks Bridge Builder is a registered trademark of Pre Engineering Software Corporation Trademarks found in the Bridge Builder program and documentation are the property of their respective holders The following is a partial list of mentioned trademarks and their holders IBM PC AT is a registered trademark of International Business Machines Inc Windows Microsoft and MS DOS are registered trademarks of Microsoft Corporation iii Table of Contents TABLE OF CONTENTS Subject Chapter Installation amp Setup Quick Start Important Points to Remember How Engineers Think About Forces Bridges Tension Forces Compression Forces Failure Modes Menu Options Interpreta
13. h the arrow located off of the grid area Clicking on the grid will cause the start of a new bridge member 9 11 Interpretation of Results INTERPRETATION OF RESULTS This chapter discusses the numerical results of the analysis Results are available after calculation or animation Click on a bridge member to list the results for that member at the bottom of the window No numerical results are possible for unstable bridges If your bridge has stable geometry the following results are available for each member Member This number is used to identify the member on the picture of the bridge when results are printed The member is assigned as each member is drawn When a member is deleted the member numbers are adjusted accordingly Size This number represents the size of the member as described in the chapter entitled The Member Library Length This is the length of the member in feet from the starting joint to the ending joint as measured along the member Actual Force The actual force is the maximum force tension or compression generated in the member by the truck driving across the bridge plus any additional load specified under the Options menu If the actual force is negative the member is in compression If it is positive the member is in tension The member will fail if the absolute value of the actual force is greater than the corresponding allowable force 10 1 Interpretation of Results
14. he weight shown on the Build screen is actually the weight of one of these two identical bridge trusses 13 2 Technical Support TECHNICAL SUPPORT We are always looking for ways to improve our product If you have any questions concerns or suggestions please do not hesitate to call or email Phone 225 769 3728 Fax 225 769 3661 E mail mail pre engineering com URL http www pre engineering com 14 1
15. int connection e Deck Load That portion of the concrete road floor beams and stringer weight to be applied at that joint e Traffic Load Additional load added to the bridge to simulate bumper to bumper truck traffic e Jtl The starting joint of the member o Jt2 The ending joint of the member e Self Wt Actual weight of that truss member e Deck Wt Deck Load between Jtl amp Jt2 e Traffic Traffic Load between Jtl amp Jt2 11 2 The Member Library THE MEMBER LIBRARY To build your bridge select members from the library on the next three pages All of the members have a square hollow cross section and are made from steel plates The thickness of the steel plate is noted TYPICAL BRIDGE MEMBER LOOKING AT END OF MEMBER VARIES SEE TABLE CROSS SECTION 12 1 The Member Library Size Cross Section Cross Sectional in x in Area of Steel in 3 a 1 3 1 4 2 75 6 2 e Jia 5 75 g u ii 12 A a 11 75 15 5 15 29 0 1 2 The Member Librar The Member Library Size Cross Section Cross Sectional Size Cross Section Cross Sectional in x in Area of Steel in in x in Area of Steel in 30 18 6 n 35 0 18 y 10 30 116 0 q1 21 7 21 60 75 33 3 24 1133 189 0 11 2 8 24 69 75 3 36 27 12 36 207 0 9 27 104 0 1 11 2 12 3 12 4 The Structural System THE STRUCTURAL SYSTEM The Bridge B
16. nly analyzes the bridge it also animates the results The truck attempts to drive across the bridge and if the bridge is not of adequate strength the bridge collapses into the river Review This mode is for viewing the results of the latest analysis This mode is automatically entered after analysis To exit this mode select the Build Mode from this menu Menu Options Options Include Truss Dead Load Include Deck Weight Include Max Traffic Load Use Min Member Force Include Truss Dead Load Use this option to add realism to the Bridge Builder simulation With this option the actual weight of each truss member is added into the calculations when determining the actual forces in the members The larger the member size chosen the more weight the bridge must support Include Deck Weight This is another option intended to add realism to the program Selecting this option adds the actual weight of the bridge deck to the analysis Add Max Traffic Weight This is another option intended to add realism to the program Selecting this option adds the equivalent weight of a bridge full of trucks bumper to bumper stopped on the bridge to the analysis 9 8 Menu Options Use Min Member Force This is another option intended to add realism to the program Selecting this option sets a minimum design load of 6 kips for a member of the bridge 9 9 Menu Options Special Make DXF Make VRML Copy to Clipboard
17. ny Windows compatible printer The information printed will depend on the Mode that the program is in If the program is in the Review Mode the information printed will be the results of the analysis When the program is in the Build Mode the printout will contain member and joint information corresponding to the bridge design Printout From the Review Mode The printout from the Review Mode labeled RESULTS contains a picture of the bridge and the information provided at the bottom of the window plus the following e Angle The orientation of the member measured clockwise from a right extending horizontal e Bridge name and weight e Overall efficiency ratio of the bridge e Options used e Total number of joints and members e Maximum downward deflection of the truss Printer Output Printout From the Build Mode The printout from the Build Mode includes a drawing of the bridge plus additional member and joint information not available when printed from the Review Mode The additional information is listed below e Joint Coordinates The location of the connections given in Cartesian coordinates e Joint Type 1 Normal connections not a hinge roller or roadbed 2 Hinge connection at the left abutment 3 Roadbed connections 4 Roller connection at the right abutment e Dead Load That portion of the truss self weight to be applied at that jo
18. oth magnitude M and direction ma W 3000 Pounds 3 Kips Weight W of an object is a force directed toward the center of Forces are sometimes measured in pounds or kips the earth 1 kip 1000 pounds 4 1 4 2 Bridges BRIDGES Force Force Roller Joint Abutment Abutment Pinned Joint Members Truss Bridge There are many types of bridges Bridge Builder is a program about truss bridges Bridges support forces such as the weight of trucks and cars The elements of a truss bridge are Abutments supports at the ends of the bridge Members framework of the bridge truss Joints connect the members together and connect the bridge truss to the abutments Bridge Builder uses pinned joints which are like single metal rods through the ends of the members to connect the members together see the Important Points to Remember chapter for a picture of a pinned joint Bridge Builder also uses a hinged joint at the right abutment connection The roller joint allows the bridge to expand on hot days and contract on cold days 5 1 Bridges EXTERNAL EXTERNAL FORCE FORCE INTERNAL COMPRESSION FORCE INTERNAL TENSION FORCES The external forces on the bridge create two basic types of internal forces in the bridge members These two internal forces are tension and compression 1 WA Z SK This member C This member is is in tension T in compression Tension Forces TENSION FORCE
19. ption by selecting another option or by selecting the Build option from the Mode menu 9 5 Menu Options Display Grid Refresh Reset Window Joint Snap Coordinate Snap Grid Select this option to toggle the grid The grid points will roughly match the current coordinate snap settings Refresh Use this option to clear the screen after using selected member options Reset Window This option will reposition the window to its default location The window should not be reduced or expanded when using the Build mode Select this option to properly size the window for the Build mode Joint Snap This is a drawing assist option This option sets the size of the hot zone around an existing joint When drawing a new member the program will find an existing joint if the end of the member is placed in the joint s hot zone Coordinate Snap This option changes the incrementing of the X Y coordinate readout at the lower left corner of the window from a default setting of 5 to 1 2 4 or 10 9 6 Menu Options Mode Build Calculate Animate Review Build The program must be in this mode to add members to your bridge Also you may want to select this mode as a means for exiting other options Calculate Select this option to calculate the results without having the truck drive across the bridge That is this mode analyzes the bridge without running the animation Animate This option not o
20. t to detect and use a sound card If you do not wish to run the program with sound after the program starts pull down the Special menu and toggle the Sound option to turn the sound off To start the program double click the Bridge Builder icon in the Pre Engineering Software group or for Windows 95 users click Start then point the cursor to Programs Pre Engineering Software and then Bridge Builder v4 0 Release the mouse button 2 1 uick Start Bridge Builder v4 0 File Joints Members MEJE V Mode Options Special Help Grid Refresh X Reset Window Joint Snap gt Coordinate Snap gt GRID COORDINATES DELTA X DELTA LENGTH ANGLE BRIDGE BRIDGE S LEbal TEG ift ift ift degrees Wt kips 085 100 0 0 D 0 After the introduction amp copyright information clears the window will appear as shown above The program is already in the Build Mode Before starting to draw your bridge it s a good idea to select Grid from the Display menu Note This example problem uses the program s start up defaults If you are not just starting the program verify that the Default Size under the Members menu is set to 5 Let s start drawing a bridge uick Start S Bridge Builderva o O CH File Joints Members Display Mode Options Special Help p GRID COORDINATES DELTA X DELTA LENGTH ANGLE BRIDGE BRID
21. ther selecting another option or selecting Build from the Mode menu Show Sizes This option allows viewing of all member sizes at once Exit this option by selecting another option or selecting Build from the Mode menu You may clear the screen by selecting Refresh from the Display menu 9 4 Menu Options Default Size When you draw a new member a default member size is automatically used To change the default member size select this option from the Member pull down menu Next select the new default member size from the associated pop up menu Number Members Use this option to place the number assigned to each member at the center of the member for reference purposes You may clear your screen by selecting Refresh from the Display menu Reflect Member This is a drawing assist option To make a mirror image of a member across the mid span of the bridge select this option and click the left mouse button with the cursor over the member Members that straddle the mid span of the bridge cannot be reflected Members that just touch the mid span of the bridge can be reflected To exit the option select another option or select Build from the Mode menu Show Stats Use this option to display member information in the display at the bottom of the window Select the option then position the mouse cursor over the member and click the left mouse button Exit this o
22. tion If this option is selected before using Save As it will default to Save As Ifa bridge already exists with this name a replace option will be given Save As Use this option the first time you save a newly constructed bridge or when you want to change the name of the current bridge Before typing the filename double click on the Library folder The Library is the subdirectory where the bridge files should be saved 9 1 Menu Options Print The information contained in the printer output depends on the current Mode of the program See the Mode menu When Bridge Builder is in the Build mode the printout includes a picture of the bridge along with detailed member and joint information When Bridge Builder is in the Review mode the printout includes a picture of the bridge and the results of the latest analysis Exit Bridge Builder This option exits Bridge Builder and returns control to Windows If you have a current bridge that has changes and has not been saved you will be given the chance to save your work Menu Options Joints Delete Move Show Delete Use this option to delete joints member connections First select the Delete option then click the left mouse button with the cursor arrow over the joint When a joint is deleted all members connecting to that joint are also deleted Exit this mode by selecting the Build option under the
23. tion of Results Printer Output The Member Library The Structural System Technical Support iv oN DAUA ies O 11 12 13 14 Installation amp Setup INSTALLATION amp SETUP Part I Installing the Software Verify that there is at least 5 megs of free space available on your hard drive It s a good idea to reserve an additional 10 to 20 of your drive s capacity for general work space To install Bride Builder load the Bridge Builder CDROM in the drive The INSTALL program should run automatically If INSTALL does not autorun click the Start button select Run then type X INSTALL where X represents your CD ROM drive letter Click OK Configuring the Software for Sound The install routine in this chapter automatically configures Bridge Builder to work with an MCI compatible sound card You may turn the sound off by de selecting Sound from the Special menu Software Removal To remove Bridge Builder run the uninstall routine found in the BB4 folder 1 1 uick Start QUICK START Bridge Builder v4 0 ind File Joints Members Display Mode Options Special Help Pre Engineering Software Corporation presents Bridge Builder c Copyright 1987 1996 Pre Engineering Software Corporation All Rights Reserved The purpose of this chapter is to introduce the Bridge Builder commands necessary for running the program Note Bridge Builder will attemp
24. uilder gameboard is two dimensional In other words you can see the length and the height of the bridge but you cannot see the width of the bridge or the roadway that it supports Take a look at the three dimensional picture of the bridge below Now you can see the width of the roadway and the structural system the structural system is the path that the load of the truck uses to reach the truss and ultimately the abutments p LATERAL STRUT La TOP CHORD Ma OF TRUSS NO 2 BOTTOM CHORD OF TRUSS NO 1 Here s how the weight of the truck gets to the truss Assume the a truck is positioned between two floor beams as shown above two transverse beams one just behind the vehicle and one in front of the vehicle 13 1 The Structural System The weight of the truck travels along the floor slab into the stringers then along the stringers into the floor beams Then the floor beam delivers the weight of the truck into the truss at a joint Note that the bottom chord of the truss never gets any of the direct weight of the truck Therefore the bottom chord only experiences an axial load load directed along the axis of the member of either tension or compression In Bridge Builder you are designing one of the two identical trusses that carry the load into the abutments Each bridge truss must be capable of supporting an 80 000 pound truck plus any additional load that you have specified in the Options menu T
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