Snow Loads — ASCE/SEI 7-22

1 · Input data

The snow load of a roof to ASCE/SEI 7-22 Chapter 7 — the flat roof snow load p_f of Eq. (7.3-1), the slope factor C_s and the sloped roof load p_s of §7.4, the separate minimum load p_m of §7.3.3 with Table 7.3-4, the four unbalanced cases of §7.6, the drifts of §7.7, and the sliding snow of §7.9. Choose the procedure first — the nine procedures do not all need the same fields, and every field the chosen procedure does not need may be left blank. The ground snow load p_g and the winter wind parameter W₂ are INPUTS on this card: Figures 7.2-1A to 7.2-1D and Figure 7.6-1 are raster maps with no text layer, so no value can be read from them here. Take p_g from the ASCE Hazard Tool or the Design Ground Snow Load Geodatabase, from Table 7.2-1 for the listed Alaskan locations, or from a site-specific case study approved by the Authority Having Jurisdiction, and record the source. There is no importance factor in ASCE 7-22 — Eq. (7.3-1) is p_f = 0.7 C_e C_t p_g, and the string “importance factor” does not occur anywhere in the chapter. Press Compute the snow load when the card is complete. A value the engine needs and does not have is refused by name, with the clause that refuses it — it is never approximated.

The procedure, and where the ground snow load comes from

The snow procedure to run (Chapter 7)

Ground snow load p_g, lb/ft² — an input

Risk Category, 1 to 4 — Table 7.3-4 is indexed by it

The exposure factor C_e — state it, or look it up in Table 7.3-1

Exposure factor C_e, as stated (§7.3.1)

Surface roughness category (Table 7.3-1)

Roof exposure (Table 7.3-1)

The thermal factor C_t — give ONE of these three

Thermal factor C_t, as stated (0.85 to 1.3)

Thermal condition (Table 7.3-2)

Roof thermal resistance R_roof (Table 7.3-3, heated and unventilated)

The roof slope — give the angle, or the rise per 12 in. of run

Roof slope θ, degrees

Roof rise per 12 in. of run

The roof surface and its form

Roof surface family (Figure 7.4-1 / C7.4)

Roof form (§7.3.3 decides whether p_m applies)

The roof geometry the chosen procedure needs

Eave-to-ridge distance W, ft

Winter wind parameter W₂ (Figure 7.6-1) — an input

Vertical angle from the eaves to the crown, degrees

Roof slope at the eaves, degrees

Azimuth from the downwind centreline, degrees (0 to 180)

Height from the valley to the ridge, ft

Horizontal extent of the overhang, ft

The upper and lower roofs — for the drifts of §7.7

Length of the roof upwind of the drift l_u, ft

Length of the lower level roof, ft

Width of the lower level roof, ft

Clear height h_c above the balanced snow load, ft

Ground snow load used for the snow density, lb/ft² — optional

Horizontal separation distance s, ft

Vertical separation distance h, ft

Length of the adjacent higher structure, ft

The two drifts at an intersection — for §7.7.3

First snowdrift depth h_d1, ft

Width of the first snowdrift w₁, ft

Second snowdrift depth h_d2, ft

Width of the second snowdrift w₂, ft

Conditions that change the load

The roof is unventilated and drains over its eaves — with C_t ≤ 1.1 it sustains 2 p_f on the overhang (§7.4.4)

Snow retention devices are fitted — the area is not unobstructed (§7.4)

The roof area is a valley — a valley obstructs sliding (C7.4)

Simply supported prismatic members from ridge to eave — with W ≤ 20 ft the windward side is unloaded (§7.6.1)

2 · Solution

No solution yet — choose the procedure in the card above, enter the values it needs, and press Compute the snow load. This card stays empty until a solution is computed, and it is emptied again the moment any input changes, so a displayed solution always belongs to the inputs on screen.