Basics of Structural Design Load Calculations | One-Way Vs Two-Way Slab

Summary

This 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

00:00

🧱 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.

05:05

🏋️‍♂️ 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.

Keywords

💡Design Loads

Design loads refer to the forces or weights that a structure is expected to support or resist. In the video, the structural engineer must carefully assign these loads, as they dictate the structure’s weight, size, and reinforcement. The video emphasizes that understanding and calculating these loads are essential to achieving an economical design.

💡Dead Load

Dead load represents the permanent, stationary weight of the structure, including its self-weight. In the script, it is calculated by multiplying the density or unit weight of the material by the thickness of the structural element. The video highlights the importance of accurately estimating dead loads as they contribute to the overall weight the structure must support.

💡Superimposed Dead Load (SDL)

Superimposed dead load (SDL) includes non-structural elements added after the construction, such as partitions, finishes, and fixtures. These loads are distributed over the slab and are calculated in a manner similar to the dead load, using the material's unit weight and area. SDL is crucial for determining the total load on structural components, as discussed in the video.

💡Live Load

Live load refers to the movable loads that a structure must carry, such as people, furniture, and equipment. The video explains that live loads depend on the building’s usage or occupancy, with higher loads in areas like gyms or auditoriums. These loads are governed by standards like ASCE 7-16, which provides guidance on the required live load for different building types.

💡One-Way Slab

A one-way slab transfers load primarily in one direction, usually supported by beams or walls on two opposite sides. The video explains that this type of slab bends along the shorter span and distributes the load to the perpendicular beams. This concept is important when calculating how the loads on the slab are transferred to the supporting structure.

💡Two-Way Slab

A two-way slab distributes loads in two directions and is supported on all four sides. The video distinguishes this slab type by showing how it bends in both directions and transfers loads to beams or walls along all edges. The ratio of the longer to shorter span determines whether a slab is one-way or two-way, impacting load distribution calculations.

💡Uniformly Distributed Load

Uniformly distributed load (UDL) refers to a load that is spread evenly across a structural element, such as a beam. In the video, the slab's loads, including dead, superimposed, and live loads, are calculated and distributed along the beams supporting the slab. This load is expressed in kilonewtons per meter (kN/m) and is crucial for determining beam design.

💡ULS Factor

Ultimate Limit State (ULS) factors are used to account for uncertainties in load assumptions, ensuring the structure’s safety. In the video, dead loads are multiplied by a factor of 1.2, and live loads by 1.6, to provide a conservative estimate of the maximum potential load. These factors are crucial for ensuring that the structure remains safe under extreme conditions.

💡Footing

Footing is the structural element that transfers the loads from the columns or walls of a building to the underlying soil. In the video, the footing is where the loads from the beams and columns are ultimately transferred. It is an essential part of the load path, ensuring that the structure's weight is safely distributed to the ground.

💡Load Path

The load path describes how loads move through a structure, from the point of application to the foundation. The video illustrates the load path by explaining how the dead load, superimposed dead load, and live load are transferred from the slab to the beams, then to the columns, and finally to the footing. Understanding the load path is crucial for structural integrity.

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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