Solid web members are the vertical and diagonal elements that connect the top and bottom chords of a truss or built-up beam, and their placement directly determines how well the structure handles loads. Getting the spacing, angle, and orientation right affects everything from shear resistance to buckling prevention. Here’s what matters when deciding where and how to place them.
Diagonal vs. Vertical Web Members
The choice between diagonal and vertical web members depends on the type of truss and the forces you need to manage. In a Pratt truss, the diagonal members carry tension under gravity loads, while the vertical members handle compression. This is efficient because shorter vertical members resist compression better than long ones would. In a Warren truss, diagonal members alternate between tension and compression, eliminating vertical members entirely in the basic configuration.
Vierendeel trusses use only vertical members between the chords, with no diagonals at all. This creates a clean, open look that works well when diagonal members would block doorways, windows, or ductwork. The tradeoff is significant: members in a Vierendeel truss experience bending, shear, and axial force simultaneously, while members in conventional trusses with diagonals primarily handle axial loads only. That makes Vierendeel designs heavier and more expensive. The vertical members closest to the supports carry the highest bending moments and need larger cross-sections than those near midspan.
Optimal Angles for Diagonal Members
When placing diagonal web members, the angle relative to the chords should fall between 35° and 55°. Angles outside this range create either very long compression members (prone to buckling) or very short, steep members that don’t efficiently transfer load across the span.
A practical rule: orient the diagonals so the longest members end up in tension and the shorter ones handle compression. Tension members can be slender because they’re being pulled taut, while compression members need to be stocky enough to resist buckling. Keeping compression members short lets you use smaller, lighter sections without sacrificing stability. This single principle drives the layout logic for most standard truss configurations.
How Web Members Resist Shear
The primary job of web members is transferring shear forces between the top and bottom chords. In any beam or column under load, shear is highest near the supports and decreases toward midspan. That’s why web member spacing is typically tightest near the ends of a truss and can be wider in the middle.
Diagonal web members resist shear the way bent-up reinforcing bars do in concrete: by crossing the path of diagonal cracking or deformation at an angle. The shear capacity they provide depends on the cross-sectional area of the member, the yield strength of the material, and the angle between the diagonal and horizontal. Steeper angles (closer to vertical) contribute more to shear resistance in the vertical direction, while shallower angles spread the force over a longer horizontal distance. Horizontal web members, when present, function more like stirrups, providing distributed shear resistance along the height of a column or deep beam.
For built-up columns with web members on multiple faces, the total shear capacity is the sum of what the web members carry plus what the surrounding material (concrete, if encased) provides. Engineers size and space web members so their combined capacity exceeds the maximum shear demand at every section along the member’s length.
Spacing to Prevent Buckling
Web members also serve as lateral bracing points for the compression chord. Without them, a long compression flange can twist and deflect sideways in a failure mode called lateral-torsional buckling. The maximum distance you can leave between bracing points before this becomes a problem is the critical unbraced length.
Testing on castellated beams (beams with openings cut into the web) gives a sense of the numbers involved. For a 24-inch deep section weighing about 26 pounds per foot, the critical unbraced length ranges from roughly 20 to 27 feet depending on the analytical method and whether you use the properties of the full section or just the tee section at a web opening. For a heavier 27-inch deep section at 40 pounds per foot, those values climb to about 24 to 31 feet. These are upper bounds for ideal conditions. Real-world connections, load positions, and imperfections typically reduce the safe unbraced length.
When loads are applied to the top flange (as with a floor or roof sitting on top of a beam), the critical unbraced length drops compared to loads applied at the beam’s center of gravity. A load pushing down on the top of a beam that’s trying to buckle sideways makes the problem worse, so web members or other bracing need to be spaced closer together in that scenario.
Material Considerations: Timber vs. Steel
The material you’re building with changes how you think about web member placement. Wood handles compression well but is relatively weak in tension. Steel handles both, but excels in tension. This is why many hybrid trusses use timber for compression members (the top chord and short vertical posts) and slender steel rods or bars for the tension diagonals.
If your structure has no tension members at all, timber is the clear choice for minimizing embodied carbon. But most real trusses have at least some members in tension, and that’s where steel earns its place. The combination of chunky timber compression members and thin steel tension members is a pattern you’ll see in many existing buildings, and it represents an efficient use of each material’s strengths.
For solid timber web blocks (as used in some engineered wood beams), the blocks need to bear tightly against the top and bottom chords to transfer vertical loads through direct compression. Any gaps mean the load path relies on fasteners alone, which dramatically reduces capacity. Steel web members, by contrast, can be welded or bolted and still develop their full strength through the connection rather than through bearing contact.
Practical Placement Sequence
When physically installing solid web members during assembly, a few principles keep the process accurate and the final structure sound:
- Work from the supports outward. The members nearest the supports carry the highest shear and, in Vierendeel configurations, the highest bending moments. Placing these first and verifying their alignment sets the geometry for everything else.
- Check plumb and square at each panel point. A web member that’s even slightly out of plane introduces eccentricity, which means the load hits the member off-center. This is especially damaging for compression members, where eccentricity accelerates buckling.
- Maintain consistent spacing. The spacing between web members was calculated to keep shear capacity above demand and unbraced lengths below critical thresholds. Shifting a member even a few inches can create a weak panel where buckling or shear failure could initiate.
- Secure connections before releasing temporary bracing. Until every web member is fastened to both chords, the truss or built-up section doesn’t have the lateral stiffness the design assumes. Removing clamps or temporary supports too early lets the compression chord move freely over a longer unbraced length than intended.
The connection type matters as much as the member placement. Welded connections in steel provide full moment transfer. Bolted connections introduce some flexibility and potential for slip, which can change how forces distribute through the web. In timber, metal connector plates with pressed teeth or through-bolted gussets are standard, and the capacity of the connection often governs the design rather than the wood member itself.