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ETABS – 08 Wall Modeling: Watch & Learn

ETABS – 08 Wall Modeling: Watch & Learn


This tutorial will illustrate some of
the options available when modeling walls in ETABS namely
meshing adding an opening and assigning pier and spandrel labels as discussed in the drawing tools
Watch & Learn there are a number of different ways to
draw wall objects the focus of this tutorial will be to discuss the
options available once the walls have been drawn first we will show some of the meshing options we will select a wall object and
then go to the Assign shell wall auto mesh options command the default setting is that the straight
walls are not meshed the physical object and the analytical element are the same
however there may be times when meshing is
desired and ETABS allows the user to select the meshing one option is to specify the
number of vertical and horizontal elements the user may
also specify an auto rectangular mesh if we select another
wall object and then this option the Modify show
button shows that the mesh size is set to four
feet here we will reduce it to two feet note that this setting applies to all
shells assigned the automatic meshing option note that the assignments are shown on
the wall objects if we go to the set display options form we can have the actual mesh displayed again the default is no mesh which typically gives good results due
to the sophisticated finite element formulation of the wall
objects the good performance of the unmeshed
walls can be observed by reviewing these side by side walls on the left we have meshed the wall
objects into 2 by 2 elements and on the right we are using the
unmeshed default settings we have applied a 25 kip lateral load
at the top of each wall after running the analysis a right-click on a top joint shows
that the wall on the left has a deflection of 0.020378 inches and the wall on the right a deflection of 0.019619 inches a difference of only about 3
percent well within engineering tolerances however there are cases where proper
meshing is very important such as modeling walls with
discontinuities here we have a planer wall with an offset at the first level the
load on the model will be a static earthquake lateral load it is the connectivity and behavior at the offset corners that is of
particular interest if we run the model with the default no meshing assigned to the wall objects
we see that the structure is unstable this is
not an appropriate model to rectify this situation we will modify the model in two
different ways in the first analysis we will use a
refined mesh with no edge constraints this will be the standard for comparison
in the second model we will use a semi refined mesh with connectivity at the corners along with edge constraints for the first
analysis select all of the wall objects and
assign an internal mesh of one by five reselect the wall objects and turn off
the edge constraints option if we go to the set display options
command we can view the mesh on-screen note that the mesh lines up
not only at the corners but that all of the interior mesh lines are continuous next we rerun the
analysis and a right click on a top joint shows a displacement of
0.014 note this for further reference a plot of the forces in the wall shows a
pattern that seems appropriate with the expected concentrations around
the corners in the next analysis we will use a mesh
that does not line up in the interior region but does provide
connectivity at the corners select an object and assign edge constraints making sure that the applied to full
structure is checked this will help to lock the mesh
together in the interior of the wall where it does not align running the
analysis this time gives the displacement that is approximately only five percent
different than that for the refined mesh and a force plot that is reasonable thus this is an effective use of the
edge constraint option using a mesh and edge constraints is a
powerful feature for connecting walls with discontinuities as a side note had we not used the edge
constraints to tie the two meshes together we would have had a sizable gap where
the two misaligned meshes meet however with the edge constraints on the objects are tied together even
though the meshes do not line up next we will show how to draw an opening in a wall object this can easily
be done using the draw wall openings command clicking on this
displays a properties of object form with width height and distances to the first corner note also the
lock icons we will explain these in a moment there
are a number of ways this command can be used the first can be done without entering
any values in the properties of object form make sure that the locks are unlocked and
then click on the wall object that will have the opening this displays the dimensioning lines the next click
locates the first corner of the opening and the third click sets the opposite corner and finishes
the opening note that the size of the opening is displayed in the properties of object
form if we lock the size dimensions of the
opening just drawn the first click on a wall object will again display the
dimensioning lines but the size of the opening is now fixed a second click completes the drawing of the opening
lastly if we enter dimensions for the bottom left hand corner of the opening and then lock these dimensions a single click on a wall object will draw the opening
at the specified location regardless of where we click on the wall we
could also unlock the width and height dimensions
but leave the corner position locked which results in the starting corner
being locked but allows the size of the opening to be
dynamically adjusted we can also add openings by eliminating sections of walls
using the draw reference points and draw reference planes commands here we have a building where the
elevator core walls have been drawn but with no door openings to get a
better view we will activate the 3d view and select
the walls we can then right click away from the
model and choose show selected objects only we want to
add two doors per story along grid line 2 switching back to the
plan view we right click again away from the model
and select the snap options command set the plan fine grid
spacing to 48 inches and check the fine grid
option this will allow the cursor to snap to
points 48 inches on center anywhere on the plan now click the draw reference points command
and we will add reference lines along the grid line 2 walls at the fine grid points we can see these reference lines better
if we switch to an elevation view the next step is to add reference planes to mark the
top of the door openings our doors will be eight feet tall we set
the vertical offset to 8 feet or 96 inches and click once at each story starting at the base
through story four if the locations of either the reference
points or planes needs to be reviewed this may be done by going to the Edit
stories and grid systems command clicking the Modify button and then the
appropriate reference button the next step is to divide the walls up
into sections that match our reference lines and planes select all the wall objects along
grid line 2 and go to the Edit edit shells divide shells command here we select
the intersections with visible grids option now it is simply a matter of selecting
the wall objects where the openings should be and then deleting next we will assign pier and spandrel
labels to the wall objects these labels must be assigned before we
can obtain either output forces or design the
objects piers will report the forces and perform the design at the top and
bottom of each object for each story spandrels on the other hand output the
forces and the design at the left and right ends of the
objects and not at the top and bottom objects may be assigned both a pier and
spandrel label but no object may have more than one of
each there are many ways to arrange the labels and we suggest you review the shear wall
design manual for more details a right click on a wall object shows
that initially it is not assigned either a pier or spandrel label to assign a pier label simply select the wall objects and go to
the Assign shell pier label command here we can use an
existing label or create a new label if we run the analysis we can display the forces based on the
pier labeling and then run the design this was a simple wall and pier
arrangement and next we will look at a more complex wall layout here we have a wall system created using
the wall stack feature unlike the other wall drawing commands
in ETABS when drawing wall stacks pier labels are
automatically assigned we can see the pier labels by going to
the set view options and on the other assignments tab
clicking the labels checkbox in this case P2 was
assigned note that the wall stack command assigns
the same label to all walls in the stack the analysis has already been run so we can go straight to the shear wall
design note that in the 3d view that the
reinforcing is reported at the centroid for the entire section that is the total reinforcing required
for the three wall sections as a whole is reported in this case
10.08 a right-click displays the pier
reinforcing report however it might be useful to
know the reinforcing required in each of the three wall segments and not
just the total to do this each segment should have its
own pier label we will assign the two vertical legs different pier labels we will create a P3 three label and then a P4 label assign them to the walls and then rerun the shear wall design on the 3d view we now have three
different reinforcing values reported one for each wall segment although the
three different values shown total up to the number previously
reported we now have a better understanding of how the reinforcing should be distributed throughout the
stack lastly let’s look at the pier labeling
when openings are present for this model we’re going to assume
that we want the wall objects on the left of the openings assigned one
pier label and the wall objects to the right of
the openings assigned another pier label so that we will have design output for
each side an alternative labeling scheme would
have been to only assign pier labels to the wall
objects that lie directly adjacent to the openings so that reinforcing would be reported at
the top and bottom of the openings for the wall objects directly above and below the openings we could assign
another pier label or a spandrel label or both in this case
we will apply a spandrel label which will provide us design results for
these shells in terms of spandrel behavior we run the analysis and then the shear wall design longitudinal reinforcing required for
the walls on each side of the openings is displayed if we go to the design menu we can display the spandrel reinforcing a right click on a spandrel displays a
report showing the particulars of the spandrel design with design complete we can go one step
further by running detailing based on the design
reinforcement calculated and the detailing parameters
schematic drawings are generated this concludes this tutorial on
modeling walls

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