13-52-220 Roof loads.

   Wind load, snow load and rain load are environmental loads. Live loads on a roof are those produced (1) during maintenance by workers, equipment and materials. Reductions in roof live loads and/or environmental roof loads shall not be permitted. Design roof loads shall be determined by the provisions of Sections 13-52-230 to 13-52-290 inclusive, 13-52-350 and 13-52-380.

(Prior code § 68-3; Added Coun. J. 11-29-89, p. 8387; 6-14-95, p. 2841)

13-52-230 Minimum roof live loads.

   Roofs shall be designed for a live load of 15 pounds per square foot on the horizontal projection or for other controlling combination of environmental loads.

(Prior code § 68-3.1; Added Coun. J. 11-29-89, p. 8387)

13-52-240 Special purpose roofs.

   Roofs used for roof gardens or assembly purposes shall be designed for a minimum live load of 100 pounds per square foot. Roofs used for other special purposes shall be designed for appropriate loads, as directed or approved by the building commissioner.

(Prior code § 68-3.2; Added Coun. J. 11-29-89, p. 8387; Amend Coun. J. 9-13-89, p. 4604; Amend Coun. J. 3-5-03, p. 104990, § 20; Amend Coun. J. 10-1-03, p. 9163, § 4.5; Amend Coun. J. 11-13-07, p. 14999, Art. II, § 1)

13-52-250 Nonload bearing partitions and suspended ceilings.

   Nonload bearing partitions and suspended ceilings shall not interfere with the deflection profile of the roof system.

(Prior code § 68-3.3; Added Coun. J. 11-29-89, p. 8387)

13-52-260 Ponding loads.

   Roofs shall be designed to preclude instability from ponding loads.

(Prior code § 68-3.4; Added Coun. J. 11-29-89, p. 8387)

13-52-270 Rain loads.

   Roofs having a pitch of less than 30 degrees shall be designed for an environmental rain load or snow load or live load considered to act on the horizontal projection of the roof surface for the appropriate roof drain head system with or without controlled flow as described in Chapter 18-29 of this Code. Roofs having a pitch of 30 degrees or more shall be designed for snow or lateral pressures as required in Sections 13-52-280 and 13-52-290.

   (a)   For roof areas equipped with roof drain heads referenced in Chapter 18-29 of this Code, the design load shall be 25 pounds per square foot of roof area. Maximum ponding shall not exceed three inches above the surface of the roof immediately adjacent to the roof drain head.

   (b)   For roof areas equipped with roof drains for controlled flow as described in Chapter 18-29 of this Code, the design load shall be 35 pounds per square foot of roof area. Maximum ponding shall be limited so that the overflow shall not exceed six inches above the surface of the roof immediately adjacent to the roof drain head.

(Prior code § 68-3.5; Added Coun. J. 11-29-89, p. 8387; Amend Coun. J. 11-9-16, p. 36266, § 13)

13-52-280 Snow loads.

   Roofs shall be designed for the controlling of environmental snow loads. Live load reductions do not apply to environmental loads.

   (a)   Flat-Roof Snow Loads. The snow load pf on an unobstructed flat roof shall be calculated as 25 pounds-force per square foot. “Flat” as used herein refers not just to dead-level roofs but to any roof with a slope of less than one in./ft. five degrees).

   (b)   Sloped-Roof Snow Loads, ps. All snow loads acting on a sloping surface shall be considered to act on the horizontal projection of that surface. The slope-roof snow load ps shall be obtained by multiplying the flat- roof snow load pf by the roof slope factor Cs:

   ps = Cspf    Equation 68-5.3

   Values of Cs for warm roofs and cold roofs are as follows:

 

Slope	Cs
less than 30°	1.00
less than 40°	0.75
less than 50°	0.50
less than 60°	0.25
70° and greater	0.00

 

   (c)   Roof Slope Factor For Curved Roofs. Portions of curved roofs having a slope exceeding 70 degrees shall be considered free from snow load. The point at which the slope exceeds 70 degrees shall be considered the “eave” for such roofs. For curved roofs, the roof slope factor Cs shall be determined by basing the slope on the vertical angle from the “eave” to the crown.

   (d)   Roof Slope Factor For Multiple Folded Plate, Sawtooth, And Barrel Vault Roofs Including Bowstring Trusses. No reduction in snow load shall be applied because of slope (that is, Cs = 1.0 regardless of slope, and therefore ps = pf).

   (e)   Unloaded Portions. The effect of removing half the balanced snow load from any portion of the loaded area shall be considered. For unsymmetrical loading refer to ANSI A58-1-1982.

   (f)   Unbalanced Roof Snow Loads. Winds from all directions shall be considered when establishing unbalanced loads. See ANSI A58.1-1982 for unbalanced snow load configurations.

   (g)   Drifts On Roofs. Roofs shall be designed to sustain localized loads from snow drifts that can be expected to accumulate on them in the wind shadow of higher portions of the same structure. The surcharge load due to snow drifting shall be determined as provided for in Section 7.7 of ANSI A58.1-1982.

   (h)   Roof Projections. A continuous projection longer than 15 feet may produce a significant drift on a roof. The loads caused by such drifts shall be considered to be distributed triangularly on all sides of the obstruction that are longer than 15 feet. The magnitude of drift surcharge loads and the width of the drift shall be determined by using the method developed for lower roofs in Section 13-52-280(g).

(Prior code § 68-3.6; Added Coun. J. 11-29-89, p. 8387)

13-52-290 Lateral loads.

   Wind loads shall be determined by the provisions of Sections 13-52-300 and 13-52-310.

(Prior code § 68-4; Added Coun. J. 11-29-89, p. 8387)

13-52-300 General lateral loads.

   Provisions for the determination of wind loads on buildings and other structures shall be as follows:

   (a)   The provisions apply to the calculation of wind loads for main wind-force resisting systems and for individual structural components and cladding of buildings and other structures. Special investigations shall be required to determine wind loading for buildings or structures exceeding 600 feet in height, having irregular shapes, response characteristics or site locations with shielding or channeling effects that warrant special consideration, or for cases in which more accurate wind loading is desired.

   (b)   Wind load provisions of Section 13-52-290 are based on a basic wind speed of 75 miles per hour defined as the Annual Extreme Fastest-Mile Speed ten Meters (33 feet) Above Ground. A 50-Year Recurrence Interval has been used in developing Section 13-52-310, minimum design wind pressures.

   (c)   Combined Stresses. For combined working stresses due to dead, live and wind loads, the allowable working stresses in material may be increased one-third, provided the section thus determined is at least as strong as that required for dead and live load alone except that this increase in stresses shall not be permitted for structures as given in Section 13-52-310(f), signs, tanks, towers and chimneys.

   (d)   Overturning And Sliding.

   Overturning. The overturning moment due to the wind load shall not exceed two-thirds of the moment of stability of the building or other structure due to the dead load only, unless the building or other structure is anchored so as to resist the excess overturning moment without exceeding the allowable stresses for materials used.

   Sliding. When the total resisting force due to friction is insufficient to prevent sliding, the building or other structure shall be anchored to withstand the excess sliding force without exceeding the allowable stresses for the materials used. Anchors provided to resist overturning moment may also be considered as providing resistance to sliding.

   (e)   Anchorage. Adequate anchorage of the roof to walls and columns, and of walls and columns to the foundations to resist overturning, uplift and sliding shall be provided in all cases.

   (f)   Stresses During Erection. Refer to Chapter 13-124; Safeguards During Construction.

(Prior code § 68-4.1; Added Coun. J. 11-29-89, p. 8387)