How to design trusses in Calcs.com

How trusses carry load: tension, compression, and the truss versus frame distinction

Laurent opened by distinguishing trusses from frames. A truss resists loads through tension and compression in its individual members: loads are assumed to act only at the joints, so each member carries only axial force. A frame resists loads through bending. In Calcs.com, the same workflow supports both: the Truss Analysis wizard is used for trusses, and the Portal Frame wizard for frames. The design-only linking approach is identical for both structure types.

The two-stage workflow in Calcs.com: analysis then design

Designing a truss in Calcs.com follows two linked stages. First, create a Truss Analysis wizard calculation. Enter the truss geometry (type, span, pitch, bay count) and apply loads. Run the analysis to get axial forces, bending moments, shear, and displacement for every member.

Second, create design-only calculations linked to the analysis using the Link to Analysis Module. When creating a design-only calculation, select the member type and choose the envelope option to design for the worst-case forces across all members of that type. The envelope selects the longest member length (conservative for buckling) and the highest force from any individual member in the group. Alternatively, individual members can be linked for a sharper design.

Laurent confirmed that linking is one-way: changes in the analysis propagate automatically to linked design calculations, but changing a member size in a design calculation does not update the analysis automatically. After finalising member sizes, go back to the truss analysis and update the member inputs to match, then check that the design calculations still pass with the recalculated forces.

One caveat: Calcs.com checks buckling using the member length between panel points, not the full chord length. This means global buckling of an entire chord acting as a single strut is not checked. For residential trusses braced by a diaphragm, this is generally not a concern. For larger or unbraced trusses, global buckling should be checked separately.

Worked example: 50-foot residential Howe roof truss

Laurent designed a 50-foot-span Howe roof truss at a 4:12 pitch, with a 2-foot eave, under 720 PLF snow load and 660 PLF uplift. Self-weight was excluded for this demonstration.

For the snow load case, the top chord design-only calculation was linked to the top chord envelope. With Douglas fir large, number 1 grade, the 8 by 8 and 10 by 10 failed under the combined bending and compression interaction. An 8 by 16 passed at 87% utilisation. The bottom chord, which carries approximately 45,000 pounds in tension, was initially sized at 8 by 10. After updating the truss analysis with the larger members and recalculating, the bottom chord failed at 106%. Applying a duration factor of 1.15 for the short-term snow load case brought utilisation to 92%, and the final selection was an 8 by 12. Web members were sized at 8 by 8, passing comfortably.

For the uplift case, Laurent copied the analysis and design calculations into a separate group and updated the truss loads to 660 PLF uplift. Under uplift, the top chord is in tension. Applying the wind duration factor provided additional capacity, and all members passed. The snow load governed.

Worked example: 120-foot steel cantilever Warren truss

The second example was a 120-foot Warren truss with five bays and a two-bay cantilever. The pin support was moved from node 12 to node 8 using the Advanced Loads section to create the cantilever configuration. A 1,000-pound point load was applied at the tip of the cantilever by setting the distance from start node to the full element length (removing the default L/2). A 100 PLF distributed load was applied over the cantilever span elements using element-by-element input.

The bottom chord was designed as a steel member linked to the bottom chord envelope. A W12x45 passed, but the target was an HSS section. With a maximum depth of 8 inches, an HSS 4x4x1/4 passed initially. After updating the truss with the HSS member and recalculating, an HSS 4x4x3/8 was selected to handle the recalculated forces. Final utilisation: 97% under combined compression and bending interaction.

Advanced loads: element-by-element and node-by-node input

The Advanced Loads section provides full control over where loads enter the truss. In addition to the cantilever example above, Laurent demonstrated lateral wind load input by setting the load orientation angle to 0 degrees (horizontal) on individual elements, instead of the default 90 degrees (vertical gravity direction). He also showed how eave loads can be set independently of the main top-chord load, for example to apply twice the standard uplift over the eave length while keeping a different value on the rest of the chord. Advanced Loads is the tool for any situation where standard distributed top-chord input does not reflect how loads actually enter the structure.