Code of Massachusetts Regulations › 780 CMR - STATE BOARD OF BUILDING REGULATIONS AND STANDARDS › Chapter 16 - Structural Design › Section 1607 - LIVE LOADS › Subsection 1607.11 - Roof loads

Current through Register 1525, July 5, 2024
The structural supports of roofs and marquees shall be
				  designed to resist wind and, where applicable, snow and earthquake loads, in
				  addition to the dead load of construction and the appropriate live loads as
				  prescribed in this section, or as set forth in Table 1607.1. The live loads
				  acting on a sloping surface shall be assumed to act vertically on the
				  horizontal projection of that surface.
1607.11.1
Distribution of roof
				loads.
Where uniform roof live loads are reduced to less than 20 psf
				(0.96 kN/m2) in accordance with Section 1607.11.2.1
				and are applied to the design of structural members arranged so as to create
				continuity, the reduced roof live load shall be applied to adjacent spans or to
				alternate spans, whichever produces the most unfavorable
load
				effect
. See Section 1607.11.2 for reductions in minimum roof live
				loads and Section 7.5 of ASCE 7 for partial snow loading.
1607.11.2
Reduction in roof live
				loads.
The minimum uniformly distributed live loads of roofs and
				marquees,
L
o, in Table 1607.1 are
				permitted to be reduced in accordance with Section 1607.11.2.1 or 1607.11.2.2.
1607.11.2.1
Flat, pitched and curved
				roofs.
Ordinary flat, pitched and curved roofs, and awnings and canopies
				other than of fabric construction supported by lightweight rigid skeleton
				structures, are permitted to be designed for a reduced roof live load as
				specified in the following equations or other controlling combinations of loads
				in Section 1605, whichever produces the greater load.
In structures such as greenhouses, where special scaffolding
				  is used as a work surface for workers and materials during maintenance and
				  repair operations, a lower roof load than specified in the following equations
				  shall not be used unless
approved
by the
building
				  official
. Such structures shall be designed for a minimum roof live
				  load of 12 psf (0.58 kN/m2).
L
r
=
							 L
o
R
1
R
2
(Equation 16-25)
where: 12 <=
L
r
				  <= 20
For SI:
L
r
=
				  L
o
R
1
R
2
where: 0.58 <=
L
r
				  <= 0.96
L
r = Reduced live
				  load per square foot (m2) of horizontal projection
				  in pounds per square foot (kN/m2).
The reduction factors
R
1 and
R
2 shall be determined as
				  follows:
R
1 = 1 for
A
t <= 200 square feet (18.58
							 m2)
(Equation 16-26)
R
1 = 1.2 -
							 0.001
A
t for 200 square feet <
A
t < 600 square feet
(Equation 16-27)
For SI: 1.2 - 0.011
A
t
				  for 18.58 square meters <
A
t <
				  55.74 square meters
R
1 = 0.6 for
A
t >= 600 square feet (55.74
							 m2)
(Equation 16-28)
where:
A
t = Tributary area
				  (span length multiplied by effective width) in square feet
				  (m2) supported by any structural member, and
R
2 = 1 for
F
<= 4
(Equation 16-29)
R
2 = 1.2 -
							 0.05
F
for 4 <
F
< 12
(Equation 16-30)
R
2 = 0.6 for
F
>= 12
(Equation 16-31)
where:
F
= For a sloped roof, the number of inches
				  of rise per foot (for SI:
F
= 0.12 * slope, with slope
				  expressed as a percentage), or for an arch or dome, the rise-to-span ratio
				  multiplied by 32.
1607.11.2.2
Special-purpose
				roofs.
Roofs used for promenade purposes, roof gardens
,
assembly purposes or other special purposes, and marquees, shall be
				designed for a minimum live load,
L
o,
				as specified in Table 1607.1. Such live loads are permitted to be reduced in
				accordance with Section 1607.9. Live loads of 100 psf (4.79
				kN/m2) or more at areas of roofs classified as Group
				A occupancies shall not be reduced.