The Science of

Structures

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We learn from our mistakes . . . . . It is said engineers learn from their mistakes. One could say that is true of any field and every person. If we don’t learn from our mistakes we are bound to repeat them. Thomas Kuhn, author of The Structure of Scientific Revolution, discusses how we start with accepted facts and theories, based on what society thinks is true at the time until someone discovers an anomaly or a problem with the theory. That is followed by a hyper interest in the theory. If the error turns out to be true it changes everything humans thought about the theory. This is called revolutionary science or a new paradigm. For instance, you know the engineers thought the O-rings in the Space Shuttle Challenger were safe. They were produced by Thiokol Corporation one of the world's largest producers of solid rocket motors and other parts for the aerospace and defense industries. You know it was inspected by civilian and government inspectors and deemed within specifications and approved it for use. However, the design was found to be faulty and caused one of the worst disasters in NASA history.

Space Shuttle Challenger Disaster

On January 28, 1986, the Space Shuttle Challenger disintegrated off the

coast of Florida causing the deaths of its seven crew members. It is

believed that an O-ring seal on the right solid rocket booster failed,

causing a breach in the joint and allowing hot, pressurized gas from

within the rocket motor to escape. This led to the structural failure of the

external tank and precipitated, with the assistance of aerodynamic forces,

the complete breakup of the vehicle. The failure of the O-rings was

attributed to a faulty design.

Chernobyl Nuclear Meltdown

On April 26, 1986, the world witnessed the worst nuclear power plant

disaster in history at the Chernobyl Nuclear Power Plant in the

Ukraine. The reactor was Russian built. The resulting explosion(s)

resulted in the release of significant amounts of radiation into the

atmosphere, producing over 400X more fallout than the bombing of

Hiroshima. While human error may have played a role in the disaster,

flaws in the design of the control rods are thought to have been the

root cause of the accident.

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The control rods failed on a low power test (the design flaw) causing the reactor to start shutting

down. The operator error was they tried to by-passed

safety systems to keep the reactor up and the result was it

exploded. The authorities remain largely unsure of the

effects of the radiation exposure, and the area is still

abandoned and heavily contaminated over 20 later.

Hyatt Regency walkway collapse

Sometimes it comes down to a simple design change during

construction that causes failure. This was the case with a

design change on the Hyatt Regency walkways. The end results were that the

structural engineers

involved in the project were

found guilty of gross

negligence

On 17 July 1981, two

suspended walkways through the lobby of the in Hyatt Regency

Kansas City, Missouri collapsed, killing 114 and injuring more ,

than 200 people. The walkways were 120 feet long and

weighed 64000 pounds.

The collapse was due to a late change in design.

Instead of the walkways supported individually, as per

the plans, only the highest was secured and the

walkway below suspended below it from one set of

1.25 in steel tie rods. This made the connection was

barely able to handle the weight of the walkways, much

less people walking on them. This is a standard case

study for engineering courses, and is used to teach the

importance of ethics in engineering. Cause: insufficient

load capacity.

From these disasters you know the job of a Structural Engineer is extremely important. We are

going to do a simple exercise in structural engineering called, The Pavilion to see if you might be

interested in becoming a Structural Engineer.

Page – 3 The Pavilion

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The CAD Academy

Page - 4 We were having a great game of volley ball at a family reunion when a fast moving storm came in and we all ran for a nearby pavilion for shelter. It was great to watch the pelting rain come down and listen to the thunder from the relative protection of the pavilion. We see pavilions and gazebos in parks where picnics, band performances and even wedding are performed. With such valuable people sheltered there having the structure hold in all kinds of conditions becomes very important. A STRUCTURAL ENGINEER does the calculations to make sure structures will not fail and usually builds redundancy (backup or more than needed) into the calculations. Our project is to make a pavilion and calculate the LOAD FACTORS to make sure our structure is safe. Find the ARCHICAD ICON and launch the program. Be sure and match the screen on the right so we all have the same menu systems. Select NEW PROJECT, RESIDENTIAL TEMPLATE and s STANDARD PROFILE.

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First, we are

going to create

a 36’’ x 24’

SLAB.

Change the

material to

CONCRETE 03.

Left mouse

.

click on the

2nd and 3rd

material type

and use

The connector at right that

changes all of the material to be

the same.

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Make sure the rectangle

option is selected in the

INFO BAR. Select a

beginning point and move

up and to the right. Tap

the TAB key to get to

DIMENSION 1: type in –

36 – for 36 feet. TAB

again and type in 24 and

ENTER.

To check out the

SLAB go to

GENERIC

PERSPECTIVE in

the NAVIGATOR.

You can use the

icon below to turn

the slab into

various

orientations.

Then return back to the FIRST FLOOR in the

navigator.

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Next we will place a few columns.

Select COLUMN from the TOOLBOX.

We are going to place a column in each

corner and then construct the roof.

After we place the roof we will CHECK

the structure to make sure it will be

safe and add additional columns if

necessary.

The column will be 1’x1’ (the default). Place

in corner when you see the check mark.

We want the column placed on the CORNER so we need to MOVE it.

To CHANGE ANYTHING select the ARROW in the TOOLBOX and

then select the column. Notice the DOTS (GRIPS). We are going to

select and top left GRIP and align the column to the corner.

When you see a CHECK MARK you are on the endpoint or

corner of the object.

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You may want to zoom in closer. This can

be accomplished by using the ROLLER on

the mouse or these tools found at the

bottom of the screen.

Use the PLUS to zoom in. The circle with four points zooms out completely.

Place and move columns the other 3

columns to the corners as shown.

You may want to go to the GENERIC PERSPECTIVE

in the NAVIGATOR to see how the pavilion is

progressing.

Then, return to the 1st floor using the NAVIGATOR.

Select ROOF from the TOOL BOX and

then select OK.

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We want to make a GABLE roof. Make sure you make the changes in the INFO BOX to

change to a GABLE roof and also change the eight to 10’, which is the height of the walls.

Select the two corners as shown below and then mouse the mouse to the right (it will DRAG

the ROOF) to 36’.

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The results should be as follows:

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Select the roof.

If we want to

select specific

items you can

select the item in

the TOOLBOX – in

this case THE

ROOF and then

HOLD the CTRL

key down and

TAP the A key for

all. This works

for any item in the

toolbox.

Select the PULL DOWN

menus and SELECT

DESIGN and then ROOF

EXTRAS and then

ROOFMAKER and finally

the ROOF WIZZARD.

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Change the layer to A-PIC as shown and

then select OK.

The results should be as shown.

Select GENERIC PERSPECTIVE in the NAVIGATOR.

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We are going to learn another

“short cut” Hold down the

CTRL key and TAP the L key for

layers. Find the A-ROOF layer

and turn it into wire frame.

Select OK.

First, we are going to figure out

the weight of the wood in the

rafters.

Because ArchiCAD is a BIM tool (BUILDING INFORMATION MODELING) we should be able

to get some information about the model. Select COMPONENTS in the NAVIGATOR and

then COMPONENTS by ELEMENTS.

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However, I hear all of the time that if a number if off, the operator still has to have an idea

of what it should be.

We are going to do some

manual calculations. Select the

MEASURE TOOL and then make

the two picks as shown.

We are going to call this 15’8” and we have 31 per

side or 62 total 15’8” 2/4 rafters. I brought up the

calculator that is in

the start menu on

ACCESSORIES and

multiple 62 by 15.8

and came up with

979.6 feet of wood in

the rafters. Now we

need to calculate the

Collar Tie. I show

them at 15.2 x 31 or a total of 471.2. Total 1,450.8 ft.

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Double check my calculations to see what you come up with. Then go to the website below

and check out the weight of the rafters.

http://www.csgnetwork.com/lumberweight.html

I came up with 4,352.4 pounds of wood in the rafters!

Can our columns hold up this weight? There are many factors that go into figuring the load.

Material strength varies by wood type and it depends on how the column is secured, etc.

But, in general one column could hold up 5,173 pounds. We have 4 columns so that would

hold up 20,692 pounds.

Material Pine

Height (Feet) 10.0

Diameter (Inches) 12”

Style Name Colonial

Column Design Square

Load Bearing limit (lbs.) 5173.0

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We still have plywood, and tiles for the roof and other weight issues. Wood bends as well.

Just to make sure the pavilion would not fail in

wind or with the weight of snow on its roof I

added 2 additional columns for better weight

distribution. We could probably go with 6x 6 x

10 column as well. Did the weight of just the

rafters surprise you?

.

Hold down the CTRL day and tap

the L key to bring up layers. Turn

the A-ROOF layer back on by

selecting the “eye”, and press OK

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I placed a large square MESH for grass on my project and then went to OBJECTS and

placed some trees.

Go to the DOCUMENT pull-down and then CREATIVE IMAGING and then PHOTORENDER

PROJECTION to see a RENDERING of YOUR project.

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Please watch the two videos included in this unit on the Instructor Portal that will give you

an understanding of structural failure.. The Citicorp skyscraper was built lighter to save

money. A student noticed an issued and contacted the architect who discovered a design

modification could cause the building to fail under certain conditions. The other is the

classic, “Tacoma Narrows Bridge”. Build exactly like the San Francisco Bay Bridge, but

lighter to save money. Find out why it failed.

Structural Engineers carry a lot on their shoulders. Failure is not an option. As we learn

from our failures, structures become safer. Calculating loads, stresses and other factures

make this a job for someone who likes working numbers.

Citicorp skyscraper, New York Tacoma Narrows Bridge

Lesson is complete