A large girder truss is a prefabricated or field-assembled frame that carries roof, floor, or bridge loads across spans too wide for solid beams. Aircraft hangars, warehouses, arena roofs, and industrial plants use these members in the 40 to 120 foot range, while bridge girders extend well beyond 200 feet. Designers who study the plate girder components and functions used in solid-web members will recognize how a truss version adapts that anatomy: deep flanges take bending, a web resists shear, and stiffeners control local buckling.
A truss achieves the same job with less steel by replacing the solid web with open diagonal framing. The open layout leaves room for ductwork, conduit, and lighting, which is why mechanical contractors favor the arrangement. Erection speed matters too: a complete girder truss can arrive on one truck and lift into place in a single crane pick, saving weeks of site welding.
How Girder Trusses Carry Heavy Roof Loads
Every girder truss transfers load through the same chain: roof deck to purlins, purlins to panel points, panel points through web members into the chords, and chords into columns or bearings. Loads should arrive at the frame’s panel points, where web members meet the chords, so each member works in pure tension or compression instead of bending. The same load path shows up in lighter frames, because timber roof trusses rely on identical geometry with wood chords and metal connector plates.
Load Path From Deck to Foundation
Follow the forces on a typical 80 foot girder truss supporting a metal roof. Snow and dead load on the deck travel down through purlins spaced 5 to 8 feet apart. The purlins land on top-chord panel points, never mid-panel, because a panel-point load keeps the chord segment between nodes in simple compression.
- Dead load: roofing, insulation, deck, and the self-weight of the truss itself.
- Live load: snow, rain, and maintenance traffic, taken from local code maps.
- Collateral load: suspended ceilings, ductwork, sprinkler pipe, and light fixtures.
- Lateral load: wind and seismic forces that enter through the roof diaphragm.
Depth-to-Span Ratios That Work
Truss depth drives both weight and deflection. A depth-to-span ratio of 1/10 to 1/15 handles most roof applications, so a 100 foot truss stands 7 to 10 feet deep. Ratios below 1/20 push member sizes up sharply, and ratios above 1/8 cost more in wall height and cladding than they save in steel.
Deflection limits usually control the final sizes. Roof trusses commonly hold live-load deflection to L/360, and frames carrying plaster ceilings or sensitive equipment tighten that to L/480 or L/600. Camber, a slight upward pre-set built into the chord, offsets dead-load sag so the finished roof reads flat.
Connection Details and Field-Built Girder Trusses
Connections decide how a girder truss behaves in service. Bolted gusset plates remain the most common shop and field connection because they allow on-site adjustment and inspection. Welded connections save material and look cleaner, but they demand qualified welders and careful quality control. For wood frames, high-capacity structural screws, including FlatLok screws for built-up beams, let carpenters assemble engineered members without glue, heavy clamps, or a laminating press.
Bolted Connections
A bolted gusset connection transfers force through shear in the bolts and bearing in the plate. Standard practice uses ASTM A325 or A490 bolts for steel, with snug-tight installation for most roof trusses and slip-critical joints where vibration or load reversal occurs. Edge distance and bolt spacing follow AISC rules to prevent tear-out.
Field Splices
Trusses too long for one truck arrive in pieces and splice at the site. Splices belong at points of low moment, usually the third points of the span. A 90 foot truss splits into three 30 foot pieces, with flange splice plates bolted on both sides of the chord and web members joined at the panel point.
| Connection Method | Material | Typical Use | Strengths | Limits |
|---|---|---|---|---|
| Bolted gusset | Steel | Shop and field splices | Adjustable, inspectable | More bolts and plates |
| Welded | Steel | Shop fabrication | Clean, light joints | Needs certified welders |
| Structural screws | Wood | Built-up beams and trusses | Fast, no glue | Capacity limited to wood |
| Nail and plate | Wood | Light roof trusses | Cheap, quick | Lower load capacity |
Design Principles for Steel Girder Trusses
The design principles of steel trusses start with one assumption: every member carries axial load only. Chords resist the global bending moment, with the top chord compressing and the bottom chord tensioning under gravity. Web members shuttle shear between panel points, alternating tension and compression as the load pattern moves.
Choosing a Web Configuration
Three patterns dominate. Warren trusses use alternating diagonals and suit uniform loads with the fewest members. Pratt trusses angle diagonals toward the center and keep long compression members short, a good fit for heavy point loads. Howe trusses reverse the diagonals and work well when the top chord carries the load directly.
Buckling Checks That Govern Sizing
Compression governs most truss designs. A slender top chord or diagonal buckles well before its material reaches yield, so designers check slenderness ratios, unbraced lengths, and torsional stability. Lateral bracing at panel points shortens the unbraced length of the compression chord, and that single decision often trims member size more than any other.
- Model the frame and apply code loads: dead, live, snow, wind, and seismic.
- Run the analysis and record the axial force in every member.
- Size chords for combined moment and axial demand.
- Size diagonals and verticals for axial force and slenderness.
- Design each connection for the forces at its panel point.
- Check deflection, camber, and stability, then add bracing.
Girder Trusses in Bridge and Heavy-Civil Applications
Bridge work pushes girder trusses to their limits. The same open-web logic appears in plate girder bridges, where solid-web girders handle spans of 60 to 200 feet and truss girders extend the range further with less steel weight. Floor beams frame between the main girders, and stringers carry the deck down to the floor beams.
Deck Systems and Load Distribution
A composite concrete deck shares load with the top flanges of the girders through shear studs. Older structures use a non-composite system with the deck resting on stringers, which simplifies replacement but wastes steel. Either way, cross frames at intervals of 15 to 25 feet hold the girders plumb and distribute wind and braking forces.
Maintenance access matters on long bridges. Walkways, inspection ladders, and anchor points inside the truss web turn a periodic visual check into a safe operation. Painted steel needs recoating every 15 to 25 years depending on exposure, while weathering steel skips the paint cycle entirely in the right climate.
Erection, Bracing, and Site Handling
A girder truss earns its reputation on the ground, not in the drawings. The erection sequence starts with a lift plan that matches crane capacity to the heaviest pick, which is often the first end of the truss rather than the whole member. Rigging spreads the load across two or more pick points so the truss does not buckle sideways under its own weight.
- Set temporary towers or cribbing at splice points before the first piece lands.
- Lift the first truss and bolt it to its bearing seats, leaving the top flange free.
- Install temporary guy wires or a crane-held tag line before releasing the hook.
- Place the second truss and connect the first bay of purlins or roof deck.
- Release rigging only after permanent bracing ties the pair into a stable unit.
Temporary Bracing
Wind is the biggest threat to a bare steel frame. A row of unbraced trusses acts like a line of dominoes once the top flange starts to twist. Erection drawings specify the minimum number of purlins and cross braces to install before the crew can leave a bay overnight.
Lateral Stability During Lifting
Long compression chords want to buckle in the weak direction during a lift. Spreader bars and multiple slings reduce the unsupported length, and a strongback, a small truss bolted across the top chord, keeps the member straight until the permanent bracing lands.
Selecting the Right Girder Truss System
Selection comes down to span, load, budget, and delivery. The roof truss selection criteria that guide light frames scale up to girder trusses: compare material cost, fabrication time, shipping constraints, and erection cost, not just the price per ton of steel.
For very long spans or heavy point loads, a truss beats a solid girder on weight. For shallow depth or a tight envelope, a plate girder wins. When torsion dominates, as it does on curved alignments and single-girder systems, a closed section handles twist far better than an open truss; box girder bridges demonstrate the torsional advantage of a closed cell.
Whichever system the analysis favors, the decision deserves the same rigor as the member design: shop drawings reviewed before fabrication, connection details checked against the model, and an erection sequence written before the first truck arrives. Large girder trusses reward that planning with spans that stay flat, quiet, and stable for decades.