SAFE - 08 Cracked Section Analysis: Watch & Learn
Summary
TLDRThis 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
- đ 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.
- đ SAFE automates the process of finding cracked bending moments and curvatures using an iterative method to adjust flexural stiffness modifiers for each element.
- đ ïž The initial step involves calculating forces based on elastic properties, and determining reinforcing either by the program or user input.
- đ 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.
- đ The program iterates to adjust bending properties until the change in maximum displacement is minimal, ensuring accuracy in the analysis.
- đ SAFE provides options for calculating both immediate and long-term deflections, important for different stages of a structure's life.
- đïž 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.
- âïž Load patterns are defined with specific multipliers, and load cases are set up for nonlinear cracked analysis, considering immediate deflections.
- đą The tutorial demonstrates setting up load combinations for displaying elastic deflections and for the analysis of cracked deflections.
- đ§ Users can specify the source of reinforcement and adjust minimum reinforcing ratios and modulus of rupture within the cracking analysis options.
- đ The deformed shape display helps visualize the results of the analysis, showing the deflection changes from elastic to immediate and long-term cracked conditions.
- đ 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
đ 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.
đ§ 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.
đ 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.
Mindmap
Keywords
đĄSAFE program
đĄCracked section analysis
đĄIterative process
đĄFlexural stiffness modifier
đĄLoad patterns
đĄLoad cases
đĄNonlinear cracked
đĄDeflections
đĄModulus of rupture
đĄCreep and shrinkage
đĄLoad combinations
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
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