L5: Steel Frame Systems Flashcards
How is steel different from other materials?
Brittle materials fail abruptly at the fracture point but steel is ductile and continues to strain
What happens when steel stretches and reaches the plastic region?
It cannot return to its original shape
How is steel made?
Produced from cast iron and altering carbon content
What happens when there is the wrong amount of carbon in steel?
Too much - hard, brittle
Too little - soft, weak
How can steel be made corrosion resistant?
Weathering steel (low-alloy) Coated in zinc
How are structural shapes produced with steel?
Running hot steel through a series of rollers
Why is New Zealand’s steel industry unique?
Uses magnetic ironsand formed from volcanic activity in Taranaki
What does New Zealand’s steel industry focus on?
Sheet steel products rather than structural steel members
What are the key advantages of steel frames?
Ease and speed of erection (no formwork)
Dimensional control (factory fabricated)
Ability to modify frame (extensions/strengthening)
Superior strength/weight ratio (allows large column free spans)
What are the key disadvantages of steel frames?
High cost (depending on market; more volatile)
Fireproofing (weakens rapidly in high temperatures)
Tendency to corrode (painted regularly or expensive corrosion-resistant)
Susceptible to buckling (require support)
What are the main steel structural shapes?
Wide flange Channel Angles Structural tee, Bars/plates Pipes/tubes
What makes up a wide-flange beam?
Top flange
Web
Bottom flange
How are steel sizes written?
Shape letter depth x weight
What is an alternative to a wide-flange beam?
Joist girder (constructed from angles)
What makes up a bar joist/open-web steel joist?
Top chord (angles) Web (bar) Bottom chord (angles)
What do wide-flanges and joist girders do and what is the difference between them?
Supports loads from bar joists
Joist girders are more lightweight and economical than wide flanges, but are also deeper so may not suit high rise
What do bar joists commonly require?
Bridging with horizontal bars/angles welded to bottom chords
OR
Cross bridging placed at midspan to maintain correct orientation and help support loads
What are the different types of connections?
Rivets (obsolete) Bolts Welding Beam-to-column Column-to-column
What are the methods for verifying bolts?
- Turn of nut
- Load indicator washer
- Tension control bolts
What makes up common beam-to-column connections?
Shear tab at the joint
Bolts provide shear resistance
Stiffener plate used if flanges not strong enough to handle moment forces
Dog bones cut into flanges to create weak zone
What makes up common column-to-column connections?
Plates and bolts connecting the flanges
Shim plates used if slightly different size
Can be welded at web
Butt plate welded if big difference in size
What are the three stabilising mechanisms?
Diagonal bracing
Sheer panels
Moment connections
What are the three types of diagonal bracing
K-bracing
X-bracing
Eccentric bracing (earthquakes)
What are sheet panels (stabilising mechanism)
Steel or concrete typically surrounding the core
What are two common methods to stabilise the frame and how do they work?
Rigid core or rigid perimeter
They stabilise the frame as the floor can provide rigidity to non-rigid areas by connecting them to rigid areas
What is the construction process of a steel frame system?
Fabricator Ironworkers Erect first tier Erect upper tiers Placing last beam
What does the fabricator do?
Delivers steel members to site ready to be assembled without further processing
What do the ironworkers do?
Raising gang - position components
Bolt-up gang - tighten and plumb
How is the first tier erected?
Install columns
Beams/girders
Plumb and tighten
Decking if necessary
How are columns installed when the first tier is erected?
Smaller columns prefab with base plates set in footing with anchor bolts and levelling nuts and filled with grout
OR
Can put column on thin levelling plate which is on a bed of grout on the footing
How are the upper tiers erected?
Columns installed and spliced to previous tier
mobile crane or tower crane
What happens when the last beam is placed?
Topping out ceremony
What are the two main types of decking?
Metal and concrete
What is the spanning capability of metal decking based on?
Thickness of sheet, depth, spacing of corrugations
What are the types of metal decking?
Single sheet
Cellular decking
Composite metal decking
What is cellular decking?
Corrugated sheets welded to flat sheets providing voids for services
What is composite metal decking and how does it bond?
Metal deck (tensile strength) with concrete topping (compression strength)
Metal bonds with concrete through deformed ribs or welded rods; shear studs can be used to force concrete topping to work with beams to resist bending
What are the types of connections to frame?
Puddle welds
Self-drilling, self-tapping screws
Power-driven pins
What are the two types of concrete decking?
Site cast decking (not compatible with steel framing)
Pre-cast concrete planks
What do building codes say about fireproofing steel frames?
Majority requires fireproofing; high rise structures typically require full frame fireproofing
Which elements are not fireproofed?
Those with concrete tied against them as the concrete absorbs the heat, thus protecting the steel
What are the methods of fireproof columns?
Spray-on
Encasement in reinforced concrete
Loose insulating fill inside sheet material
Enclosure in metal lath and plaster
Water filled box/pipe column
Enclosure in layers of gyp board (preferred)
What are the methods of protecting beams/girders?
Encased in reinforced concrete
Encased in metal lath and plaster
Spray on fireproofing (most common)
Plaster/multi layer gyp board suspended ceiling
What is the newest generation of fireproofing and how does it work?
Intumescent coatings; thin coatings which expand when exposed to fire and form thick char the insulates the structural member
How are steel frame systems used to improve beams?
Castellated beams - wide flange with cut through web which increases depth without extra weight
Plate girders - custom made steel plates angles; can be placed where anticipate forces require and can provide openings for pipes etc.
How can steel be used in other frame types?
Rigid frames - rectangular frame/arch which requires steel rods at their base to resist thrust
How is steel used in composite columns?
Combines strength and economies of steel and reinforced concrete
What are three types of composite columns?
Steel column surrounded by reinforced concrete (concrete carries loads and protects steel)
Pipe column filled with reinforced concrete (pipe column carries load, steel may need fireproofing)
Pipe column with wide flange column and reinforced concrete inside (added steel increases load bearing capacity)
