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)

13-52-310 Minimum wind design pressures – Buildings and portions thereof and other structures.

   (a)   Buildings and Portions Thereof. All buildings, cladding and components shall be designed and constructed to resist a horizontal wind pressure on all surfaces exposed to the wind, allowing for wind in any direction, in accordance with Table 13-52-310 as set out in this section. Other structures shall be designed and constructed to comply with the applicable provisions of this section. Reductions in wind pressure due to neighboring structures and terrain shall not be considered. The height is to be measured above the average level of the ground adjacent to the building or structure. Wind pressures shall vary linearly between pressures established in Table 13-52-310 as follows:

Table 13-52-310
Minimum Design Wind Pressures – Buildings and Portions Thereof

Height – Feet	Column (A) Main Wind Force Resisting System Wind Pressure (pounds per square foot)	Column (B) Wind Pressure – Components/Cladding (pounds per square foot)
		Other Than Corner	Corner
200 or less	20	25	30
300	21	27	32
400	25	32	38
500	28	35	41
600	31	39	45
700	33	42	49
800	36	45	54
900	39	49	58
1,000	42	53	63

 

   Notes For Table 13-52-310:

   1.   The value set forth in Column (A) shall apply to the structural wind-force resisting system. Examples include two or three dimensional rigid and braced frames, roof and floor diaphragms, trusses and shear walls.

   2.   The values set forth in Column (B) shall apply to components and cladding of enclosed structures that are either directly loaded by the wind or receive wind load as relatively close locations and that transfer these loads to the main wind-force resisting system. Examples include curtain walls, exterior glass windows and panels, purlins, girts and studs. The pressures indicated may act inward or outward.

   3.   The corner pressures set forth in Column (B) shall apply at each corner of the building for a distance equal to ten percent of the least building width or 0.50 height above grade, whichever is smaller.

   4.   The pressure exposure category distribution for design of components and cladding in Column (B) are similar to A.N.S.I. A58.1-1982 Figure 3, pages 38 and 39 and Figure 4, page 40. “Other than Corners” refers to Zone 4, Figure 3 and to Zone 5, Figure 4. “Corner” refers to Zone 5, Figure 3 and to Zones 6 and 7b combined in Figure 4. The pressures are considered acting toward and away from the surfaces and may signify plus and minus signs, respectively.

   (b)   Roof Structures Over Enclosed Building Or Other Structures. All main roof framing structures shall be designed and constructed for the following pressures:

      1.   Flat roofs: an outward pressure acting normal to the surface equal to 75 percent of those established in Table 13-52-310, Column (A) for the corresponding mean height of the roof and applied to the entire roof area.

      2.   Sloped roof, slope equal to or less than 30 degrees: an outward pressure acting normal to the surface equal to 100 percent on the windward side and 75 percent on the leeward side of those established in Table 13-52-310, Column (A) for the corresponding mean height of the roof.

      3.   Sloped roofs, slope greater than 30 degrees: an inward pressure acting normal to the surface equal to 100 percent on the windward side and an outward pressure acting normal to the surface equal to 75 percent on the leeward side of those established in Table 13-52-310, Column (A) for the corresponding mean height of the roof.

      4.   Overhanging eaves and cornices: an upward or downward pressure acting normal to the surface equal to 200 percent of those established in Table 13-52-310, as set out in this section, column (A) for the corresponding height of the eave or cornice.

      5.   Roofing sheathing and membranes: an outward pressure acting normal to the surface equal to the pressures set forth in Section 13-52-310 b.1, b.2 and b.3 except within an area at the edge of the roof equal to ten percent of the width of the structure parallel to the wind direction being considered, outward pressure equal to 200 percent of those established in Table 13-52-310, Column (A) as set out in this section, for the corresponding mean height of the roof.

   (c)   Monoslope Roofs Over Unenclosed Buildings.

      1.   Slope less than ten degrees: shall be designed and constructed to withstand an inward or outward pressure acting normal to the surface equal to 100 percent of those established in Table 13-52-310, Column (A).

      2.   Slope greater than ten degrees shall be designed and constructed to withstand an inward or outward pressure acting normal to the surface equal to 150 percent of those established in Table 13-52-310, Column (A).

   (d)   Projecting Elements. All companies, balconies and parapets shall be designed and constructed to withstand wind pressures in any direction equal to 200 percent of those established in Table 13-52-310, Column (A) as set out in this section.

   (e)   Flagpoles. All flagpoles shall be designed to resist a wind pressure of one and one-half pounds per square foot of flag area applied at the top of the pole and an additional pressure of 50 pounds per square foot on the vertical projection of the pole.

   (f)   Signs, Tanks, Towers and Chimneys.

      1.   Signs. All signs and outdoor display structures shall be designed and constructed to withstand wind pressure applied to the projected exposed area allowing for wind in any direction in accordance with the following:

         (a)   Solid Signs. 30 psf up to a height of 100 feet. Above 100 feet, add 0.025 lb. per foot of additional height.

         (b)   Open Signs. increase wind pressure established for solid signs by one-third. Signs in which the projected area exposed to wind consists of 70 percent or more of the gross area as determined by the over-all dimensions shall be classed as solid signs; those in which the projected exposed area is derived from open letters, figures, strips and structural framing members, the aggregate total area of which is less than 70 percent of the gross area so determined, shall be classed as open signs.

      2.   Tanks, Solid Towers and Chimneys. Tanks, solid towers, chimneys and similar structures shall be designed and constructed to withstand pressures established in Table 13-52-310, Column (A) as set out in this section applied to the projected area multiplied by the following factors:

 

Shape in Horizontal Cross Section	Factor
Square or rectangular
H/D less than 7	1.0
H/D greater than 7	1.4
Polygonal – all values of H/D	1.0
Round – all values of H/D	0.7
Where:    H = height of structure    D = diameter of least horizontal dimension

 

      3.   Antenna towers and antenna supporting structures shall comply with the requirements of this Code and with “Structural Standard for Steel Antenna Towers and Antenna Supporting Structures” EIA Standard RS-222-C March 1976, published by Electronic Industries Association Engineering Department, Washington, D.C.

      4.   The increase in the basic allowable unit stresses permitted for wind loads per Section 13-52-300 c, shall not apply to structures considered in Section 13-52-310 f.

(Prior code § 68-4.2; Added Coun. J. 11-29-89, p. 8387)

13-52-320 Interior non-load bearing partitions.

   Interior non-load bearing partitions shall be capable of sustaining a horizontal uniform pressure of not less than five pounds per square foot between lateral supports.

(Prior code § 68-4.3; Added Coun. J. 11-29-89, p. 8387)

13-52-330 Tornados.

   Tornados have not been considered in developing the basic wind-speed distributions of this Code. For those structures or buildings for which design to resist tornadic winds are considered by the owner, the designer is referred to the references in the ANSI A58.1-1982 Appendix (A6.5) on tornado-resistant designs.

(Prior code § 68-4.4; Added Coun. J. 11-29-89)

13-52-340 Earthquakes.

   Special provisions for seismic design shall not apply. The basic wind design provisions for buildings, portions thereof, cladding and components and other structures shall apply.

(Prior code § 68-4.5; Added Coun. J. 11-29-89, p. 8387)

13-52-350 Soil and hydrostatic pressure.

   Sections 13-52-360 and 13-52-370 set out provisions on soil and hydrostatic pressure.

(Prior code § 68-5; Added Coun. J. 11-29-89, p. 8387)

13-52-360 Pressure on basement walls.

   In the design of basement walls and similar approximately vertical structures below grade, provision shall be made for the lateral pressure of adjacent soil. Due allowance shall be made for possible surcharge from fixed or moving loads. When a portion or the whole of the adjacent soil is below a free-water surface, computations shall be based on the weight of the soil diminished by buoyancy, plus full hydrostatic pressure.

(Prior code § 68-5.1; Added Coun. J. 11-29-89, p. 8387)