Truss Analysis (Mechanics vs. Hypermesh)

TRUSS ANALYSIS:
SOLUTION BY MECHANICS
VS. HYPERMESH
Created By:
Akshay Mistri

Structure under analysis:
 Gabrel truss as shown has following
properties:
 Youngs Mod., E=28.5e6 psi
 Mod. Of Rigidity, G=11.5e6 psi
 Poisson’s Ratio, v=.29
 Density = 0.283 lb./𝑖𝑛𝑐ℎ3
 Cross-Section: Circular, 2 inches in radii.
 Truck stopped in the middle of the bridge
weighs 7500 lbs. with its CG in the center.
 Objective is to find stresses in members of
the truss using Hypermesh and compare it
with solution by Mechanics.

Steps for Analysis in Hypermesh
 First thing to keep in mind is we can see
only one side of the bridge, hence we will
compute the stresses assuming the half side
of the bridge supports exactly half the load
of the truck, i.e. 3750 lbs.
 We start with making nodes in Hypermesh in
Opti-Struct user profile.
 Taking care of the units, distance between
each node is kept in inches.
 1 feet = 12 inches, so we multiply each
distance by 12 to get all the distances in
inches.

Creating Material
 Click on create/edit.
 Enter the material properties as given, E, G, v, density. Click on return.

Defining cross-section of the truss members
 Go to 1D > HyperBeam.
 Select Standard Section as
shown and click on create.
 Enter radii as 2.0 and click
on the Model View (first
icon under the word
Model).

Creating Property for Truss Members
Create the property using the inputs as shown in the picture. Ensure to give a
different color code for every material/property.
Creating Component for Truss Members
Click on create.

Creating Truss Members between the Nodes
 Go to 1D > Rods. Select property and element type as shown.
 Click on the first and second node to create a rod member between them.

Creating Load Collectors
 Create Load Collector named SPC
(Single Point Constraint) with red
color code.
 Similarly, create Load Collector
named Force with green color code.

Creating Constraints (SPC)
 Now go to Analysis > constraints.
 We will now create SPC’s on First and Last Node.
 For first node, it is free to rotate about z-axis (perpendicular to screen). So we will
uncheck box dof6.
 Also, make sure your current Load Collector is SPC (check bottom-right corner). If not,
click there and select SPC as current load collector.
 Click on the first and then click create to make a constraint on the node.

Creating Constraints (SPC) Continued
 Now, make constraint on the last node which is free to slide in horizontal plane along with
rotation in z-axis.
 For making it free to slide in horizontal plane we uncheck box dof1.
 Click on last node and then click create to make constraint.
 Dof1 to dof3 represents degree of freedoms
in x, y, z directions while dof4 to dof6
represent freedom of rotation along x, y, z
axes respectively.

Applying Loads
 Now, loads from vehicle tire to truss can only be applied on
nodes in Hypermesh.
 The load 3750 lbs will be applied to nodes H, G & F in ratio of
the distances from center.
 50% (1875 lbs) of the load goes to member HG and 50% (1875
lbs) to member GF.
 For member HG, load of 1875 is divided between node H (60%
of 1875 = 1125) and node G (40% or 750).
 Similarly, for member GF, node F will have load of 1125 and G
will get another load of 750 lbs.
 So, total load at G becomes 750+750 = 1500 lbs.

Applying Loads on Nodes
 Go to Analysis > Forces.
 Make Force as your current component.
 Apply loads on nodes with negative
magnitude to give them proper direction.

Make LoadSteps
 Go to Analysis > LoadSteps.
 Give Name Static, since it’s a static load.
 Click on ‘=‘ sign and select SPC for the SPC row and Force for the Load row.
 Click on create.

Run for Analysis
 Go to Analysis > Opti-Struct.
 Click on Opti-Struct. You will get a message Analysis completed. (if no errors)
 Go to the folder where your files are. You can check the folder by clicking on save as (near Opti-Struct button).
 In that folder you will find a Altair MVW file. Double clicking on that will open hyperview player used for analyzing results.
 Close the message log which pops out. Click on apply and click on yes for any warnings.
 Then Click on the contour button as shown below. Again click on Apply to see the displacement contour. You can also change
views of your model by different view buttons given below.

Results
 Click on query results, and then click on any node in the structure to see its displacement.
 You can again go back to contour and change Displacement to Element Stresses.

Result
 Change displacement to
Element Stresses from
drop down menu and
click apply.
 Again click on query
table. Now you can click
on members in which you
want to find the stresses.
 Negative sign indicates
compression.

Solution by Mechanics
 The element stresses
found by mechanics
can be compared to
results of
Hypermesh.

Comparison: Mechanics vs. Hypermesh

Truss Analysis (Mechanics vs. Hypermesh)

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