The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →A truss bridge carries weight through connected members and joints to its supports. But the completed bridge and a partly built bridge are not necessarily the same structural system: supports, bracing, load paths and member forces can change during erection. That is why construction-stage checks must consider the members, their connections and the stability of the whole structure—not just the finished design.
How does a truss bridge carry weight?
A truss is an arrangement of connected members, commonly organized into triangles. The deck and floor system transfer loads to points along the truss, which carries them through its members toward the supports. In an idealized pin-jointed model, members primarily carry axial tension or compression.
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Chords, verticals and diagonals share the forces, but no member has one fixed force role in every situation. The truss geometry, where and how loads are applied, support conditions and construction stage all affect whether a member is in tension or compression and how much force it carries. This model helps explain the load path; it is not a substitute for analyzing a real bridge. FHWA’s 2015 construction stability reference manual discusses structural behavior and stage-specific analysis.
Connections are part of the load path
Members transfer forces through joints. In steel trusses, gusset plates connect chords, diagonals and verticals; they are structural elements with their own capacity, not merely covers over a joint. A bridge can therefore have a connection problem even when its main members have not reached their capacity. FHWA’s gusset-plate guidance describes these plates and their role in steel truss connections.
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Why can a bridge be less stable while it is being built?
Erection changes the structure over time. A partly assembled truss may not yet have all its permanent bracing; temporary supports or falsework may hold it in locations different from the final supports. Lifting, sliding, jacking or other operations can change the load path and produce forces that differ from those in the completed bridge. A construction-stage configuration can also be more exposed to lateral instability.
FHWA recommends evaluating each stage of construction for the loads and stability conditions that actually exist at that point. Its guidance includes:
- Dead load from members already installed and live loads from construction activity.
- Environmental effects, including wind and thermal forces.
- Demand and capacity of individual members, as well as lateral strength and overall stability.
- Concentrated loads from operations such as lifting or jacking, including the stability of the truss and falsework during lifting or sliding.
Concentrated jacking forces can lead to local buckling, yielding or crippling. FHWA notes that placing jacks at panel points, where practical, can distribute their loads into the framing. If a stage check identifies overstress, the manual describes responses such as temporary bracing, increasing member cross-section or revising the erection plan. These are engineering considerations—not instructions for carrying out bridge work without project-specific professional design. Read FHWA’s construction stability reference manual.
What can make a bridge collapse during construction?
A collapse can involve inadequate capacity in a member or connection, local instability, loss of overall lateral stability, or a load or support condition that was not correctly represented in analysis. More than one factor may contribute. It is useful to distinguish the component that failed from the loads and decisions that contributed to the demand; there is no universal sequence that explains every bridge failure.
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I-35W: inadequate gusset-plate capacity under combined loading
The I-35W bridge in Minneapolis collapsed in 2007. The National Transportation Safety Board (NTSB) determined that gusset plates at the U10 nodes had inadequate load capacity. The plates failed under the combined effect of increased bridge weight from previous modifications, traffic and concentrated construction loads present on the day of the collapse. NTSB’s investigation finding identifies a connection-capacity problem under combined loading; it does not attribute the collapse to defective steel or poor erection.
The case shows why connection capacity and construction loads belong in the analysis alongside member forces. Materials or equipment placed in a concentrated area can add demand to a structure, so their location and the work-stage configuration matter.
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FIU pedestrian bridge: calculation errors at a node and deck connection
On March 15, 2018, a partially constructed pedestrian bridge collapsed at Florida International University while a crew was retensioning post-tensioning rods. NTSB’s probable-cause finding was load and capacity calculation errors in the design of the main-span truss member 11/12 nodal region and its connection to the deck. NTSB’s investigation page describes the finding.
The relevant lesson is that an operation changing forces in a partly built structure needs to be evaluated in that actual work-stage configuration. Retensioning was occurring when the collapse happened, but NTSB’s probable-cause finding was the calculation errors at the node and deck connection—not retensioning alone.
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Can a gusset plate fail even when the truss members are still intact?
Yes. A connection plate can have inadequate capacity for the forces transferred through it even when the connected members have not themselves failed. The I-35W investigation is a documented example: NTSB identified inadequate capacity in the U10 gusset plates under the combined loading present at the time. FHWA’s gusset-plate guidance also recounts the undersized-plate finding.
That does not mean every truss connection is more vulnerable than its members. It means the connection, member and overall stability checks answer different questions; adequacy in one does not establish adequacy in the others.
Does redundancy mean a damaged truss will stay standing?
Not necessarily. An alternate load path can allow surrounding members to take on forces after sudden damage to a critical member, potentially preserving stability. Whether that redistribution is possible and effective depends on the specific structure and its design. FHWA’s study concerns long-span steel truss bridges and should not be read as a guarantee for every truss or bridge form. FHWA’s alternate-load-path study examines this subject in that context.
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