Trellis Engineering
Structural trellis engineering: column loads, wind and package limits
These packages are calculated to IBC 2018 and ASCE 7-14, Risk Category I for open structures.
What makes a trellis structural rather than decorative?
A structural trellis has posts on footings and a sealed calculation behind it. The stamped 18 ft by 3 ft trellis package is calculated to the same criteria as the larger structures, and it can be submitted for permit review the same way. Acceptance still rests with the local permitting jurisdiction.
What design criteria is the trellis calculated to?
The same criteria as the larger structures, even though it carries far less. Ground snow is run at 30 psf and 50 psf, becoming 23 psf and 38 psf at the roof plane. Dead load is 15 psf, live load 20 psf. The wind input is 115 mph ultimate in Exposure B — the figure the calculation starts from, not a survival promise — over soil assumed at 1,500 psf.
What loads does the stamped trellis carry?
The full-dimension 10 in. by 10 in. columns carry 1,800 lb at 30 psf ground snow and 2,100 lb at 50 psf, against 62,500 lb of bearing capacity — 2.9 percent and 3.4 percent. The governing wind case totals 457 lb, or 114.25 lb at each of four load points, which the package reports rounded to 114 lb.
Why is the load column higher than the plan pressures suggest?
All nine of our sealed packages print column loads above dead plus roof snow spread over the plan — between one and three and a half times higher. That pattern holds across every package rather than being particular to this one, so the load column carries inputs the plan pressures on this page do not show. The sealed calculation prepared for your address shows that work; this page reports its result.
What does that 62,500 lb capacity mean?
It is the bearing capacity printed in the sealed packet: 625 psi of compression perpendicular to grain across the stated bearing area, which is 100 sq in. for a 10 in. by 10 in. post. All members are Douglas Fir #1 or better, the species group whose published compression-perpendicular-to-grain design value is that 625 psi. Read 2.9 percent as crushing at the bearing surface only. It is not the column’s axial, buckling, bending or connection utilization, and any of those four can govern at a far higher percentage. Bending, shear, deflection and the connections are checked separately in the sealed package.
Where do the load figures come from?
The bearing table in the sealed packet. They are the reactions the calculation reports for each member, on the tributary areas the engineer assigned. The trellis sheet prints a 54 sq ft tributary area and an 810 lb unfactored dead total, which is 15 psf over the 54 sq ft plan and 45 lb per foot along the 18 ft length. That figure reproduces exactly.
What about the line loads printed on the same sheet?
The sheet also prints 120 lb per foot dead, 160 lb per foot roof live, 184 lb per foot roof snow and 272 lb per foot wind. Those line loads carry the same 8 ft tributary length the pavilion sheet uses, and an 8 ft tributary width is not possible on a plan 3 ft wide. The wind line load does not reconcile either: 272 lb per foot over the 18 ft length is 4,896 lb, and even at the true 3 ft width the implied 34 psf over 54 sq ft is 1,836 lb, against a governing wind total of 457 lb on the same package. We publish the line loads as printed rather than dropping them, and we use the 457 lb governing case, which is the figure the package carries forward into the post loads and the foundation check. Do not size anything from the line loads.
What plan and post size does the stamped trellis cover?
The stamped package covers an 18 ft by 3 ft plan on full-dimension 10 in. by 10 in. posts with a 3 in. by 8 in. classic knee brace. A longer open span is outside that package and needs its own sealed calculation rather than an extension of these numbers.
Why is trellis wind load so much lower than a pavilion?
Footprint, almost entirely. The pavilion wind model puts 1,265 lb at each load point against the trellis’s 114 lb, about eleven times as much. The two structures are nowhere near the same size: the trellis package models 18 ft by 3 ft, which is 54 sq ft, and the pavilion’s wind sheet models 24 ft by 23 ft — the analysis dimensions behind the 20 ft by 30 ft pavilion — which is 552 sq ft, a little over ten times larger.
Per square foot of plan the two land close together: 9.2 lb for the pavilion and 8.5 lb for the trellis, about 8 percent apart. Since the plan areas differ by a factor of ten and the per-square-foot figures differ by 8 percent, the eleven-to-one gap at each load point is the size of the structure, not the open roof. The slat allowance is real — the packages take 50 percent off projected wind area and 25 percent off wind load for a slatted roof — but across these two packages it sits inside that 8 percent residual rather than driving the difference.
What foundation does a structural trellis need?
The same post base as the larger structures: a quarter-inch aluminum knife plate set inside the post and through-bolted, anchored into concrete over soil assumed at 1,500 psf. That detail is proven once against a bounding uplift demand of 3,299 lb rather than recalculated for each plan.
How is bearing capacity calculated across every structure?
One method, every package: 625 psi times the member’s bearing area. That 625 psi is the published compression-perpendicular-to-grain design value for Douglas Fir-Larch, and every member in these packages is Douglas Fir #1 or better. Sixty-two of the sixty-two bearing capacities published across our nine structures reproduce from it exactly.
| Member | Bearing area | Capacity |
|---|---|---|
| 4 in. by 8 in. | 32 sq in. | 20,000 lb |
| 4 in. by 10 in. | 40 sq in. | 25,000 lb |
| 4 in. by 12 in. | 48 sq in. | 30,000 lb |
| 4 in. by 14 in. | 56 sq in. | 35,000 lb |
| 4 in. by 16 in. | 64 sq in. | 40,000 lb |
| 6 in. by 16 in. | 96 sq in. | 60,000 lb |
| 6 in. by 20 in. | 120 sq in. | 75,000 lb |
| 10 in. by 10 in. | 100 sq in. | 62,500 lb |
The 625 psi figure is the published compression-perpendicular-to-grain design value for Douglas Fir-Larch in the American Wood Council’s National Design Specification for Wood Construction (NDS) Supplement, Table 4A. The snow conversion follows the flat-roof snow equation in ASCE 7-14 Chapter 7. Both are named here so the figures above can be checked against their source rather than taken on trust.
Read every bearing percentage on this page as crushing at the bearing surface only. It is not a member’s axial, buckling, bending, shear, deflection or connection utilization, and any of those can govern at a far higher percentage. Those checks are separate calculations in the sealed package.
How does ground snow become roof snow in these packages?
Every package runs two ground snow cases and converts each to a roof snow load. The conversion is identical across all nine structures.
| Ground snow | Roof snow | Ratio |
|---|---|---|
| 30 psf | 23 psf | 0.767 |
| 50 psf | 38 psf | 0.760 |
Both ratios sit within one percent of each other, which is what a single set of exposure, thermal and importance factors produces. A site outside those two ground snow cases gets its own sealed calculation rather than an interpolation from this table.
What wind load does the package calculate at each load point?
Wind enters every package at 115 mph ultimate in Exposure B — the figure the calculation starts from, not a survival rating. Each package resolves its governing wind case to four load points.
| Structure | Plan modeled | Total | At each load point |
|---|---|---|---|
| Trellis | 18 ft by 3 ft | 457 lb | 114.25 lb |
| Gazebo | 14 ft by 14 ft | 2,493 lb | 623.25 lb |
| Cabana 16 by 16 | 16 ft by 16 ft | 1,484 lb | 370.90 lb |
| Cabana 20 by 20 | 20 ft by 20 ft | 2,202 lb | 550.40 lb |
| Cabana 16 by 20 | 16 ft by 20 ft | 2,967 lb | 741.79 lb |
| Pergola | 16 ft by 20 ft | 3,387 lb | 846.78 lb |
| Cabana 20 by 30 | 20 ft by 30 ft | 4,451 lb | 1,112.69 lb |
| Pavilion | 24 ft by 23 ft | 5,059 lb | 1,264.75 lb |
Each packet prints both the total and the per-load-point figure. On five of the eight the per-point figure does not equal the printed total divided by four — 370.90 lb times four is 1,483.60 lb against a printed 1,484 lb, for example — because the packet rounds the total for display and computes the per-point figure from the unrounded value. We publish both as printed. Read them as calculation results for the plan each package models, not as ratings for a structure of any other size. The arbor package resolves its main wind force to zero because that structure has no gable length and no roof slope.
Does this page work as permit documents by itself?
No. A permit office needs the sealed package prepared for your project address and accepted by the local permitting jurisdiction. Packages are site-specific, single-use, and non-transferable, and they are void without a proper engineer’s seal. Bigger spans, a different exposure, higher snow or wind, or soil that does not match the assumption get new site-specific calculations rather than a copy of this summary.
What if my site needs more snow or wind than the standard package?
The standard packages run two ground snow cases, 30 psf and 50 psf, at a 115 mph ultimate design wind speed. A site beyond those limits needs site-specific engineering sealed by a professional engineer licensed where the structure is being built. The engineer may change member sizes, framing spacing, connections, anchors or foundations as the project loads and soil conditions require. We fabricate to sealed drawings; we do not perform structural design through this website.
See the full stamped design criteria on our engineering and permitting page, or compare with arbor engineering and pergola engineering.
Shane Watson, PE (Utah #5206456-2202) seals our structural engineering packages and holds professional engineer licenses in 21 states. Sealed drawings for a permit are issued by an engineer licensed in the project state — Watson where he is licensed, or a coordinated in-state engineer where he is not. Western Timber Frame has completed jobs in all 50 states. Figures on this page are summarized from the stamped engineering package (Project 5C 25-022, calc job SSE WTF-026-03) and are calculated to IBC 2018 and ASCE 7-14. The sealed PDF prepared for your address controls.
See sizes, options and pricing on our timber arbors and trellises page.