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Structural Steel Design

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Sam Berman

on 27 July 2015

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Transcript of Structural Steel Design

Structural Steel Design
Strains and Moments
The Building Process
Bridge Design
Beam Theory
Properties of Structural Steel
Parts of a Bridge
Beginnings (foundations)
Types of Bridges
My Building
240 feet tall plus roofing
Twenty 12' stories
96'x85' or 8160 square-foot floors
two utility floors
Core-wall design
2 interior columns + utility shaft
X-brace (full bracing) girders in shaft
7026.25 square feet unoccupied floor space
13.5'x10' emergency stairwell
8'x10' freight elevator
six 4'x6' passenger elevators
23 vertical columns
2 foot floor systems
Intro- Basics
Sam Berman
Vocabulary!
Member
Stress
Strain
Plasticity
Ductility
Deflection
Truss
Brace
Beam
Column
Girder
Tie
Strut
Cantilever
Moment
-or: the dance of Stress & Strain-
fig 1
fig 2
fig 3
Stress-Strain Curves
A graph of stress (in kips) against strain (in percentage index of deflection) -Figure 1-
Structural steel is very hard to bend, so its stress-strain curve is different looking from most other materials -Figures 2 and 3-
MANY different kinds of steel with MANY different stress-strain curves -Figure 4-
fig 4
Neutral Axes and Bending
When a beam is subject to bending, it is really subject to compression on the load side and tension on the support side
The point in the beam that is subject to a zero net stress is called the "neutral axis"
The further from the neutral axis, the higher the stress
Understanding Cantilevers
An analysis of the loads in a simple cantilever
One side entirely in tension
The other entirely in compression
Diagonals in ALTERNATING tension and compression between the two sides
Plasticity vs. Ductility
Ductility: the malleability of something that can be drawn into threads of wires
(cheese strings)
Plasticity: the ability to retain a shape attained by pressure deformation
(rubber erasers)
Steel beams are almost always made with the process of
rolling
Large drums are rolled across them, shaping them through
pressure deformation
Higher tensile strength steels are MORE ductile but LESS plastic
Means that it is easier to change the original shape at higher stresses that otherwise won't break it
Lower strength steels retain their shape all the way until they break
-or: why things break-
-What happens when things bend?-
Types of Strain
Tension- strain that stretches a member by pulling two points away from each other
Compression- strain that condenses a member by pushing two points towards each other
Shear- lateral strain along the length of a member where its layers are slid across each other
Torsion- strain that twists a member along an axis
Strains in Combination
Things break when strains are combined in ways they shouldn't be
Compression and bending- compression members are considered to be inefficient in lateral loading (bending.) Combining these two strains will cause
buckling
Shear and tension- connections subject to shear within their load specifications will break because of exceeded TENSION loads in individual parts of the connection (rivets, welds, etc.)
Bridges!
Deck
Floor system
Support
Span
Stringer
Floor beam
Post
Portal
Sway system
Chords
Laterals
Strut
Arch Bridges
Arched bridges are the oldest kind of bridge
Used in ancient Greece and Rome
Carry loads through uniform compression of the arch
Suspension Bridges
Developed with the advent of wrought and cast iron
Carries loads through the tension of wires, chains, or cables, and through compression of the main tower members
Flexural Bridges
Carry loads through the flexing of the main members
Truss bridges are considered flexural
Gravity loads that are permanent parts of the bridge (eg. The weight of its members, or snow piled up)
Gravity loads that are constantly changing and active (eg. Cars driving over a road, or a train crossing the bridge)
Loads that push the bridge from side to side (eg. Wind)
High concentration loads resulting from an impact (eg. A car collision)
Trusses and Bracing
Trusses are used to carry a bridge's primary loads (vertical dead and live)
Many different kinds used for specific purposes or types of members
Bracing is used to prevent any undesired motions (like sway or thrust) caused by secondary loads
Design Considerations
Some bridges are better suited for certain situations, but most choices are completely at the engineer's discretion
The most economical option is usually the one chosen
A safety factor of 1.5 is required for all bridges (able to support up to 1.5 times the expected maximum load)
My Bridge
Flexural-truss type
Curved-chord Pratt-Parker truss (camel back truss)
Main span of 461.5 ft
55 degree diagonals
40 feet of clearance under the portal
Designed for 75 feet of clearance below the bridge
71.5 feet tall at its highest point
146.5 feet total height
Model scale: 2.5in:50 ft or 1:240
Buildings, and
Popsicle Stick Construction

Materials
To represent steel members, I chose to use popsicle sticks
very cheap
very versatile
To represent a concrete foundation, I used three layers of foam board
works well as a functional base
For connections, I used hot glue
only for its effectiveness
not very strong connections

Bridge Construction
Built for continence, not accuracy
largest members first, smallest last
gusset plates to strengthen popsicle stick connections, not steel
Cut each member from individual sticks with coping saw
except for double width stringers
Roughly
120 sticks used
Building Construction
Triple layer of foam board foundation
shape & size based on available materials (realistic)
Built long logs of triple and double wide beams
staggered the breaking points
cut the members I needed from these "logs" with the band saw
Lined the outside of utility shafts with basswood
Built up one whole floor at a time (about 6 hours per floor)
Roughly
1200
sticks used
Works Cited
Cridlebaugh, Bruce S. "C. L. Schmitt Bridge - Bridges and Tunnels of Allegheny County and Pittsburgh PA."
C. L. Schmitt Bridge - Bridges and Tunnels of Allegheny County and Pittsburgh PA.
Bruce S. Crilebaugh, 2 May 2000. Web. 15 Sept. 2013

Gordon, J. E.
Structures: Or, Why Things Don't Fall down.
New York: Plenum, 1978. Print. First Paperback Printing

Johnston, Bruce Gilbert, Fung-Jen Lin, and T. V. Galambos.
Basic Steel Design.
2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1980. Print

"Structural Engineering."
Wikipedia
. Wikimedia Foundation, 12 Oct. 2013. Web. 15 Oct 2013.

Tall, Lambert, and Lynn S. Beedle.
Structural Steel Design
. 2nd ed. New York: Ronald, 1974. Print.
Number one goal: unrestricted floor space
keep columns and trusswork out of the main floor space
live loads are more localized than bridges
UTILITIES
recources (water, electricity, and
people
) must be able to reach all floors easily
emergency exit route for people in case of a fire (stairwell)
Building Theory
The taller a building is, the more considerable the wind loads are
There are two main ways to resist shear and
relative axial shortening
Core-wall structures
Bearing-wall structures
Core-wall uses the utility shaft as a "spine for the building
Bearing-wall uses closely spaced columns as a shell around the building
acts like a vertical cylinder