15.2 - Vertical Loads
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Introduction to Vertical Loads
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Today, we're discussing vertical loads. Can anyone tell me what vertical loads are?
I think they are the forces acting downwards, like weight.
Absolutely! Vertical loads act downwards due to gravity. They primarily include dead loads and live loads. Student_2, can you explain what a dead load is?
Dead loads are the weights of permanent structural elements, right?
Correct! Dead loads include everything from walls to roofs. Now, Student_3, what about live loads?
Live loads are temporary weights, like people or furniture.
Exactly. It's crucial to calculate these loads for safe structural design. Remember: DL = Dead Load, LL = Live Load. Let's keep that in mind.
Material Weights
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Let's look at the weights of some common building materials. Student_4, how much does reinforced concrete weigh?
I remember it’s around 100-150 lb/ft².
Yes! And what about a steel deck?
The steel deck weighs between 50 to 80 lb/ft².
Exactly. When designing structures, knowing these weights helps determine the total dead load. Let’s summarize: Table 15.2 lists typical weight values. Why is it important to know these?
It helps ensure the structure can carry the weight safely!
Perfectly said! Knowing these weights is key to structural stability.
Snow Loads
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Now let’s discuss snow loads, which can vary significantly. Student_3, how would you describe snow loads?
They vary by location and can be heavier based on the roof's pitch.
Exactly. In areas with heavy snowfall, the load can range from 20-45 psf. Why does roof geometry matter here?
A steeper roof retains less snow, so the load can be reduced!
Yes! So remember: Steeper roofs = less snow retention. There’s an equation for that reduction we can use, which we’ll explore next.
Calculating Loads
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How do we calculate the total load on a structural column? Student_2, what do you think?
We add dead loads and live loads together, right?
Correct! Now, let’s talk about a real-world example. If your building has a total dead load of 600 kips and the live load is 740 kips, how do we find the total load?
It’s the sum of both loads!
Exactly, but what if we reduce the live load due to higher occupancy? Student_4, how would we do that?
We can apply a reduction factor based on the building's design.
Exactly! Always account for load reductions to ensure accurate design.
Introduction & Overview
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Quick Overview
Standard
In the realm of structural engineering, vertical loads are a key focus due to their impact on building stability. This section categorizes vertical loads into dead loads and live loads, and provides detailed information, including material weights, that are essential for engineers to consider during the design phase. Additionally, it discusses how snow loads vary based on geographic context.
Detailed
Detailed Summary
Vertical Loads in Structural Engineering
Vertical loads, crucial for the integrity of any structure, are the forces acting on a structure in a downward direction due to gravity. They are categorized into:
- Dead Load (DL): This encompasses the weight of all permanent structural elements, including walls, roofs, and fixtures.
- Live Load (LL): These refer to transient forces that arise from occupancy and usage, like furniture and people.
- Snow Load: This is an environmental load that varies significantly based on geographic location, with typical values ranging from 20 to 45 psf, influenced by roof geometry and snow retention capabilities.
Understanding vertical loads is essential for designing structures that can safely transfer loads efficiently from one component to another—like from a roof to walls, and finally to the foundation. Knowledge of material weights is crucial in calculating these loads, as exemplified in Tables 15.2 and 15.3. For engineers, accurate assessment of these loads, including their reduction factors in multi-story buildings, is key to ensuring structural safety.
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Introduction to Vertical Loads
Chapter 1 of 5
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Chapter Content
For closely spaced identical loads (such as joist loads), it is customary to treat them as a uniformly distributed load rather than as discrete loads, Fig. 15.1.
Detailed Explanation
In structural engineering, when dealing with loads that are closely spaced and identical, such as the loads supported by joists, we often simplify our calculations by treating these loads as a single, evenly distributed load instead of calculating each load separately. This simplification helps in designing and analyzing structures more efficiently.
Examples & Analogies
Think of this like a group of people standing in a line to get on a bus. If everyone stands right next to one another, it’s easier to think about how many people fit on the bus in total rather than counting each individual person. This is similar to how engineers treat these identical loads.
Weights of Building Materials
Chapter 2 of 5
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Chapter Content
Material
lb=ft2
Ceilings
Channel suspended system 1
Acoustical (fiber tile) 1
Floors
Steel deck 2-10
Concrete-plain 1 in. 12
Linoleum 1/4 in. 1
Hardwood 4
Roofs
Copper or tin 1-5
5 ply felt and gravel 6
Shingles asphalt 3
Clay tiles 9-14
Sheathing wood 3
Insulation 1 in. poured in place 2
Partitions
Clay tile 3 in. 17
Clay tile 10 in. 40
Gypsum Block 5 in. 14
Wood studs 2x4 (12-16 in. o.c.) 2
Plaster 1 in. cement 10
Plaster 1 in. gypsum 5
Walls
Bricks 4 in. 40
Bricks 12 in. 120
Hollow concrete block (heavy aggregate)
4 in. 30
8 in. 55
12 in. 80
Hollow concrete block (light aggregate)
4 in. 21
8 in. 38
12 in. 55
Detailed Explanation
This section provides a detailed table of the weight of various building materials. The weight is measured in pounds per square foot (lb/ft²). Heavy materials such as bricks and concrete exert a significant vertical load on structures, which engineers need to account for in their designs. For example, a 12-inch brick wall weighs 120 lb/ft², which adds considerable weight to the structure it supports.
Examples & Analogies
Imagine a stack of books. If you have thick textbooks, they weigh a lot, while a few thin magazines weigh much less. Just like the books, different building materials have varying weights, and engineers must choose materials wisely based on how they will support structures.
Average Gross Dead Load in Buildings
Chapter 3 of 5
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Chapter Content
Material lb=ft2
Timber 40-50
Steel 50-80
Reinforced concrete 100-150
Detailed Explanation
This table gives the average dead load of common materials used in construction. Timber has a dead load of about 40-50 lb/ft², steel ranges from 50-80 lb/ft², and reinforced concrete can weigh between 100-150 lb/ft². These values help engineers understand how much weight a building's structure has to support, known as the 'dead load'.
Examples & Analogies
Consider a backpack. If you fill it with books, it will get heavy quickly, just like how the weight of materials in a building adds up. The heavier the materials (like reinforced concrete), the stronger our backpack (the building) needs to be to carry the load.
Snow Loads
Chapter 4 of 5
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Chapter Content
Roof snow load vary greatly depending on geographic location and elevation. They range from 20 to 45 psf, Fig. 15.2. Snow loads are always given on the projected length or area on a slope, Fig. 15.3.
Detailed Explanation
Snow loads on roofs can vary significantly based on the region and how high the building is located. For example, roofs in snowy areas might have design snow loads ranging from 20 to as much as 45 pounds per square foot (psf). This measurement is crucial because it helps engineers determine how much snow weight a roof must safely support without collapsing.
Examples & Analogies
Think of a picnic table in winter. If a heavy snowfall covers it, you can imagine how much weight is pressing down on it. Just like we clear snow off our tables to prevent collapses, engineers need to ensure buildings can withstand similar snow loads.
Reducing Snow Load on Steep Roofs
Chapter 5 of 5
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Chapter Content
The steeper the roof, the lower the snow retention. For snow loads greater than 20 psf and roof pitches more than 20°, the snow load p may be reduced by p = ((θ - 20) * 0.5) psf (15.2).
Detailed Explanation
On steep roofs, snow doesn't build up as much compared to flat roofs, due to gravity pulling the snow down. Engineers use a formula to calculate how much they can reduce the anticipated snow load based on the steepness of the roof. If a roof has a pitch above 20 degrees and a snow load greater than 20 psf, it can be safely assumed that less snow will stay on the roof.
Examples & Analogies
Imagine a slide at a playground. The steeper the slide, the faster the kids go down and the less likely they'll pile up at the top. This is similar to how snow behaves on a sloped roof compared to a flat one.
Key Concepts
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Vertical Loads: Forces acting downwards on a structure due to gravity.
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Dead Load (DL): Permanent weight of the structure.
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Live Load (LL): Temporary weight from occupancy and use.
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Snow Load: Load from accumulated snow on structures affected by roof slope.
Examples & Applications
Example of calculating dead load for a residential building using weights from Table 15.3.
Snow load calculations adjusted for a steeply pitched roof reducing the effective snow accumulation.
Memory Aids
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Rhymes
When the roof is steep, snow can leap, less it keeps, a load we now reap.
Stories
Imagine a snowy day, where the weight on a roof didn’t care about the pitch. It made a big difference, with all the snow sliding off steep roofs, while flat ones sagged.
Flash Cards
Glossary
- Dead Load (DL)
The weight of all permanent structural elements within a building.
- Live Load (LL)
Temporary loads acting on a structure, including occupancy and furniture.
- Snow Load
A type of load that varies based on geographic location and roof pitch related to snow accumulation.
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