50 psf = 2.4 kN/m^2. presentation of snow loads in the form of isolines of equal snow load on a map is the more common approach. It's been requested a few times before, usually for the Canadian codes, to pattern a portion of the snow load only. Last Revised: 11/04/2014. Immediately after falling, however, the snow crystals start to change. Again, there are other differences but these are notable. No. Owners of curved- or arched-roof buildings designed prior to this change should consider having the structures evaluated by a qualified structural engineer. For unbalanced snow loads with the eave to ridge distance less than 20 ft, what loads do we apply on the leeward roof? Keep an eye out for more on this in part two of this blog series! (86 mm) tube of 250 mL volume. Uneven distribution can also be a result of the shape and slope of the roof. Section 8.3. Report Errors or Make Suggestions . Engineered design, through ASCE 7, is required in instances where the limitations on … Section 8.1. Measurements of depth and density are taken with metre sticks and sampling tubes. 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 . VI-3 Unbalanced Snow Load and Elevation for Ex. S. now comes in many forms and is affected by numerous atmospheric and geographic conditions. 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:firstname.lastname@example.org Abstract Building codes and standards often indicate that unbalanced loading of arched roofs need not be considered if the roof is flat enough. The process of measuring snow loads is relatively imprecise. Only relatively recently have drifting and sliding snow loads been addressed in building codes. 4 Drift Losses 7 Drift Losses In the US most snow related … Snow Load Design Resources. 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. For example: A roof truss has the loading shown below. Rain Load. 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. The structural analysis of the steel moment frame system shows the collapse load is approximately 5 to 7 psf of snow/ice load. 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). 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. Curved roofs show similar or even more unbalanced distributions (little snow on top and heavy snow near the base of the arch) (Figure 2). Total truss load (live + dead) = 50 psf. 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. 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. 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). Nova Scotia Labour and Advanced Education promotes safe and healthy workplaces by developing and enforcing safety standards, and providing education, and consulting support. Unbalanced roof snow loads were not introduced in UBC until 1988. 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. The resulting unbalanced snow loads can lead to sliding and drifting that can threaten the roof’s structural system. 300. In colder … Following the code in most cases when you have a gable roof you have a pretty severve unbalanced load requirement. This means that on surface, snow accumulates in varying depths depending on the roof profile. Unbalanced snow load from drifting and sliding snow. The snow load provisions of ASCE 7-05 provide guidance for determining the magnitude of those loads based on geographic … - 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. 7.5 yes no . This varies somewhat based on the assumption of load distribution. 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. 6.3 52 VI-6 Balanced and Unbalanced Snow Loads for Sawtooth Roof in Ex. … Using the following equations: γ = 0.13p g + 14 (snow density) W = Load due to wind pressure. A building constructed 40 years ago may not have been designed for snow loads as they are understood today. Unbalanced Snow Load : Drifts: Sliding Snow: Example Problems: Homework Problems: References. Unbalanced Snow Loads for Dome Roofs. An unbalanced snow load is just like it sounds – a snow load that is not balanced, or uniform, across a roof surface. p f = Flat Roof Snow Load in psf; Calculate unbalanced snow load for hip and gable roofs as shown in ASCE Figure 7-5 below. Each collapse was started by an unbalanced snow load that was imposed on the structure – usually at the lower panels of the domes. • Rain-on-snow load. 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. Are asphalt shingles, wood shingles, and shakes considered as slippery surfaces? 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. The only work around (which isn't great) is to enter some of the live load as dead load instead. There are a few conditions required to create an unbalanced snow load. 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. What is the ground snow load for Sitka, Alaska? Loading diagrams for these load cases are shown in Figure 7-5. Live Load Lr=20 psf, Dead Load (top chord + bottom chord) = 30 psf. I'll be sure to pass it along again. 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. Regretfully the program always skips or pattern loads the full live load including snow load on cantilevers and back spans. Drifting Snow: Wind can easily transport lighter snow from one area of a roof to another. 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. The ground snow load is required to calculate the corresponding snow component of the resulting Roof Snow Load. Unbalanced snow load from Fig. This data is specific to the area in which the building is to be located. Hazard location. Gable Roof Drifts (aka Unbalanced Load) 4. 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. 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. 6.2 49 VI-4 Typical Curved Roof Drift Formation 51 VI-5 Balanced and Unbalanced Snow Loads for Curved Roof in Ex. Leeward roof snow load =I*pg (ASCE 7-10 Figure 7-5) 200. 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. Many, but not all, of these changes will be demonstrated within the new examples. 3. The unbalanced snow loads load requirements in ASCE 7-02 raises a lot of questions. Last Revised: 11/04/2014. 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 … The ASCE 7-16 standard requires both balanced and unbalanced snow load case scenarios for a structure's design consideration. Unbalanced snow loads are required for roof pitches between 1/2 on 12 to 7 on 12. Overview. 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. 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 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. 4 Check drift loads Is roof in the wind shadow of (1) higher portions of the structure or (2) adjacent structures or terrain features? Snow loads are prevalent in northern and/or mountainous regions all over the world. V asd = Nominal design wind speed (3-second gust), miles per hour (mph) (km/hr) where applicable. 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. V ult = Ultimate design wind speeds (3-second gust), miles per hour (mph) (km/hr) determined from Figure 1609.3(1), 1609.3(2), 1609.3(3) or ASCE 7. 200. Unbalanced Snow Load : Drifts: Sliding Snow: Example Problems: Homework Problems: References. The values provided in this Building Calculator are shown in Appendix C of the applicable building code. ASCE 7 Section 7.6.1 defines the snow load cases, balanced and unbalanced, for gable and hip roofs. Load and Snow Load are turned on in Truss Loading. Density of snow Freshly fallen snow is very loose and light and has a specific gravity of about 0.05 to 0.1. Flat Roof Snow Load. Several resources are available that are helpful in obtaining snow load design information, as well as determining appropriate snow design loads. Snow load. W i = Wind-on-ice in accordance with Chapter 10 of ASCE 7. (Influence lines are very helpful). Expo brings the industry to you and it is the only place where all the resources you need are available at the same time. The unbalanced loads in Section 7.6 of ASCE 7-16 are in most cases drift loads. T = Self-straining load. In the past I considered unbalanced snow … 2.2 Snow Types. Other Structural Systems Select areas to carry half the balanced snow load to produce the greatest effect on the member being analyzed. Report Errors or Make Suggestions . This situation is termed unbalanced because the leeward portion has more snow than the windward portion.