ARCH 1162 - Law Building Bridge Tutorial
This is the bridge between spaces of
the Sydney University Law building
by the architects, Francis-Jones et al.
We will examine an approximation of
its structure.
Go through this procedure; we will
explain what the steps mean as we
navigate the course. First, access
the Multiframe application from the
UNSW “my access" portal (Maxsurf
Multiframe)
Initially, set up four windows on
the screen in tile mode as
follows:
Restore-up (mid-size) four windows
only; Frame, Data, Load and Plot.
Go Window/ Tile Vertical then make
the frame window operative by
clicking in the top band (grey to
blue). Ensure that you are using the
correct metric units as follows: View
/ Units / Australian, Edit/
Preferences/ Numerics / Decimal
(1place). Set up all the windows as
2-D (view/front).
Make a two dimensional frame
in the Frame Window:
View/Front. Pull down the geometry
menu and add a horizontal member to
the right from the origin, Geometry
/ Add member (there are shortcuts;
icons & right click options). Click on
the right hand end of the first member
and again about a quarter of a window
upwards to add a post. Add another
horizontal member to the right of the
first. You should now see a wonky
upside-down “T”
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Correct the dimensions as follows:
Click the top band of the "Data"
window. Go into the joint data table
and modify the x, y and z coordinates
of Joint 1 to be 0,0,0. Change the
coordinates of joint 2 so they are
12,0,0, joint 3 so they are 12,3.5,0
and joint 4 so they are 24,0,0. Reactivate the frame window and see the
corrected frame made up of its three
parts by View /Size to Fit. Note that
the default joints are rigid (can
transfer moment and shear).
(Display/symbols/joint nos) to see or
remove the joint numbers.
In the Frame window, select the
horizontal nodes at each end and
provide a “pin support” for each
(triangle with circle above in toolbar).
Select the middle joint and the top of
the post and make those “Pin joints”
(small circle in toolbar). Select the
two horizontal members and define
their sections as “360UB50.7”
(Frame/Section type/UB/
360UB50.7). Highlight the post.
Define it as “Grade 350 355.6x6.4
CHS” (circular hollow section)
Activate the Load window; go Case/
Edit case and change “load case 1” to
“G, dead load”. Select both
horizontal members and apply a
“global distributed load” (4th icon
from left in toolbar) of 1.8kN/m to
both. Use even load - left icon in
pop-up window.
Still in the Load window; go
Case/add case/ static ; call it “ Q,
live load” and apply 9kN/m to both
horizontals.
Again in the load window, with all
members highlighted; go Case/add
case/ Self weight call it “self wt”
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Again in the Load window;
Case/add case/ Static combined
change its name from “load case
x” to “W* Ultimate”. In the
dialogue box, assign factors as
follows: to dead load, 1.25; to
live load, 1.5; to self wt, 1.0.
Again in the load window;
Case/add case/ Static combined
change name from “load case x” to
“Ws Service”. In the dialogue box
assign factors as follows: to dead
load, 1.0; to live load, 0.5; to self
wt, 1.0; to W*, 0
Activate the frame window and
select all three members. Go
Geometry/duplicate .
In the dialogue box,, set y spacing: 3.5m, No. of times: 4.
Enter . See the structure go up 4
times. View/ size to fit, view
/shrink then delete the top column.
Because the top beams form the
roof, we need to remove the floor
live load from them. Activate the
load window and set Case to “Q,
live load”.
Highlight the top beams, right
click and go “unload member” or
press delete to remove the floor
live load from the roof.
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Now we need to add the diagonal
ties to hold the bridge up. In
Frame window, geometry/add
member . Insert members as
shown between the fourth level
supports and the bottom centre
point. Define their sections as
Grade 350 219 x 6.4 CHS.
Highlight each of the top supports
of the ties and define them as pins
(small circles; bottom icons).
Now “Analyse” the structure;
Analyse/Linear. If all goes OK,
a red diagram should appear in the
plot window. Size to fit and
shrink it with “Case” set to “W*
Ultimate” and the left “Numbers”
icon in the plot toolbar activated to
see the Ultimate Bending Moment
diagrams with maximum values
shown. See that the greatest
value of bending is 292kN.m.
This is a large bending moment.
Still in the W* case with the plot
window active, Display/member
stress/bending bottom. The red
diagram is now showing
maximum stresses of 368 MPa in
tension at the bottom of the
beams; this is greater than the
steel yield stress (300 MPa) so the
beams are undersized. We need to
increase the beam size.
Activate the frame window and
highlight all the beams. Right
click and define Section type
460UB74.6.
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Analyse linear again and see that
the Ultimate bending stresses in
the plot window are now around
203 MPa which is OK.
Now change the case to “Ws,
service” and click on the blue δ
deflection icon in the plot toolbar.
Maximise the plot window, Click
on a grey trace of the most bent
beam and move the cursor along
to the worst deflection.
See that the maximum deflection
of the beam under service load
(Ws) is about 35mm in the middle.
(numbers at bottom left)
This represents a span/deflection
ratio of (12000/35=) L/343 which
is not great but acceptable. It may
make sense to go up a size for the
beams. Click on the little sketch
at the bottom to get back to the
main window.
Axial stress in columns and ties:
Change the case to W* Ultimate
and go Display member stresses
/axial Sx to see the tension stress
in the ties and the compression in
the posts. Note that all are well
below the 300 MPa yield of steel
so it is not going to collapse at
Ultimate. There is however a lot
of compression in the bottom post
so that needs to be checked for
buckling.
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Finally, still in the Plot
window, change the case
to Ws, click on the blue
deflection icon, set the
view to 3-D, click on the
blue “Render” icon (Like
a bit of an I-beam) and go
Display /Animate
/Diagram /OK to see an
exaggerated deflection
animation. This is useful
to get an idea of structural
behaviour.
For explanatory notes, see the Law Building Structures Tutorial Commentary.
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