Basics of Structural Design Load Calculations | One-Way Vs Two-Way Slab
Summary
TLDRThis video focuses on the fundamentals of calculating structural design loads, with an emphasis on gravity loads. It explains the different types of loads, such as dead loads, superimposed dead loads, and live loads, and their importance in structural design. The video walks through examples on how to calculate these loads, especially for concrete slabs, and how to distribute them across beams and columns for an economical design. Additionally, it highlights key concepts like one-way and two-way slabs and concludes by encouraging viewers to engage with the content for more detailed insights.
Takeaways
- 🏗️ Structural engineers must carefully consider and assign design loads for accurate and economical designs.
- ⚖️ Design loads directly influence the weight, reinforcement, and size of structural members.
- 📚 This video focuses on calculating structural design loads and understanding basic load paths.
- 🏢 The main loads in a structure include Dead Loads, Superimposed Dead Loads (SDL), and Live Loads.
- 🧮 Dead Load is the self-weight of the structure and can be calculated by multiplying the unit weight of the material by its thickness.
- 🧱 Superimposed Dead Load includes interior walls, floor finishes, ceilings, and MEP systems, calculated similarly to Dead Load.
- 🚶♂️ Live Loads account for movable items like furniture, equipment, and occupants, varying based on building use.
- 📖 Codes like ASCE 7-16 provide minimum live load values based on the type of structure or occupancy.
- 🔄 The load distribution in slabs depends on whether it’s a one-way or two-way slab, affecting how loads are carried by beams.
- ✍️ To apply service loads in design, structural engineers multiply by ULS factors: 1.2 for Dead Loads and 1.6 for Live Loads.
Q & A
Question 1: What are the types of loads a structure typically carries?
-A structure typically carries gravity loads and resists horizontal or lateral loads. The main gravity loads include Dead Load (self-weight), Super-imposed Dead Load (SDL), and Live Loads (movable loads).
Question 2: How is the Dead Load of a structure calculated?
-The Dead Load is calculated by multiplying the density or unit weight of the material by its thickness. For example, for a concrete slab of 0.25m thick with a unit weight of 25 kN/m3, the Dead Load is 6.25 kN/m2.
Question 3: What does Super-imposed Dead Load (SDL) include?
-The Super-imposed Dead Load includes partition or interior walls, floor screed, floor finish, ceiling loads, facade and internal cladding, and MEP (Mechanical, Electrical, Plumbing) pipes and fixtures.
Question 4: What are Live Loads and how are they determined?
-Live Loads are movable loads such as equipment, movable partitions, furniture, and occupants. The magnitude of Live Loads depends on the type of building or occupancy, and the minimum requirements are specified in relevant codes and standards, like ASCE 7-16.
Question 5: How do one-way and two-way slabs differ in load distribution?
-A one-way slab transfers loads in one direction, typically to two opposite edges, while a two-way slab transfers loads in two directions and is supported on all four sides.
Question 6: How do you determine if a slab is one-way or two-way?
-A slab is considered one-way if the ratio of the longer span to the shorter span is greater than 2.0. If the ratio is less than or equal to 2.0, it is classified as a two-way slab.
Question 7: What is the procedure for distributing loads on a two-way slab?
-For a two-way slab, draw an isosceles triangle in the shorter direction and a trapezoid in the longer direction to visualize how loads are transferred to the supporting beams. The loads are distributed proportionally to these areas.
Question 8: How do you calculate the load distribution on beams?
-To calculate load distribution on beams, calculate the area of the slab adjacent to each beam (triangle, trapezoid, or rectangle) and multiply it by the Dead Load, Super-imposed Dead Load, and Live Load. Divide the result by the length of the beam to get the uniformly distributed load in kN/m.
Question 9: What load factors are used for Ultimate Limit State (ULS) design?
-For ULS design, the load factors are 1.2 for Dead Loads and 1.6 for Live Loads. These factors are applied to the service loads to ensure safety under extreme conditions.
Question 10: How are loads transferred from the slab to the foundation in a structure?
-The loads carried by the slab are transferred to the perimeter beams, which distribute the loads to the columns. The columns transfer the load to the footing, which then transfers it to the underlying soil.
Outlines
🧱 Introduction to Structural Load Design
The first paragraph highlights the importance of correctly assigning design loads in structural engineering. The structural engineer must carefully consider these loads as they determine the weight and reinforcement of the structure. This episode will explain how to calculate structural design loads and provide an understanding of load paths and key considerations for achieving an economical design. The focus will be on gravity loads, including dead loads, superimposed dead loads, and live loads, as they influence the overall structure.
🏋️♂️ Understanding and Calculating Dead Loads
This section covers the concept of dead load, which is the self-weight of the structure. To calculate the dead load, the unit weight of the material is multiplied by its thickness. For example, a 0.25m thick concrete slab with a unit weight of 25 kN/m³ results in a dead load of 6.25 kN/m². The paragraph explains how the basic loads in a structure are carried from the slab to the beams, columns, and finally to the footing, resting on the soil.
🏠 Calculating Superimposed Dead Loads
Superimposed dead loads include non-structural elements like partition walls, floor finishes, and fixtures. The same principle used in calculating dead loads is applied to superimposed dead loads, where the density or unit weight of materials is multiplied to determine the total load. It emphasizes that material properties can be sourced from codes, standards, or lab testing to ensure accuracy.
🚶♂️ Introduction to Live Loads
Live loads are explained as the movable loads that the structure can carry, such as furniture, equipment, and occupants. These loads depend on the building's function, with higher live loads in assembly areas compared to residential areas. Standards like ASCE 7-16 provide minimum live load requirements, and a residential concrete structure typically has a live load of around 2 kN/m².
⚖️ Summing Up Basic Loads for a Structure
The paragraph summarizes the total dead load (12.25 kN/m²) and live load (2 kN/m²) for a typical structure. These loads are carried by the slab and transferred to the beams. A two-way slab distributes loads through an isosceles triangle and trapezoid, while a one-way slab distributes them in a linear direction. The calculated areas of adjacent sections are used to determine load distribution on beams, which are then transferred to the columns and footing.
🔢 Load Distribution and Design Calculations
This paragraph focuses on calculating the load distribution for beams, taking into account different geometric areas such as triangles and trapezoids. It explains how to compute the uniformly distributed load on the beams by dividing the total load by the length of the beam. Furthermore, it mentions the application of ULS factors (1.2 for dead loads and 1.6 for live loads) to design service loads, which are transferred to columns and ultimately the soil through the footing.
💬 Conclusion and Viewer Interaction
The final paragraph invites viewers to engage with the video by sharing their thoughts in the comments section. It encourages them to like, share, and subscribe for more content, and directs them to the website for detailed calculations and additional resources.
Mindmap
Keywords
💡Design Loads
💡Dead Load
💡Superimposed Dead Load (SDL)
💡Live Load
💡One-Way Slab
💡Two-Way Slab
💡Uniformly Distributed Load
💡ULS Factor
💡Footing
💡Load Path
Highlights
Structural Engineers must carefully consider design loads, as they impact the structure's weight, reinforcement, and dimensions.
This episode will teach you how to calculate structural design loads and the key considerations for an economical design.
Loads on buildings include Dead Loads (self-weight), Superimposed Dead Loads, and Live Loads (movable).
Dead Load is calculated by multiplying the material's unit weight by its thickness, e.g., a 0.25m concrete slab at 25 kN/m3 results in 6.25 kN/m2.
Superimposed Dead Loads include interior walls, flooring, ceiling, and fixtures, calculated similarly to Dead Loads.
Live Loads account for movable items like furniture and people and vary depending on the building’s use.
Live load assumptions are based on building occupancy type, with higher loads in gym or assembly areas compared to residential areas.
Referring to ASCE 7-16 standards, a typical live load for a residential building is around 2 kN/m2.
To determine load distribution, slabs are classified as one-way (load transfer on two edges) or two-way (load transfer on all four edges).
One-way slabs are used when the ratio of the longer span to the shorter span exceeds 2.
Two-way slabs are used when the ratio of the longer span to the shorter span is less than or equal to 2.
In a two-way slab, the load is distributed by drawing isosceles triangles in the short direction and trapezoids in the long direction.
An example of load distribution shows a slab carrying Dead Loads of 12.25 kN/m2 and Live Loads of 2 kN/m2.
Loads on beams can be calculated by multiplying the areas adjacent to the beams with the calculated loads (Dead Load, Superimposed Load, Live Load).
Service loads must be multiplied by ULS factors of 1.2 for Dead Loads and 1.6 for Live Loads to be used in design.
Transcripts
Perhaps the first thing for the Structural Engineers to be aware of in his/her structural
design is the assumptions and consideration of the design loads.
As a structural Engineer, we should be very careful assigning these loads in the structure
we are designing for.
Because these loads will dictate how heavy our structure is and the reinforcement and
the size or dimension of each of the structural members will vary according to our load assumptions.
In this episode, we will tackle how to calculate structural design loads on our structures
and what considerations we need to do in order to achieve an economical design.
After watching this video, you will learn at least the basics of load and its load path,
what are the considerations in assigning loads in a structure, and the load calculation procedure
necessary at the start of Structural Design.
But before we proceed further, please subscribe if you haven’t yet and hit the notification
bell for you to be updated with the latest video like this!
We all know that a given structure carries gravity and resists horizontal or lateral
loads.
In this video, we will focus on the gravity loads that a structure is carrying.
Loads in structures/buildings are composed of the self-weight of the structures or the
DEAD LOAD, the Super Imposed Dead Load or SDL, and the LIVE LOADS or the movable loads.
These loads are the basic loads of a certain structure/building.
These basic loads are carried by the slab which will be distributed in beams and transfers
to the columns to be resisted by the footing which is rested to the underlying soil.
So how are these loads calculated?
To understand how these loads are being calculated, Let us know first what type of material that
our structure will carry and how we will differentiate each accordingly.
In a typical residential concrete structure, for example, the load of the building is to
be carried by slab; we will start our calculation from there.
Dead Load.
Dead Load is the self-weight of the structure.
To calculate dead load, the density or unit weight of the structure should be multiplied
by the thickness, which will give us the weight of the structure per given area.
For a concrete slab of 0.25m thick for example, that will give us the following:
Considering the unit weight of concrete to be 25 kN/m3
• 25 kN/m3 multiplied by 0.25m = 6.25 kN/m2
Super-imposed Dead Load.
Super-imposed Dead loads include the partition or interior walls, floor screed, floor finish,
ceiling loads, facade and internal cladding, and M.E.P.
pipes and fixtures.
To calculate, let us assume that a slab is carrying a total of 6kilo Newton per square
meter.
Note that the same principle as calculating dead loads can also be adopted in determining
the weight of the Superimposed Dead Load making up the construction, with the given density
or unit weight of the material.
These densities or unit weight of materials can be sourced from the relevant codes and
standards, material data sheets, or obtained via laboratory testing.
Live Load.
Live Loads are the movable or moving loads that the structure can carry.
It can include the movable equipment, movable partitions, furniture, and other people occupying
the Structure.
Live load assumptions depend on the usage of the building or the type of occupancy.
It has obviously bigger live loads in assembly or gym areas compared to the residential areas.
The minimum live load requirement is given in the codes and standards that we are using.
Referring to ASCE 7-16 for example, Table 4.3-1 will give you all the recommended live
loads of the structure being designed.
Let's take the Live load to be 2 kN/m2.
So our Live/imposed loads on a residential concrete structure can be summed up to:
Dead Load/Self Weight = 6.25 kN/m2 Superimposed Load= 6 kN/m2
Live Load= 2 kN/m2
How the Basic Load is Being Distributed in the Structure.
Before we proceed, you have to differentiate whether our slab is a One-way or a Two-way
slab.
One way slab transfers the imposed loads in one direction only or on 2 edges to the perpendicular
beam or walls.
This tends to bend the slab in a short direction only.
The two-way slab has four edges support.
It is supported by beams or walls on all four sides in which the bending occurs in two directions
of the slab.
To verify using calculations, we can use the following formula:
• For a one-way slab, we can consider a one-way slab if the ratio of the longer to
the shorter span of the slab is greater than 2.
• And for two-way slabs, the ratio of longer to the shorter span of the slab is less than
or equal to 2.0 Applying the formula for a slab classification,
let’s take a look at this slab framing layout as an example:
To distribute the loads let us consider the result of the Loads that we previously calculated:
Total Dead Loads = 12.25 kN/m2 Total Live Load would be equal to 2 kN/m2
The Dead Load, Superimposed Dead Load and Live Load that we consider will be carried
by the slab.
It will then be distributed to the perimeter beam supporting it.
To distribute it on the perimeter beams, let’s take a look at this figure.
To distribute the load on a two-way slab, simply draw an isosceles triangle in its short
direction and a trapezoid in its long direction as shown.
A one-way slab simply cuts the slab into two along its length.
For the beam to carry the slab, calculate the areas adjacent to it.
Let’s calculate the loads on beams B3 and B4 respectively as an example.
Area of the triangle at B3 = 1 m2 Areas adjacent to B4 = area of trapezoid plus
area of rectangle = 14 m2 To distribute it along the beam, multiply
these areas with the Dead Load, Superimposed Dead-load, and live load to get the actual
load distribution in kilonewton.
Dividing the actual load distribution into the length of the beam will give you the uniformly
distributed load in kilonewton per meter.
To use in design these service loads should be multiplied by the ULS factor, 1.2 for Dead
Loads and 1.6 for Live Loads.
The load reactions on each support of the beam will be carried by the column joining
them and eventually transferred to the footing supported by underlying soil.
Visit our website link in the description below to know more about the detailed calculations.
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