Snow Loads for Structural Design in Montana (Revised 2004) Gregory P. Theisen Michael J. Keller Jerry E. Stephens Fred F. Videon James P. Schilke December 2004 Driven by recent changes to the ASCE 7 snow load provisions, this guide describes: The second example is a one-story L-shaped roof. Heat flow from poorly insulated roofs melts some of the roof snowpack. Allows up to 10 different sloped roof snow loads. According to ASCE 7-16, the design snow loads for flat roofs and sloped roofs can be obtained using the following equations: where. (Note: for reference, a 1:12 roof slope equates to 4.76 degrees, and the program allows a slope up to 1.05:12.) Snow Loading Analysis Calculator. 2. Where flat roof snow loads exceed 30 psf (1.44 kN/m 2), 20 percent shall be combined with seismic loads. = live roof load W = wind load S = snow load E = earthquake load R = rainwater load or ice water load T = effect of material & temperature H = hydraulic loads from soil F = hydraulic loads from fluids. However, I still have a hard time making sense of it when comparing the calculated drifting snow loads of building with very steep (say 12:12) roof and that with flat roof :) 2. 3. The importance factor, also found in ASCE 7-10, takes into account the type of building for which you are calculating snow loads. ASCE702S is a spreadsheet program written in MS-Excel for the purpose of flat roof snow loading analysis for buildings and structures per the ASCE 7-02 Code. Ground snow loads do not discount that actual snow loads may exceed them, only that the risk of snow-load-induced failure is reduced to an acceptably low level. Question 3: Equation 16-21 (alternative basic load combination) also uses D+L+S+E/1.4. For C t = 1.0 and I s = 1.0, Equation 7-1 produces ground-to-roof snow This one hour webinar presents design roof snow loads as per ASCE 7 for four increasing complex residential roofs. Flat roof snow loads of 30 psf (1.44 kN/m 2) or less and roof live loads of 30 psf (1.44 kN/m 2) or less need not be combined with seismic loads. The ASCE 7-16 standard requires both balanced and unbalanced snow load case scenarios for a structure's design consideration. Calculate flat roof snow load p f using the following equation: p f = 0.7C e C t I s p g where: p f = Flat Roof Snow Load in psf C e = Exposure Factor, as determined by ASCE Table 7-2 below. I s is important factor. The International Code specifies design snow loads are to be determined according to Section 7 of a document called ASCE 7. р s = design snow load for a sloped roof. See Table 2.9 for importance factor values, depending on the category of the building. in ASCE 7-10, unbalanced snow load is not required for roof with slope > 30.2deg (or 7:12) The basis for ASCE 7-05 snow load computations is the ground snow load, p g. This value is modified to become a flat roof snow load, p f, by multiplying by a constant that accounts for roof snow loss that ground measurements don't see. Exposure factor, C e: Exposure factor, C e is based on wind exposure of structure. or 1 in./ft.) Load Calculations. 2. Allows up to 10 different flat roof snow loads to be calculated by entering values for all of the parameters in accordance with ASCE 7. The complexity arises from the variable upwind fetch for unbalance or across-the-ridge drift loads. This document provides for all roof snow loads to be calculated from ground snow loads, however not every Building Department follows this procedure. ASCE 7 in fact clarifies that S may be the flat roof snow load or the sloped roof snow load, depending upon the slope of the roof. Section 7.4 of ASCE 7-16 explains how to convert flat roof snow loads into sloped roof snow loads using the roof slope factor, C s. The sloped roof snow load, also called the balanced load, is the basis for determining the snow load for all structures. I = importance factor. Where flat roof snow loads exceed 30 psf (1.44 kN/m 2), 20 percent shall be combined with seismic loads. The standard guide in the U.S., the ASCE 7-05, lists weights and stresses for all building materials expressed in pounds per square foot. ASCE705S is a spreadsheet program written in MS-Excel for the purpose of flat roof snow loading analysis for buildings and structures per the ASCE 7-05 Code. EXAMPLE CALCULATIONS F Section 7.3.1 Table 7-2 Section 7.3.2 Table 7-3 Section 7.3.3 … Snow Loads is an essential supplement to ASCE/SEI Standard 7 for all engineers, architects, and construction professionals who work on projects in regions subject to snow. Step 4 - Determine the Flat Roof Snow Load Flat Roof Snow Load (p f) p f = 0.7C e C t Ip g p f = 0.7(1.0)(1.0)(1.0)(20) = 14 psf But not less than p f =(I)p g = 20 psf ASCE 7-05 Section 7.2 Figure 7-1 Section 7.3 Eq. The first is a simple, one story hip roof. Calculating the load on a flat roof is a challenging task for the layperson that involves knowledge of building materials, their weights and local building codes. Specifically, coefficients and related and required parameters are selected or calculated in order to compute the net design snow loads, including snow drift due on lower roofs and rain-on-snow surcharge. 2. ARCH 331 Note Set 13.1 S2014abn 2 Determining Dead Load from Material Weights Material density is a measure of how much mass in a unit volume causes a force due to gravity. About the Author Michael O'Rourke, Ph.D., P.E., is a professor in the Department of Civil and Environmental Engineering at Rensselaer Polytechnic Institute, where he teaches and pursues snow load research. If slope ≤ ½” in 12”, add rain-on-snow surcharge (Section 7.10 ASCE 7) 2 yes . This program specifically follows Section 7.0, Snow Loads, of the ASCE 7-10 Standard, "Minimum Design Loads for Buildings and Other Structures". This program assumes only snow loading analysis for buildings with a flat roof, or low slope roof <= 5 degrees. Snow load: (ASCE 7-05, 10, IBC 2006, 2009, 2012) ... Flat roof snow load: p f = 0.7 C e C t I s p g. C e is exposure factor. Sloped Roof Snow Loads. Specifically, coefficients and related and required parameters Ground snow loads map (Figure 7-1 of ASCE 7-05 Code) 1. Specifically, coefficients and related and required parameters are selected or calculated in order to compute the net design snow loads, including snow drift due on lower roofs and rain-on-snow surcharge. I would agree with you if we use ASCE 7-05, which requires unbalanced snow on leeward side of the gable regardless of roof pitch. Like its predecessors that accompanied earlier versions of ASCE 7, this new edition of Snow Loads discusses flat roof loads, sloped roof loads, partial loads, and all types of conventional drift loading. ASCE 7 Table 7-3 Calculate flat roof snow load pf 0.7CeCt Is pg no Is roof flat? This program specifically follows Section 7.0, Snow Loads, of the ASCE 7-10 Standard, "Minimum Design Loads for Buildings and Other Structures". Input Data. Herein the exposure and importance factors are taken to be 1.0. Snow Load - Snow loading analysis for buildings with flat or low slope roofs Snow Map - Ground snow loads map (Figure 7-1 of ASCE 7-10 Code) Program Assumptions and Limitations: 1. Snow Loads: A Guide to the Use and Understanding of the Snow Load provisions of ASCE 7-02 is the only book that provides a detailed, authoritative interpretation of the snow load provisions of Minimum Design Loads for Buildings and Other Structures, Standard ASCE 7-02. Pg = ground snow load Pf = flat roof snow load = Pg*I*Ce*Ct Ps = sloped roof snow load = Pf*Cs The "minimum" Pf is defined in ASCE-7-98 as: I*Pg when Pg is less than or equal to 20psf I*20 when Pg is greater than 20psf +++++ The "minimum" that is specified is for cases when the Ce*Ct<1.00. Non-shaded (CS) areas require site-specific Case Studies to establish ground snow loads. Flat roof snow loads of 30 psf (1.44 kN/m 2) or less and roof live loads of 30 psf (1.44 kN/m 2) or less need not be combined with seismic loads. Maps of ground snow loads in IBC and in ASCE 7 indicate a 2 percent probability of the indicated load being equaled or exceeded in any given year. 7-1 F-6 PROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds A Multi-Hazard Foundation and Installation Guide. Step 3 - Determine the Importance Factor. "ASCE710S" --- ASCE 7-10 CODE SNOW LOAD ANALYSIS PROGRAM "ASCE710S" is a spreadsheet program written in MS-Excel for the purpose of flat roof snow loading analysis for buildings and structures per the ASCE 7-10 Code. Most residential structures are situated in a B or C terrain category and are partially exposed with a C e =1.0 C t = Thermal Factor, as determined by ASCE Table 7-3 below. C t is Thermal factor. The flat roof snow load in ASCE 7-10, P f, is a function of the ground snow load and three factors related to the building and its surroundings: P f = 0.7C e C t I s P g. Where: C e is the exposure factor, C t is the thermal factor, and I s is the importance factor. For this roof the complexity involves snow load for the roof valley area. If slope ≤ ¼” in 12”, design to prevent ponding. ASCE 7-05 Code for Buildings with Flat or Low Slope Roofs (= 5 deg. I have the same question as Q2 about the snow load used. You can click on the map below to determine the design ground snow load, latitude and longitude and elevation for that location. (Section 7.11, ASCE 7) 2 Go to partial loading 1 If slope > 5°, calculate sloped roof snow load. 3. While this may be more intuitive for flat or even gable/hip type roofs, the determination of snow loads is increasingly more difficult for arch roofs due to complex geometry. Flat roof snow load — The flat roof snow load, p f, is calculated in pounds per square foot (psf) from Section 7.3 of ASCE 7 and is given by Equation 7-1: p f = 0.7C eC tI sp g 0.7 is the combined exposure reduction factor for ground-to-roof conversion for normal conditions. Specifically, coefficients and related and required parameters Ground snow loads map (Figure 7-1 of ASCE 7-10 Code) Figure 7-1, pages 34-35 and Table 7-1, page 30 Table … Warm roofs lose heat in such a way that it melts snow, while cold roofs lose heat that doesn’t contribute to melting. (Note: for reference, a 1:12 roof slope equates to 4.76 degrees, and the program allows a slope up to 1.05:12.) This program specifically follows Section 7.0, Snow Loads, of the ASCE 7-05 Standard, "Minimum Design Loads for Buildings and Other Structures". "ASCE705S" --- ASCE 7-05 CODE SNOW LOAD ANALYSIS PROGRAM "ASCE705S" is a spreadsheet program written in MS-Excel for the purpose of flat roof snow loading analysis for buildings and structures per the ASCE 7-05 Code. р f = design flat roof snow load. Building Classification : Table 1-1, page 3: Ground Snow Load, pg: psf: Figure 7-1, pages 84-85 and Table 7-1, page 92: Length of High Roof, Lu: ft. Flat Roof Snow Loads. 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