A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements, usually forming triangular units.
Overview of the Activity | Truss Bridge
In this learning activity, we will design, build, and test a model truss bridge. We will analyze the Owner’s
needs, then formulate specific design requirements. We will develop a truss configuration, analyze the structure, design each individual member and connection, then develop plans and specifications. Finally, we will build the bridge and test it to verify that it can carry load safely.
Why? | Truss Bridge
In Learning Activity #1, we played the role of the Constructor and built a model bridge that had been
designed by someone else. In Learning Activity #5, we will assume the role of the Design Professional and
design a new bridge with the same span length but with a different loading and a very different geometric configuration. In doing so, we will learn a process that can be used to design a bridge with practically any span length, loading, or configuration.
This project provides an opportunity to apply everything we have seen in the previous four learning activities. We will see how the various elements of the engineering design process fit together—how scientific principles, mathematic tools, engineering concepts, experimental data, and practical considerations contribute to the final product. We’ll see how the truss configuration is tailored to the Owner’s needs; how the structural model is derived from the truss configuration; how structural analysis results and experimental data contribute to the design of structural members; how the size and shape of connections are determined; how constructability considerations affect the final design; and how engineering computations are translated into the drawings and schedules required for construction. Finally, we will build the bridge we designed—a great way to check the validity of the design and the accuracy of the plans and specifications.
As a result of this learning activity, you will be able to do the following:
- Explain how design-build project delivery differs from design-bid-build project delivery.
- Explain how the factor of safety is used in design.
- Explain how scientific principles, mathematic tools, engineering concepts, experimental data, and practical considerations contribute to the engineering design process.
- Design a model truss bridge to meet a set of design requirements.
- Build a model truss bridge, consistent with a set of plans and specifications.
To successfully complete this learning activity, you must understand the following key terms and concepts
from previous learning activities:
If you need to refresh your memory on any of these terms, see the Glossary in Appendix D.
Using the Factor of Safety in Design
When we analyzed a structure in Learning Activity #3, we used the following definition for the factor of
To use this equation, we first determined the internal force in each member (by doing a structural analysis) and the strength of each member (by using our experimental data from Learning Activity #2). Then we used these numbers to calculate a unique factor of safety for every member in the structure. In short, we used known values of internal force and strength to calculate unknown factors of safety.
When we design a structure, we need to select members that are strong enough to carry load safely. Thus, in design, the unknown quantity in the equation above is the strength. The known quantities are the internal forces and the factor of safety. As before, the internal member forces are determined by a structural analysis; but in design, we will simply specify the factor of safety. We might use a design code as the basis for deciding what the factor of safety should be, or we might simply use our experience and judgment. In either case, we will choose a value that appropriately reflects the level of safety required for our structure.
Since strength is the unknown quantity, it makes sense to algebraically rearrange the equation above by
multiplying both sides by the internal member force. The result is
The product on the right-hand side of this expression—the factor of safety times the internal member
force—is called the required strength. This expression tells us that the actual strength of a member must be greater than or equal to its required strength. We use > because it’s always OK for a member to be “too strong.” Indeed, as we saw in Learning Activity #3, sometimes it makes good economic sense for some members in a structure to be stronger than they really need to be.
We will use the expression above as the basis for determining the size of each structural member in our
Design-Build Project Delivery
As we discussed in Learning Activity #4, most public works projects in the United Sates use design-bid-build project delivery. In this system, (1) the Design Professional develops a complete design and provides it to the Owner, (2) the Owner advertises the project, (3) construction contractors submit bids, and (4) the Owner awards the construction contract to the lowest responsive, responsible bidder. Owners typically use designbid-build project delivery because the competitive bidding process tends to keep the construction cost low.
However, this system has some significant disadvantages as well:
- In design-bid-build project delivery, the Design Professional often has only minimal involvement in the construction phase of the project. Thus the designer is not able to ensure that that structure is built as intended.
- The Constructor is never involved in the design process. Thus constructability issues may not be fully considered in the design.
- The period of time required for advertising, collecting contractors’ bids, and awarding the construction contract can be quite substantial. At this point in the process, the design is complete, and construction activity has not yet begun. Thus this entire period is essentially non-productive.
For these reasons (and others), an alternative system called design-build project delivery is becoming
increasingly popular. In a design-build project, a single firm contracts with the Owner to do an entire project—both design and construction. Thus, in a design-build project, there is no break in continuity between design and construction. Coordination between the Design Professional and the Constructor is likely to be more effective, because one firm has overall responsibility for the project. Eliminating the bidding phase may also speed up the project. Indeed, with design-build project delivery it is possible for construction to begin even before the design is complete—a procedure called “fast-tracking.”
Of course, design-build project delivery also has its disadvantages. Thus the best means of project delivery always depends on the nature of the project.
Recently a tractor-trailer truck lost its brakes while driving on Grant Road. The driver lost control of the
vehicle, and it collided with one of the end posts on the west end of the Grant Road Bridge. Fortunately, no one was hurt; but the bridge was damaged beyond repair. Grant Road is now closed, and the Town of Hauptville has initiated a project to replace the structure as quickly as possible.
The Town of Hauptville is the Owner for this project. On behalf of the Owner, the Town Engineer has again
hired Thayer Associates to provide design services. Thayer Associates has sent a team of engineers to begin working on the needs analysis. The engineers meet with the Mayor, the Town Council, the Town Engineer, and other Hauptville residents to work out the functional and aesthetic requirements for the new structure.
At the meeting, the engineers receive the following input:
- The Mayor says, “I don’t want another bridge failure in my town. I want you to ensure that this new bridge is not as vulnerable to a vehicular collision as the old one was.”
- The President of the Town Council adds, “We didn’t plan on having to replace a bridge when we developed this year’s budget. The cost of this project must be kept as low as possible.”
- Another member of the Town Council adds, “The residents of Hauptville are very upset about the closure of Grant Road. We need to get this project completed as soon as possible.”
- A member of the Hauptville Historical Society says, “I know money is tight. But it would be a terrible mistake to build an ugly bridge, just to save some money. We at the Historical Society think it’s important to the preserve the historic character of the town so, if possible, we’d like the new bridge to be a truss.”
- Finally, the Town Engineer adds his own input: “I am still very concerned with the ever-increasing number of heavy trucks using Grant Road. To give us an added margin of safety, I’d like the new structure to be designed for a 20% higher vehicular loading than the AASHTO bridge design code requires.” Based on this input, as well as data gathered from a thorough investigation of the project site, the engineers from Thayer Associates develop the following design requirements:
- The replacement bridge will be constructed on the existing abutments, which are 24 meters apart. [Again our 1/40 scale model bridge will have a span of 60 centimeters.]
- Like the previous bridge, the new structure will carry two lanes of traffic. However, the width of the deck will be increased by 20% to provide more space for larger vehicles. [Our model bridge will have a roadway width of 11 centimeters—2 centimeters wider than the first Grant Road Bridge model.]
- The bridge will be designed for a vehicular loading 20% larger than that required by the AASHTO bridge design code. [Our model bridge will be designed for a “traffic load” consisting of a 6 kilogram mass placed on the structure at mid-span; the first Grant Road Bridge model was designed for only 5 kilograms.]
- The factor of safety will be 2.0.
- The bridge will be made of steel. [Again, our model will use cardboard from standard manila file folders.]
- The bridge configuration will be a deck truss. With no portion of the structure extending above the roadway, the bridge will be invulnerable to a vehicular collision.
- Because of the limited project budget, the cost of the new bridge must be kept to a minimum.
- To get the bridge into service as quickly as possible, design-build project delivery will be used for this project. Consistent with this requirement, Thayer Associates enters into a partnership with Mahan Construction Company, a local contractor, to do the project.
You are the Chief Engineer for Thayer Associates. You are the Design Professional for this project. Your
responsibility is to design a replacement for the Grant Road Bridge that meets all of the Owner’s requirements. Once the design is complete, you will continue to work with Mahan Construction Company to ensure that the bridge is built correctly.
Our plan to design the new Grant Road Bridge consists of the following major activities:
- Decide on a truss configuration.
- Create the structural model.
- Check static determinacy and stability.
- Calculate reactions.
- Calculate internal member forces.
- Determine member sizes.
- Check member sizes for constructability.
- Draw plans.
- Create a schedule of truss members and a schedule of gusset plates.
- Build the bridge.
Decide on a Truss Configuration
In general, when you design a truss bridge, you may use any stable truss configuration that satisfies the
project requirements. Of course, for any given set of project requirements, some configurations are bound to be more efficient than others. An experienced engineer might be able to choose an efficient configuration based simply on what has worked well for previous projects. If you lack experience, you might try several different alternative configurations, develop a preliminary design for each one, and select the configuration that costs the least. You might also base your selection on aesthetics or constructability, rather than on structural efficiency.
For this specific project, the only constraint on the selection of a truss configuration is that it must be
a deck truss.
Fortunately, we do have previous experience with designing this particular bridge type. In Learning Activity 4, we used the West Point Bridge Designer software to design a Warren Deck Truss that proved to be quite efficient. Let’s use this same configuration for our Grant Road Bridge replacement. This configuration is also included as Truss 16 in the Gallery of Structural Analysis Results (Appendix B). By using a configuration that is included in the Gallery, we will be able to save considerable effort in our structural analysis.
Create the Structural Model
Having selected a truss configuration, we will now model the structure, by defining (1) the geometry of the truss, (2) the loads, and (3) the supports and reactions—just as we did in Learning Activity #3. We idealize the three-dimensional bridge as a pair of identical two-dimensional trusses. The geometry of one main truss is shown below. The dimensions indicate the locations of the member centerlines. Joints are identified with the letters A through M.
Note that the dimensions of our structural model are all consistent with the dimensions shown for Truss 16 in the Gallery of Structural Analysis Results. The Gallery shows that each of the six top chord members has a length L. To achieve a total span length of 60cm, as the design requirements specify, we must use L=10cm. Now the remaining dimensions are calculated using this same value of L. For example, the Gallery shows the overall height of the truss as 1.375L. Since we have defined L as 10cm, the height of our structural model is
Once we have determined the geometry of the truss, we can calculate the loads. According to the design
requirements, the bridge must be capable of safely carrying a 6-kilogram mass placed on the structure at midspan. The weight of a 6-kilogram mass is
Again we will apply this load by placing a stack of books onto the top chord of the truss. The weight of the stack will be supported on six joints—C, D, and E on each of the two main trusses. Assuming that the weight of the books will be distributed equally to these six joints, the downward force applied to each joint is
Note that we could have gotten this same result directly from the Gallery of Structural Analysis Results. The diagram for Truss 16 shows that a downward load of 0.1667W is applied to each of the three center top-chord joints. For a total load W=58.86N, the load at each joint is
The bridge will be supported only at its ends; thus, the reactions RA and RG are shown at Joints A and G.
Check Static Determinacy and Stability
Before we can use the equations of equilibrium to analyze a truss, we must first verify that it is statically
determinate and stable. As we saw in Learning Activity #3, the mathematical condition for static determinacy and stability is
where j is the number of joints and m is the number of members. Our structural model has 13 joints and 23 members. Substituting these numbers into the equation above, we find that 2j and m+3 are both equal to 26, so the mathematical condition for static determinacy and stability is satisfied.
Note once again that we could have gotten this same result directly from the Gallery of Structural Analysis
Results. The diagram for Truss 16 indicates that each reaction has a magnitude of 0.25W. For a total load
W=58.86N, each reaction is
Calculate Internal Member Forces | Truss Bridge
At this point in the design process, we must determine the internal force in each member of the truss. As
long as the truss is statically determinate, we can always calculate internal member forces by applying the
Method of Joints, just as we did in Learning Activity #3. However, when we use a truss configuration from the Gallery of Structural Analysis Results, we can determine these forces with considerably less effort.
Each truss in the Gallery is presented with a complete set of internal member forces, calculated for the
loading shown. The internal forces are shown in terms on the total applied load W. To determine the internal member forces for our specific loading, we just substitute W=58.86N for each member. For example, the Gallery indicates that Member AB in our structural model has an internal force of -0.167W. Therefore, the force in…
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