SAFE - 08 Cracked Section Analysis: Watch & Learn

Summary

This tutorial guides users through the SAFE program for cracked section analysis in structural engineering. It explains the automated process for calculating bending moments and curvatures, the iterative method to determine flexural stiffness modifiers, and how to handle both immediate and long-term deflections. The example of a flat slab model demonstrates defining load patterns, assigning loads, and analyzing with different load cases to assess deflections, showcasing the program's capability to handle complex structural analysis.

Takeaways

• 📘 The tutorial covers the use of SAFE program for cracked section analysis, which is crucial for determining bending moments and curvatures in reinforced concrete structures.
• 🔍 SAFE automates the process of finding cracked bending moments and curvatures using an iterative method to adjust flexural stiffness modifiers for each element.
• 🛠️ The initial step involves calculating forces based on elastic properties, and determining reinforcing either by the program or user input.
• 📊 Curvatures for both uncracked and fully cracked states are calculated to find the applied moment's curvature, which helps in computing the flexural stiffness modifier.
• 🔄 The program iterates to adjust bending properties until the change in maximum displacement is minimal, ensuring accuracy in the analysis.
• 📏 SAFE provides options for calculating both immediate and long-term deflections, important for different stages of a structure's life.
• 🏗️ The example model used in the tutorial is a simple two-bay flat slab subjected to various load patterns including dead, superimposed dead, and live loads.
• ⚖️ Load patterns are defined with specific multipliers, and load cases are set up for nonlinear cracked analysis, considering immediate deflections.
• 🔢 The tutorial demonstrates setting up load combinations for displaying elastic deflections and for the analysis of cracked deflections.
• 🔧 Users can specify the source of reinforcement and adjust minimum reinforcing ratios and modulus of rupture within the cracking analysis options.
• 📊 The deformed shape display helps visualize the results of the analysis, showing the deflection changes from elastic to immediate and long-term cracked conditions.
• 🕒 The long-term cracked deflection analysis includes considerations for creep and shrinkage, and involves setting up additional load cases to account for sustained and non-sustained live loads.

Q & A

What is the purpose of the SAFE program mentioned in the tutorial?

The SAFE program is used to perform a cracked section analysis, determining cracked bending moments and curvatures for every element in a structure.

How does SAFE calculate initial forces in its analysis?

SAFE calculates initial forces based on elastic properties and the determined reinforcing, either by the program or as input by the user.

What is the iterative process SAFE uses to determine flexural stiffness modifiers?

The iterative process involves computing interpolation coefficients, determining curvatures for uncracked and fully cracked states, and using a ratio of curvatures to compute a flexural stiffness modifier, which adjusts the bending properties.

What is the significance of the moment-curvature graph shown in the tutorial?

The moment-curvature graph illustrates the transition between uncracked and fully cracked states, which is crucial for understanding the behavior of elements under different load conditions.

How many options does SAFE offer for calculating cracked deflections?

SAFE offers two options for calculating cracked deflections: one for immediate deflections and another for long-term deflections.

What is the difference between immediate and long-term deflections in SAFE?

Immediate deflections are calculated based on service loads without considering time-dependent effects like creep and shrinkage, while long-term deflections account for these effects over time.

How does the tutorial's example model a simple two-bay by two-bay flat slab?

The model is loaded with three load patterns: dead, superimposed dead, and live loads, and the tutorial demonstrates how to define load patterns, assign loads, and define load cases in SAFE.

What is the purpose of the 'sdead' load pattern in the tutorial?

The 'sdead' load pattern represents the superimposed dead load and is used to apply a specific load magnitude to the slab in the analysis.

Why are nonlinear load cases used in the tutorial for calculating immediate deflections?

Nonlinear load cases are used because they allow for the calculation of immediate deflections considering the cracked section behavior, which is not superimposable like linear static load cases.

What are the steps SAFE takes to account for long-term effects like creep and shrinkage in the analysis?

SAFE sets up additional load cases that include sustained loads with creep and shrinkage, and isolates the immediate deflection component from the non-sustained live load to accurately calculate long-term deflections.

How does SAFE handle the combination of load cases for long-term cracked section analysis?

SAFE combines the long-term sustained load case with the immediate all loads case, subtracting the immediate sustained case to account for the non-sustained live load component, resulting in a long-term load combination.

Outlines

00:00

🔍 Introduction to Cracked Section Analysis with SAFE

This paragraph introduces a tutorial on using the SAFE program for cracked section analysis. It explains the automated process of determining cracked bending moments and curvatures for structural elements. The tutorial outlines the steps involved in calculating initial forces, reinforcing determination, and the iterative process to compute flexural stiffness modifiers. It also covers the calculation of immediate and long-term deflections, using a two-bay flat slab model as an example. The process includes defining load patterns, assigning loads, and setting up load cases for nonlinear cracked analysis. The tutorial emphasizes the importance of understanding the moment-curvature transition and the iterative nature of the SAFE program in achieving accurate results.

05:01

🔧 Setting Up and Running the Cracked Analysis

This section details the setup process for running a cracked section analysis in SAFE. It includes defining load cases with consideration for immediate and long-term effects such as creep and shrinkage. The tutorial explains how to unlock the model to set up additional load cases, the importance of sustained loads, and the distinction between sustained and non-sustained live loads. It also describes the creation of load combinations for displaying elastic deflections and for calculating long-term deflections. The process involves defining new load cases with specific factors to account for sustained and non-sustained loads, and the use of nonlinear load cases to account for the effects of creep and shrinkage. The paragraph concludes with the analysis run and the observation of increased deflections in the cracked section case compared to the elastic case.

10:09

📊 Comparing Deflections: Elastic, Immediate, and Long-Term

The final paragraph of the tutorial compares the deflections obtained from the elastic analysis, immediate cracked section analysis, and long-term cracked section analysis. It presents the results of the maximum deflections in the down direction for each analysis type, showing a significant increase from the elastic to the immediate and then to the long-term analysis. The tutorial concludes by emphasizing the importance of considering both immediate and long-term effects in structural analysis to ensure accurate and safe design. The comparison highlights the necessity of using the SAFE program's capabilities to account for the complex behavior of structures under different loading conditions.

Keywords

💡SAFE program

The SAFE program is a software tool used for structural analysis, specifically for performing cracked section analysis in reinforced concrete structures. It is central to the video's theme as it automates the process of determining bending moments, curvatures, and stiffness modifiers for elements under different cracking states. The script mentions using SAFE to calculate initial forces, reinforcing ratios, and to perform iterative analysis to adjust bending properties.

💡Cracked section analysis

Cracked section analysis is a method used to evaluate the structural behavior of reinforced concrete members under load, taking into account the effects of cracking. The video focuses on how the SAFE program conducts this analysis by determining bending moments and curvatures for both uncracked and fully cracked states, which is essential for understanding the structural integrity and performance of the concrete elements.

💡Iterative process

In the context of the video, the iterative process refers to the SAFE program's method of repeatedly calculating and adjusting the flexural stiffness modifiers until the change in maximum displacement is minimal. This process is crucial for achieving an accurate representation of the structure's behavior under different load conditions.

💡Flexural stiffness modifier

A flexural stiffness modifier is a factor used to adjust the bending properties of an element in the SAFE program. The script explains that this modifier is computed based on the ratio of curvatures, which helps the program to account for the reduction in stiffness due to cracking, thus refining the structural analysis.

💡Load patterns

Load patterns in the script refer to different combinations of loads applied to the structure, such as dead, superimposed dead, and live loads. The SAFE program uses these patterns to simulate various real-world loading scenarios, which is essential for a comprehensive structural analysis.

💡Load cases

Load cases are specific scenarios defined in the SAFE program that combine different load patterns with certain factors. The video describes creating load cases for immediate and long-term deflections, which are necessary for analyzing the structure's behavior under both short-term and sustained loads.

💡Nonlinear cracked

Nonlinear cracked is a type of load case in the SAFE program that accounts for the nonlinear behavior of structures due to cracking. The script mentions applying this type to calculate immediate deflections, indicating that it is used when the analysis requires considering the effects of cracking on the structural response.

💡Deflections

Deflections in the script refer to the displacements or deformations of the structure under load. The video discusses calculating both immediate and long-term deflections, which are critical for assessing the serviceability and long-term performance of the structure.

💡Modulus of rupture

The modulus of rupture is a material property that measures the stress at which a material breaks or fractures. In the context of the video, it is mentioned as an adjustable parameter in the SAFE program, which influences the cracking analysis and the determination of the structure's response to load.

💡Creep and shrinkage

Creep and shrinkage are time-dependent deformations that occur in concrete under sustained loads. The video script discusses including these effects in the long-term cracked deflection analysis to more accurately predict the structure's behavior over time.

💡Load combinations

Load combinations in the SAFE program are sets of load cases that are combined to simulate various load scenarios that the structure may experience. The script describes creating specific combinations for elastic and long-term deflections to analyze the structure's response under different conditions.

Highlights

Tutorial demonstrates how to use the SAFE program for cracked section analysis.

SAFE automates the determination of cracked bending moments and curvatures for every element.

Iterative process to determine flexural stiffness modifiers for each element.

Initial forces are calculated based on elastic properties and reinforcing determined by the program or user input.

Cracked bending moments and interpolation coefficients are computed from applied and cracked moments.

Curvatures for uncracked and fully cracked states are used to obtain the curvature for applied moment.

A ratio of curvatures computes a flexural stiffness modifier for program adjustment of bending properties.

Displacements are calculated with the program iterating until the change in maximum displacement is minimal.

SAFE offers options for calculating immediate and long-term cracked deflections.

Demonstration of both immediate and long-term deflection calculations based on service loads.

Model setup includes a simple two bay by two bay flat slab with specific load patterns.

Load patterns defined with a special pattern for superimposed dead load and self-weight multiplier set to 1.

Load cases are defined for calculating reinforcing, including a nonlinear cracked case for immediate deflections.

Load combinations are set for displaying elastic deflections and cracking analysis options are configured.

Analysis options include specifying reinforcement source and adjusting minimum reinforcing ratios.

Running the analysis may take time due to multiple iterations and program displays deformed shape.

Deflection comparison between elastic, immediate cracked section, and long-term cracked deflection.

Setting up additional load cases to include creep and shrinkage for long-term deflection analysis.

Long-term case uses sustained loads and assumes a percentage of live load as sustained.

Isolating live load deflection components for immediate and sustained loads in separate cases.

Defining load combinations for long-term analysis, combining sustained and immediate load cases.

Running long-term cracked section calculation and comparing maximum deflections.

Conclusion of the tutorial on cracked section analysis with SAFE program.

Transcripts

this tutorial will show you how to use

the SAFE program to perform a cracked

section analysis SAFE's automated

procedure determines cracked bending

moments and curvatures for every element

and then uses an iterative process to

determine flexural stiffness modifiers

here are the general steps for each

element initial forces are calculated

based on elastic properties and the

reinforcing is determined either by the

program based on the elastic forces or

as input by the user cracked bending

moments and then interpolation

coefficients are computed based on the

applied and cracked moments curvatures

for the uncracked and fully cracked

state are determined and then used to

obtain the curvature for the applied

moment a ratio of curvatures is then

used to compute a flexural stiffness

modifier which the program uses to

adjust the bending properties using the

modified stiffnesses displacements are

then calculated the program will iterate

through this process until the change in

the maximum displacement is small the

moment curvature transition between

uncracked and fully cracked is shown in

this graph

SAFE offers two options when calculating

cracked deflections one for immediate

deflections and one for long-term

deflections we will demonstrate both

options but we'll start by calculating

immediate deflections of course both are

based on service loads our model will be

a simple two bay by two bay flat slab

which we will load with three load

patterns dead superimposed dead and live

we start by defining our load patterns

in addition to the dead and live we will

add a pattern called sdead for

superimposed dead load make sure that

the self weight multiplier is set to 1

for the dead load pattern next we will

assign the loads select the slab and

assign a surface load using the sdead

pattern with a magnitude of 20 pounds

per square foot repeat the selection and

add a surface load of 80 pounds per

square foot for the live pattern

now we will define our load cases in

addition to the three default linear

static cases which are needed to

calculate reinforcing we will add a case

called immediate all loads with the type

of nonlinear cracked because we are

calculating the immediate deflection we

will apply all three load patterns with

scale factors of one in this nonlinear

load case note that when working with

nonlinear load cases results may not be

superimposable like they are with linear

static load cases

next we will define a load combination

for displaying the elastic deflections

we will call the combination elastic

we are now about ready to run the

analysis before we do however we will

set the options for the cracking

analysis here we can specify the source

of the reinforcement for calculating the

cracked inertias which can be user

specified calculated by the program from

the analysis or quickly input by the

user we will use the program calculated

values we can also adjust the minimum

reinforcing ratios as well as the

modulus of rupture with these values set

we are ready to run the analysis this

may take some time as the program runs

multiple iterations

we will switch the display to show the

deformed shape for the elastic

combination note that the maximum

deflection is approximately 0.249

inches in the down direction next we

will switch to the immediate cracked

section case

the deflection has increased to

approximately 0.862 inches

remember these values for comparison

later with the long term cracked

deflection next we will ask SAFE to

determine the long term cracked deflection

we start by unlocking the models so that

we can set up the additional load cases

needed to include creep and shrinkage

the first load case will be dead plus

sdead plus 0.25 live with creep and

shrinkage this long-term case uses only

sustained loads and we are assuming only

twenty-five percent of the live load is

sustained however this case is missing

the deflection component from the 75

percent of the live load that is not

sustained this is the part of the live

load that contributes to an immediate

deflection to isolate that live load

deflection we will use two load cases

dead plus sdead plus live- dead plus

sdead plus 0.25 live neither of these

cases will use creep or shrinkage

we will call the case with creep and

shrinkage long-term sustained and it

will have a type of nonlinear long-term

cracked in this case we apply dead and

sdead with factors of one and the live

load pattern with a factor of 0.25 the

next case needed was previously defined

as immediate all loads the last case we

will add will be called immediate

sustained and it will have a type of

nonlinear cracked no creep or shrinkage

in this case and again the dead and

sdead will be applied with factors of one

while the live load pattern will have a

factor of 0.25

next we will define a load combination

to combine these three load cases we

will call this combination long term we

will add the long-term sustained and

immediate all those cases together but

we'll subtract the immediate sustained

case the last two cases provide the live

load immediate deflection for the non

sustained component

we can now run the analysis

for the long term cracked section

calculation again this may take some

time due to the iterations switching the

display to the long term deflection

combination

we see that the maximum deflection in

the down direction is now approximately

1.439 inches compared to 0.862 from

the immediate cracked analysis and 0.249

for the elastic analysis this

concludes this tutorial on cracked

section analysis