Drifting Snow: Wind can easily transport lighter snow from one area of a roof to another. Several resources are available that are helpful in obtaining snow load design information, as well as determining appropriate snow design loads. (Influence lines are very helpful). (86 mm) tube of 250 mL volume. Prior to 1978 densities were obtained by taking samples horizontally (from a vertical section cut through the full depth), using a short 3.4 in. Section 8.3. For unbalanced snow loads with the eave to ridge distance less than 20 ft, what loads do we apply on the leeward roof? Using the following equations: γ = 0.13p g + 14 (snow density) Engineered design, through ASCE 7, is required in instances where the limitations on … Many, but not all, of these changes will be demonstrated within the new examples. Unbalanced Snow Load : Drifts: Sliding Snow: Example Problems: Homework Problems: References. Density of snow Freshly fallen snow is very loose and light and has a specific gravity of about 0.05 to 0.1. NFBA’s goal is to offer relevant, timely education and training to help attendees do their jobs more effectively and make their companies and employees more successful. Section 8.1. ASCE 7-02 requires the application of unbalanced snow loads to situations where in Minnesota, in the past unbalanced snow would not have been considered. presentation of snow loads in the form of isolines of equal snow load on a map is the more common approach. This means that on surface, snow accumulates in varying depths depending on the roof profile. The unbalanced load for a gable roof has been prescribed to be a uniform load from ridge to eave, or a load which increases from ridge to eave. Owners of curved- or arched-roof buildings designed prior to this change should consider having the structures evaluated by a qualified structural engineer. Uneven distribution can also be a result of the shape and slope of the roof. Unbalanced snow load from drifting and sliding snow. … The values provided in this Building Calculator are shown in Appendix C of the applicable building code. 2.2 Snow Types. Loading diagrams for these load cases are shown in Figure 7-5. Gable Roof Drifts (aka Unbalanced Load) 4. Each collapse was started by an unbalanced snow load that was imposed on the structure – usually at the lower panels of the domes. 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. The ground snow load is pretty much what it sounds like: A predictable load situation that is derived from snowfall records over the years in a particular region (photo 2). 300. In the past I considered unbalanced snow … Last Revised: 11/04/2014. The process of measuring snow loads is relatively imprecise. T = Self-straining load. Other Structural Systems Select areas to carry half the balanced snow load to produce the greatest effect on the member being analyzed. 3. Except for sawtooth-type roofs, the roof’s windward portion is the snow source and the leeward portion accumulates a percentage of the drifted snow. This data is specific to the area in which the building is to be located. 6.2 49 VI-4 Typical Curved Roof Drift Formation 51 VI-5 Balanced and Unbalanced Snow Loads for Curved Roof in Ex. 7.5 yes no . A building constructed 40 years ago may not have been designed for snow loads as they are understood today. The unbalanced snow loads load requirements in ASCE 7-02 raises a lot of questions. The unbalanced loads in Section 7.6 of ASCE 7-16 are in most cases drift loads. Flat Roof Snow Load. There are a few conditions required to create an unbalanced snow load. The only work around (which isn't great) is to enter some of the live load as dead load instead. The structural analysis of the steel moment frame system shows the collapse load is approximately 5 to 7 psf of snow/ice load. Unbalanced Snow Loads A new provision was introduced to the wood truss section (i.e., R810.2) of the 2006 edition of the IRC that permits the use of roof snow load computed using 0.7p g . These figures are established by the local jurisdiction, in my case the state of Massachusetts, which lists ground snow loads for each town in the state. 4 Check drift loads Is roof in the wind shadow of (1) higher portions of the structure or (2) adjacent structures or terrain features? • Rain-on-snow load. Snow loads are prevalent in northern and/or mountainous regions all over the world. Only relatively recently have drifting and sliding snow loads been addressed in building codes. Keep an eye out for more on this in part two of this blog series! ASCE 7 Section 7.6.1 defines the snow load cases, balanced and unbalanced, for gable and hip roofs. Following the code in most cases when you have a gable roof you have a pretty severve unbalanced load requirement. VI-3 Unbalanced Snow Load and Elevation for Ex. Leeward roof snow load =I*pg (ASCE 7-10 Figure 7-5) 200. Unbalanced Snow Load : Drifts: Sliding Snow: Example Problems: Homework Problems: References. Snow load. W i = Wind-on-ice in accordance with Chapter 10 of ASCE 7. This situation is termed unbalanced because the leeward portion has more snow than the windward portion. Expo brings the industry to you and it is the only place where all the resources you need are available at the same time. Nova Scotia Labour and Advanced Education promotes safe and healthy workplaces by developing and enforcing safety standards, and providing education, and consulting support. The code change was initiated because of concern that these types of roofs may not be able to support unbalanced snow loads, typically snow drifts created by high winds. The resulting unbalanced snow loads can lead to sliding and drifting that can threaten the roof’s structural system. Immediately after falling, however, the snow crystals start to change. Are asphalt shingles, wood shingles, and shakes considered as slippery surfaces? 6.3 52 VI-6 Balanced and Unbalanced Snow Loads for Sawtooth Roof in Ex. Load and Snow Load are turned on in Truss Loading. 4 Drift Losses 7 Drift Losses In the US most snow related … Unbalanced snow loads are required for roof pitches between 1/2 on 12 to 7 on 12. For example: A roof truss has the loading shown below. The snow load provisions of ASCE 7-05 provide guidance for determining the magnitude of those loads based on geographic … This varies somewhat based on the assumption of load distribution. Likewise, unbalanced snow loads — whether the result of wind drifts, uneven melting effects from the sun, or uneven snowfall based on variable protection (usually from trees) — aren't necessarily more likely to cause a roof collapse, but they do stress a roof and its connections in places that balanced loads do not. It will then explain how to enter these loads if a different outcome is desired by the user. Complete and up-to-date information is available from the local building official. The ground snow load is required to calculate the corresponding snow component of the resulting Roof Snow Load. - There are many 94 UBC / 97 UBC to SEI/ASCE 7-02 / 05 companion code provisions that have changed in application limits, values and ranges. It's been requested a few times before, usually for the Canadian codes, to pattern a portion of the snow load only. I'll be sure to pass it along again. Unbalanced Snow Loads for Dome Roofs. UNBALANCED SNOW LOADS ON ARCHED ROOFS F. M. Hochstenbach 1, P. A. Irwin 1, and S. L. Gamble 1 1 Rowan Williams Davies & Irwin, Inc., 650 Woodlawn Road West, Guelph, Ontario, Canada N1K 1B8; PH (519) 823-1311; FAX (519) 823-1316; email:fmh@rwdi.com Abstract Building codes and standards often indicate that unbalanced loading of arched roofs need not be considered if the roof is flat enough. 200. Curved roofs show similar or even more unbalanced distributions (little snow on top and heavy snow near the base of the arch) (Figure 2). Last Revised: 11/04/2014. Snow Load Design Resources. Rain Load. V asd = Nominal design wind speed (3-second gust), miles per hour (mph) (km/hr) where applicable. The maximum value of the total snow load at the high end shall be the superimposed values of drift, balanced snow load and rain-on-snow … If we assume uniform loading, then the answer will be slightly different from unbalanced snow/ice loading because the load distribution changes the stress concentration points. W = Load due to wind pressure. Hazard location. When snow accumulates at different depths in different locations on a roof, it results in high and concentrated snow loads that can potentially overload the roof structure. Report Errors or Make Suggestions . What is the ground snow load for Sitka, Alaska? Overview. No. Unbalanced Snow Loads on Gable Roof Over the past decade or so, there have been more changes to the unbalanced load provisions than any other snow load in the ASCE standard. Total truss load (live + dead) = 50 psf. Once you have the ground snow load, p g, you can compute the flat roof snow load, p f. The design snow load represents the peak snow accumulation on a roof over the a winter season. Unbalanced snow load from Fig. Report Errors or Make Suggestions . I I I I I I I I I I I I I I I I I I I 1.0 INTRODUCTION GROUND AND ROOF SNOW LOADS FOR IDAHO By R. L. Sack and A. Sheikh-Taheri Snow loading is the most severe test of roof structures in many Measurements of depth and density are taken with metre sticks and sampling tubes. Live Load Lr=20 psf, Dead Load (top chord + bottom chord) = 30 psf. S. now comes in many forms and is affected by numerous atmospheric and geographic conditions. 3 Future Directions Regional Differences –Winter windiness Revised Windward Drift Relations Snow Capture Walls 5 Current Provisions Drift Losses Leeward Drifts Windward Drifts Parapet Wall Drifts RTU Drifts Drifts on Adjacent Structures Gable Roof Drifts (aka Unbalanced Load) 6. This rain-on-snow augmented design load applies only to the balanced load case and need not be used in combination with drift, sliding, unbalanced, or partial loads. Note that this calculates only the basic roof snow load (and unbalanced loads for hip and gable roofs), and does not take other aspects of roof geometry into account (for example, surcharges due to drifting snow that could accumulate against walls, parapets, or other obstructions). Unbalanced roof snow loads were not introduced in UBC until 1988. This sometimes leads to heavy unbalanced loads, since most of the snow is blown from the windward slope to the leeward slope, producing loads that exceed the ground load on occasions. Contributing factors may have included a lack of snow removal at the base of the domes, inadequate structural inspections, construction issues with the panel glued joints and the lack of a maintenance program. Regretfully the program always skips or pattern loads the full live load including snow load on cantilevers and back spans. 50 psf = 2.4 kN/m^2. 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