Load Table Roof Technical Information

General – Presented in the load tables are maximum uniformly distributed specified loads.

Steel – Conforms to ASTM A653/A653M or A792/A792M. Grade 33/230; Yield stress 33 ksi/230 MPa and tensile stress 45 ksi/310 MPa. Grade 50 /345; Yield stress 50 ksi/345 MPa and tensile stress 65 ksi/450 MPa; Grade 80 /550; Yield stress 80 ksi/550 MPa and tensile stress 82 ksi/565 MPa.

Finishes – A25/ZF75, G90/Z275 or AZ50/AZM150. For heavier metallic coatings, refer to ASTM A653/A653M or A792/A792M.

Load Tables – The following information regarding the determination of the specified wind and snow loads is contained in the 2015 Edition of the National Building Code of Canada (NBCC). Importance factors are applied to both strength (ULS) and serviceability/deflection (SLS) limit state design considerations. A lower load factor for wind of 1.4, instead of 1.5 for live and snow
loads is used. This lower load factor for wind somewhat offsets the higher wind loads (1 in 50 year return) that are now listed in the NBCC by geographic location. The importance category of the end use of the building/structure must also be recognized, such as Normal or Low.

All of this will impact how the load tables are to be used. In an effort load tables, the information Part 4 (Structural Design) of to help the design professional with the below was taken directly from Division B, the NBCC.

Specified External Wind Load

W = Iw qCeCtCgCp [1]

Importance CategoryImportance Factor, Iw ULSImportance Factor, Iw SLS
Low0.80.75
Normal1.00.75
High1.150.75
Post-Disaster1.250.75

Specified Snow Load

S = Is [Ss (Cb Cw Cs Ca) + Sr][2]

Importance CategoryImportance Factor, Is ULSImportance Factor, Is SLS
Low0.80.9
Normal1.00.9
High1.150.9
Post-Disaster1.250.9

The importance factors, Iw and Is, have been incorporated in the load tables, as well as the importance category. The parameters in the boxed-in portion of Equations [1] and [2] must be determined by the design professional in accordance with the NBCC.

Strength – The maximum uniformly distributed specified load based on strength in the load table must be equal to or greater than (Specified live load + 0.833 times the specified dead load). Where 0.833 = 1.25/1.5. The specified live load can be either due to use and occupancy or snow load. In cases where live load and snow load act together, the load combination factors in the NBCC must be followed.

Serviceability (Deflection) – The maximum uniformly distributed specified load based on deflection in the load table must be equal to or greater than the specified live load. The effective
moment of inertia for deflection determination was calculated at an assumed specified live load stress of 0.6Fy.

EXAMPLE (Use of Load Table)

AWR Roof (Normal Importance Category)

Given: (Imperial units)
(LLF = 1.5 and Is = 0.9)
~ Panel thickness, t = 0.024 in
~ Triple span continuous, L = 4 ft each span
~ Bearing length, N = 3 in
~ L/240 deflection limit
~ Specified Loads
1) Dead load (DL) panel DL = 1.23 psf
2) Snow Live load (LL) LL = 40 psf

The live load is the value of the boxed-in portion of the specified snow load expression [2].

Solution:

Strength “S”

  1. Specified load [LL +0.833DL] = 41.0 psf
  2. Maximum specified load (from Load Table for Grade 33)
    Is 49 psf
    Since 49 > 41.0:. OK
  3. Check web crippling (N = 3 in)
    a) End reaction = 0.400(41.0)4 = 66 lb/ft
    (from section property table)
    Pe = Pe1 + Pe2 [N/t]1/2
    = 45.4 + 11.3[3/0.024]1/2 = 172 lb/ft
    Since 172 > 66 :. OK
    Interior reaction = 1.10(41.0)4= 180lb/ft
    (from section property table)
    Pi = Pi1 + Pi1 [N/t]1/2
    = 87.6 + 14.9[3/0.024]1/2 = 254lb/ft
    Since 254 > 180 :. OK

Deflection “D”
From table L/180 = 166 psf
For L/240, multiply 166 by 180/240 = 125psf
Since 125 > 40:. OK