Steel frame Construction

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November 29, 2022

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11 STEEL FRAME CONSTRUCTION

HISTORY

Making steel was labor intensive

Use of steel was limited to weapons (e.g., Damascus steel swords), cutlery, other specialties

1750 and later

Growth in use of cast iron framing in industrial buildings and other structures

Right: Coalbrookdale Bridge, 1779, first all-metal structure

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Right: Crystal Palace, 1851; cast iron and glass

Eiffel Tower, 1889 Wrought iron

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1850 and later

Steel increasingly plentiful, with large scale production methods.

After the U.S. Civil War, excess steel making capacity sets the stage for the first use of steel in U.S. buildings.

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1850 and later

Right: Home Insurance Company Building, 1885, William Le Baron Jenny

First tall building supported entirely by a fire-protected metal frame (cast iron and steel)

Modern

Steel is one of three commonly used noncombustible structural materials. Suitable for buildings of all sizes, from single family residences to the tallest skyscrapers.

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CONSTRUCTION

THE MATERIAL STELL

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Carbon Content in Iron Alloys

Greater proportions of carbon generally increase the hardness and brittleness of the iron alloy.

Cast Iron

2% – 4% carbon Strong in compression

Less strong in in tension

Brittle (vulnerable to sudden failure)

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Wrought Iron

Little or no carbon Strong in tension

Weaker in compression Malleable (easily

shaped) and

relatively soft

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Steel

Less than 2% carbon

Strong in both tension and compression

Ductile (not prone to sudden failure)

(Reliance Building, Chicago, 1895)

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Mild Steel (Low carbon steel)

Most common alloy for modern structural steel

Not more than 0.3% carbon

Small amounts of other metals improve strength, toughness, and other qualities

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Reasonably strong, highly ductile, and easily welded

Equally strong in tension and compression

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Making Cast Iron

Iron ore (oxides of iron extracted from the ground) is combined with coke (carbon derived from coal) and limestone in a large blast furnace.

Hot air forced through the furnace burns the coke.

Chemical reactions with the combustion products remove oxygen from the ore, leaving elemental iron, but with a relatively high carbon content.

The limestone combines with impurities and is drawn off as waste slag.

Steelmaking

In a traditional steel mill, iron ore is the raw

ingredient. First, it is processed into molten iron in a blast furnace, followed by conversion to steel in a second operation.

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Traditional Mills

Recycled content: 25% – 35%

Casting

As the molten steel begins to solidify, it is cast into shapes ranging from plain rectangles or rounds to more complex cross sections, such as beam blanks (right), that approximate the shape of finished beam products.

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In the continuous casting process, casting begins once the outer shell of the steel mass has solidified, while the inner portion is still molten.

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Rolling Mill

Structural shapes are produced in a rolling mill. Prior to rolling, the beam blanks are reheated to the necessary temperature.

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Rolling Mill

Blanks pass through a series of rollers and are progressively deformed into the desired final shape.

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Structural Shapes

Wide-Flange

(W-Shape): The most commonly used shape for beams and columns

Not an I-beam!

Channels, angles, tees: For trusses, lighter weight framing, and other miscellaneous uses

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American Standard: Traditional I-beam with a shape that is less structurally efficient than a contemporary wide- flange of the same weight

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Shape Designation

W10 x 30

W: Wide-flange shape

10: Nominally 10 inches deep

30: 30 pounds per lineal foot

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By varying roller sizes and spacings, various weights can be produced, all nominally 10″ in depth:

W10 x 12: 3.96″wide x 9.87″deep W10 x 30: 5.81″w x 10.5″d

W10 x 39: 7.99″w x 9.92″d

W10 x 112: 10.4″w x 11.4″d

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The unshaded portions of the diagrams illustrate how a variety of weights of beams can be rolled from the same set of rollers by opening up the space between the rollers.

Wide Flange Shapes

Taller more narrow profiles are best for horizontally spanning elements such as beams and girders.

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Profiles more square in proportion are better suited for use as vertical columns.

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AISC Steel Construction Manual

Left page: Dimensional data

Right page:

Shape properties related to structural stiffness and strength calculations

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Within groups of shapes, T, the depth in-between top and bottom flanges, does not vary.

These shapes are

rolled from the same sets of rollers, with only the spacings of the rollers changing.

Size designations are nominal, not always an accurate indication of actual depth.

A W14 x 873 is 23.6″ deep!

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Other Shape Designations

S: American Standard beam (“I-beam”)

MC:

Miscellaneous channel

C: American Standard channel

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L: Angle L4x3x3/8: 4″x3″ nominal

legs with 3/8″

thickness

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WT: T-shape cut from a W-shape WT13.5×47:

13.5″d x 47lb/ft.

(Produced by cutting a W27x94 lengthwise in half.)

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Hollow Structural Sections (HSS)

Hollow square, rectangular, round, and elliptical shapes

Made by cold- or hot- forming steel strip (sheet) and welding longitudinally

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HSS shapes are used for trusses, structurally efficient column sections, and where the simple external profile is desirable.

Example designation: HSS 8 x 8 x ½ 8″x8″x½” wall thickness

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ASTM A36 Mild Steel

Traditional structural steel Minimum 36,000 lb

per sq. in. (36 ksi)

yield strength Now used mainly for accessory steel

angles, channels,

etc.

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High-Strength, Low Allow Steels

ASTM A992: W

shapes

ASTM A572: other shapes

Produced economically in mini-

mills from scrap steel

Minimum yield strength 50 to 65 ksi

ASTM A992 steel columns lay stacked in a fabricator yard. Note the holes predrilled for connections that will be completed in the field.

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High-Strength, Low Allow Steels

Use of stronger steel alloys permits savings in weight and reductions in the size of structural elements, reducing

costs. With controlled, thermal treatments applied after the shapes are formed, structural alloys with yield strengths greater than 100 ksi are produced.

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Weathering Steel

Surface oxidation adheres to base metal, limiting further rusting

Mostly used in highway and bridge structures,

eliminating the need for a protective coating.

(Often called Cor-Ten steel, a US

Steel trade name. )

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Stainless Steel

Added nickel and chromium Forms self-

protecting oxide

layer that provides long lasting protection against corrosion

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Building

Cold-Formed Steel

Deforming steel in its cold state causes realignment of the steel crystals and increases its strength.

Corrugated steel decking (right)

Steel studs and joists (Chapter 12)

High-strength wire for concrete prestressing strands (Chapter 13)

11 STEEL FRAME CONSTRUCTION

JOINING STEEL MEMBERS

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Riveting

White-hot fastener is inserted through holes in members to be fastened.

Fastener is hammered to produce a head on the plain end.

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Joining Steel Members

As the metal cools, it contracts, and tightly clamps the steel members.

Mostly found in historic structures. Rarely used in modern construction.

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Joining Steel Members

Bolting

Carbon steel bolts

Relatively low strength
Limited uses, such as fastening light framing elements or holding temporary connections
Also called common, or unfinished bolts

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Joining Steel Members

High-strength bolts
- Stronger than common bolts
- Used for fastening primary structural members
  
  Joining Steel Members
  
  Welding
  
  Steel surfaces to be joined are heated to a molten state.
  
  Additional molten metal is added from the electrode.
  
  In finished joint, members are fully fused.
  
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  11 STEEL FRAME CONSTRUCTION
  
  DETAILS OF
  
  STEEL FRAMING
  
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  Details of Steel Framing
  
  Framed Connection
  
  Angles, plates, or tees connect web of beam to side of column.
  
  Angles are usually joined to beam in the fabricator’s shop.
  
  Beam/angle assembly is bolted to the column in the field.
  
  Details of Steel Framing
  
  Shear Connection
  
  Transfers gravity loads from beam to columns. Not sufficiently rigid to
  
  transfer bending forces.
  
  Modeled structurally as if it is free to rotate, or hinged.
  
  AISC Simple Connection
  
  Details of Steel Framing
  
  Shear tabs welded to column in shop, bolted to beam in the field.
  
  Beams cut short to allow for deviations in column positions and to ease installation.
  
  Details of Steel Framing
  
  Moment Connection
  
  Transfers gravity loads and bending forces.
  
  Beam flanges are joined to column.
  
  Column is reinforced to carry bending forces.
  
  Beam is restrained from even small rotations.
  
  AISC Fully Restrained
  
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  Details of Steel Framing
  
  “Dog bone” cut creates a zone of weakness.
  
  In extreme seismic event, this area will deform plastically, protecting the more brittle welds from failure.
  
  Single common bolt holds beam temporarily prior to welding.
  
  Details of Steel Framing
  
  A pair of beam-column moment connections.
  
  Full penetration welds join top and bottom beam flanges to column and column reinforcing plates.
  
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  Details of Steel Framing
  
  This connection creates a continuous beam condition that can fully transmit bending forces, allowing the cantilever on the right.
  
  A shear connection would not work here. (Posts at corner are temporary.)
  
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  Details of Steel Framing
  
  Stabilizing the Building Frame
  
  Rectangular geometry of the building frame must be made stable against lateral forces by:
- Diagonal bracing
- Moment-resisting frame
- Shear walls
  
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  Details of Steel Framing
  
  Braced Frame
  
  Diagonal bracing creates stable, triangular geometry.
  
  Can be constructed with shear connections between beams and columns (free to rotate).
  
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  Details of Steel Framing
  
  Eccentric bracing (bottom) allows a plastic hinge conditions to develop during extreme seismic events, absorbing the dynamic energy of the earthquake. (Same strategy as the dog bone.)
  
  Fundamentals of Building Construction, Materials & Methods,
  
  Details of Steel Framing
  
  Moment-Resisting Frame
  
  Moment-Resisting Frame
  
  Some or all of the beam-column connections are moment connections capable of resisting rotations between the members, making the frame stable.
  
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  Details of Steel Framing
  
  Shear Walls
  
  Shear Walls
  
  Rigid vertical walls or core structures resist lateral forces.
  
  Remainder of frame relies on shear connections.
  
  Usually concrete. Occasionally heavy steel plate.
  
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  Details of Steel Framing
  
  Methods can also be combined: Inverted vee
  
  bracing with
  
  beam-to-column fully-restrained moment connections.
  
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Details of Steel Framing

These are buckling-resistant braces, designed to deform plastically in a controlled manner and absorb seismic energy during an earthquake.

Details of Steel Framing

Beginnings of a heavily reinforced concrete core that will contain vertical services and act as stabilizing shear walls.

Lateral forces are greatest at the base of the building and diminish toward the top.

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Details of Steel Framing

More Connections

When connecting beam to column web, access is more constrained.

A seated connection relies on a seat angle below the beam and stabilizing angle at top.

AISC Simple Connection

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Details of Steel Framing

Bolted web and welded flanges. The shear tab is deep enough to position the bolts clear of the column flanges for easy access.

AISC Fully-Restrained

Details of Steel Framing

End plate is welded to the beam in the shop and bolted to the column in the field.

AISC Partially- Restrained, capable of transferring some but not all of the bending strength of the beam.

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Details of Steel Framing

Partial bending stress resistance is achieved, without welding in the field.

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Details of Steel Framing

Framed beam- girder connection Beam top flanges

are coped, to

allow the tops of beam and girder to be set level.

Details of Steel Framing

Bolted column or splice. Splices typically located at roughly waist height, to avoid interference with beam- column connections at floor and so that column splices are are conveniently accessible to workers standing on the floor deck.

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Details of Steel Framing

Where outer dimensions of connected column sections vary, shims or filler plates are inserted to make up the difference.

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Details of Steel Framing

Welded column splice. The bolted connector plate holds the columns in alignment prior to welding. Later, the column flanges are welded with partial penetration welds. The hole in the plate provides an attachment point for the lifting line during erection.

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Details of Steel Framing

Where inner dimensions of spliced columns differ, a butt plate or bearing plate is added to the connection.

The plate is thick enough to transfer the full bearing loads from the upper column section to the lower one.

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11 STEEL FRAME CONSTRUCTION

THE CONSTRUCTION PROCESS

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The Construction Process

Steel Framing Plan

Shows sizes and locations of steel members

(Dimensions have been omitted from the plan at right.)

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The Construction Process

Steel Framing Plan

W30 girder-column connection: Beam to column flange

W27 beam-column connection: Beam to column web

W18 to W30

connection: Coped beam-girder

The Construction Process

Steel Fabricator

Fabricator prepares steel members and delivers them to the construction site.

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The Construction Process

Traditionally, the fabricator prepares shop drawings showing the configuration of each member.

Drawings are reviewed by architect and structural engineer.

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The Construction Process

Fabricator frequently also shares responsibility for design of the steel connections, based on more general requirements provided by the structural engineer.

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The Construction Proces

Workers cut to length, cope, drill, punch, weld, and add tabs, angles, plates and other accessories to members as indicated on the approved shop drawings.

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The Construction Proces

Welding deep, full-penetration groove welds to join two heavy A913 steel column sections end-to-end.

The Construction Process

Fabricated members are stacked using overhead crane, awaiting transportation to the construction site.

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The Construction Process

Members are individually labeled to correspond to information on the erection drawings so that each piece can be assembled in the proper location once delivered to the construction site.

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The Construction Process

More recently, using building information modeling systems, steel fabrication information and details may be developed by the structural engineer in the building model as an alternative to relying on fabricator shop drawings.

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The Construction Process

The Erector

Assembles steel members delivered to the construction site.

May or may not be same entity as fabricator.

Workers are called ironworkers.

The Construction Process

As the frame is erected, temporary cables with turnbuckles are used to plumb up (make vertical) the frame.

The Construction Process

Steel Decking

Corrugated steel decking laid over the framing is the most common floor and roof decking material.

The decking is puddle welded to the framing members below.

The Construction Process

Steel decking comes in a variety of profiles and depths to suit different load and span conditions.

Top: Relatively shallow roof decking.

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The Construction Process

Composite floor decking. Deformations allow structural bond between deck and concrete poured over it, to increase the strength of the deck/concrete floor assembly.

The Construction Process

Shear Studs

Welded to tops of beams, through metal decking. Once concrete is poured, the beams, decking, and concrete act together structurally as another form of composite construction.

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The Construction Process

Concrete Fill