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15.2.3. Snow
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Today we're learning about snow loads on structures. Can anyone tell me why we care about snow loads?
I think it's because snow can add a lot of extra weight to roofs.
Exactly! Snow loads can vary greatly, usually from 20 to 45 psf depending on where you are geographically. This is important for us to consider when designing buildings.
How does the location impact the snow load?
Great question! Higher elevations typically receive more snow. For instance, mountain areas will have larger snow loads compared to coastal areas. Now, let’s remember this with a mnemonic: 'Hilly Snow Holds Heavy' — meaning more snow, more weight!
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Moving on, how does the pitch of a roof affect snow loads?
I think steeper roofs have less snow retention, right?
Correct! The steeper the roof, the less snow it retains. For roofs over 20 degrees, we can reduce the snow load by applying a specific formula. Does anyone remember what that formula looks like?
Isn’t it something like 'p = ((h - 20) 0.5)'?
Almost! The formula helps us understand how to adjust the snow load based on the roof pitch. Let’s use the acronym 'PITCH' to remember: 'P' for projection area, 'I' for incline angle, 'T' for total load adjustment, 'C' for calculations involved, and 'H' for heavy conditions. Repeat after me, PITCH!
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Now let’s apply what we learned. If a building in a snowy area has a roof pitch of 25 degrees and we know the snow load is 30 psf, how do we calculate the adjusted load?
Can we use the reduction formula from earlier?
Yes! You would first subtract 20 from the angle then multiply by 0.5. What do you get?
That's 2.5 psf off!
Exactly! So, the adjusted snow load would be 30 psf minus 2.5 psf, totaling 27.5 psf.
Overview
Short Summary
This section discusses snow loads on roofs, their variations by geographic location, and how to determine snow load reduction based on roof pitch.
Medium Summary
Snow loads are critical considerations in structural engineering, greatly varying based on geographical factors like elevation. The section explains how roof pitch affects snow retention, detailing a formula to reduce snow load for steeper slopes. Understanding these concepts is essential for accurate structural design.
Detailed Summary
Snow Loads on Roofs
Snow loads on roofs can significantly affect the structural integrity of buildings. The magnitude of these loads can vary depending on geographic location and elevation, typically ranging from 20 to 45 pounds per square foot (psf). This variability is crucial for engineers to consider during design, as inadequate calculation of snow loads can lead to structural failure. Additionally, snow loads are always applied to the projected area of a sloped roof, meaning that as the pitch of the roof increases, the retaining capacity for snow decreases. In cases where the snow load exceeds 20 psf and the roof pitch is steeper than 20 degrees, engineers can apply a reduction formula to recalibrate the snow load on the structure. This formula, which takes into account the roof pitch, allows for more accurate load assessments. Proper engineering practices ensure safety and longevity in building design.
Audio Book
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Create a free accountRoof snow load vary greatly depending on geographic location and elevation. They range from 20 to 45 psf, Fig. 15.2.
Detailed Explanation
The snow load on roofs changes based on where you are located and how high the building is. For example, in places with a lot of snowfall and higher elevations, roofs need to support more snow compared to areas with less snowfall. The range of snow load, from 20 to 45 pounds per square foot (psf), indicates the different weights roofing can expect to bear. Higher numbers indicate areas prone to heavy snow.
Examples & Analogies
Think of it like a heavy winter coat. If you live in a place where it barely snows, your coat doesn't need to be as thick as someone living in a snowy mountain region. Just like that, roofs in different areas are designed to handle different amounts of snow.
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Create a free accountSnow loads are always given on the projected length or area on a slope, Fig. 15.3.
Detailed Explanation
When we talk about snow loads, we express them based on the sloped area of a roof rather than just the flat area, because the slope affects how much snow can accumulate. A steeper roof will shed snow more effectively than a flat roof; hence this is a crucial consideration in design calculations.
Examples & Analogies
Imagine sliding down a slide covered with snow. If the slide is steep, the snow will slide off easily, while on a flat surface, the snow piles up, making it heavier. This principle directly influences how engineers calculate the snow load on roofs.
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Create a free accountThe steeper the roof, the lower the snow retention. For snow loads greater than 20 psf and roof pitches θ more than 20°, the snow load p may be reduced by p = ((θ - 20) / 40) (psf) (15.2).
Detailed Explanation
As the angle of a roof increases, its ability to hold snow decreases. This means that for very steep roofs (greater than 20°), the snow load can be calculated to be less than on a flatter roof. The formula provided helps engineers recalculate the expected snow load for sloped roofs, making designs safer and more efficient.
Examples & Analogies
Consider a steep hill during winter. If you're rolling a ball of snow down, it will easily roll down a steep incline while it gets stuck on a flat surface. This illustrates how a steeper roof won't hold as much snow, thus requiring less structural support.
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Snow Load:
The computed weight of accumulated snow on a roof, crucial for structural calculation.
- Roof Pitch:
The angle of a sloped roof that significantly influences snow retention.
- Reduction Formula:
A calculation method used to adjust snow load based on roof pitch.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
If a flat roof has a snow load of 40 psf in a snow-prone area, it must withstand this weight without structural failure.
For a building located on a 30-degree pitch roof, the original snow load of 35 psf reduces by 5 psf using the reduction formula, resulting in a safe operational snow load.
Memory aids
Once upon a time, a builder learned that the steepness of a roof could save his house from falling under heavy snow.
Remember 'PITCH' - Projection, Incline, Total adjustment, Calculations, Heavy conditions.
Flash Cards
Glossary
Snow Load
The weight of snow accumulated on a structure, expressed in pounds per square foot (psf).
Roof Pitch
The angle of a roof measured from the horizontal, affecting snow retention and load calculations.
Projected Area
The area of a roof that is considered for load calculations, particularly in terms of the impact of snow.