1. Basic System - Post & Beam (skeleton frame) and space frames.

2. Advantages of Steel Frame Construction:

1. Can build very tall and wide (tallest buildings in the world)
2. Light weight and strong (much lighter and stronger than concrete)
3. Prefabricated - frames assemble quickly
4. Precise and predictable (excellent quality control)

1. Steel is an expensive material (much more expensive than masonry or concrete)
2. Frames are unstable
3. Needs fire protection
4. Needs separate "skin" (walls and floors)

1. Rigid Core - Usually accomplished by interior masonry (or concrete) stair towers and elevator shafts that creates a vertical rigid core that resists deformation and torsion of the building due to external lateral forces.
2. Diagonal Bracing - The addition of diagonal "X" or "K" bracing that resists lateral loads. Problems - may interfere with exterior windows.
3. Moment-Resisting Beam-to-Column Connections - Typically accomplished by fabricating extra connection angles, welds and bolts that greatly increase the rigidity of the connection. Problems - extremely labor intensive and expensive.
4. Shear Walls - Exterior (or interior) walls built of masonry or concrete that act as a vertical cantilever beam resisting lateral loads. Problems - may interfere with exterior windows, labor intensive, heavy.

In general, structural steel is fabricated in a hot-rolled process under several ASTM designations, the most common being A36. This steel has a minimum yield stress of 36 KSI and a minimum ultimate (breaking) stress of 58 KSI. Many other grades are available, with A572 - 50 KSI yield stress as a choice for higher strength.  A new steel grade A992 has recently replaced A572 and A36 (for W sections) as the standard grade.  Like A572, it also has a yield stress of 50 KSI.

1. Wide Flange - The typical "I Beam" used in construction. Example - W18x35, where "W" = Wide flange, 18 = nominal depth of member in inches, and 35 = weight of beam in pounds per linear foot. Used for beams, columns, piles, bracing and other heavy applications.



 

2. Angles - Either equal legs or unequal legs. Example - L4x3x1/4 where 4 and 3 are the actual leg dimensions in inches and 1/4 = angle thickness in inches. Used for lintels, bracing, built-up beams and columns, secondary framing and other light-duty applications.


     

3. Steel Channels - These "C" shaped members are used for beams, built-up columns, bracing, secondary framing and other light to medium-duty applications. An example of a channel is C10x30 where "C" indicates channel, 10 is the actual height of the channel in inches, and 30 = pounds per linear foot.


     

4. Steel Pipe - Circular pipe is used most often for columns. Comes in three categories based upon diameter and wall thickness, and are "standard weight", "extra strong" and "double extra strong". The double extra strong pipe is the strongest because it has the thickest walls. Example - Standard Weight 4" dia. pipe.


     
 
 

5. Tube Steel - These square or rectangular sections are used most often as columns, but may also be used as beams, bracing or other applications. A typical example is TS6x4x1/4 where TS = Tube Steel, 6 and 4 are the actual width and depth in inches, and 1/4 is the wall thickness in inches.

As of 2003, tube steel is now referred to as "Hollow Structural Section" (HSS) by the American Institute of Steel Construction.  A typical designation is HSS6x4x1/4. 


     
 
 

6. Plates - Flat pieces of steel cut to size. Generally in the range of 1/8" thick up to 6" thick. Used as column base plates, built-up beams and columns (i.e., plate girders), connection pieces (I.e., gusset plates, weld plates, etc.), and any other application where specific sized pieces are required. Example - PL 6x4x3/8 where 6 and 4 are length and width of the plate in inches and 3/8 is the plate thickness in inches.
7. Cut Sections - Typically these are wide flange sections that are cut in half to form a "T" section. Used for lintels, beams, bracing and columns. Example - WT7x19 is a section cut from a W14x38 wide flange.

1. Rivets - Generally not used any more for reasons such as low strength, safety and poor quality control.
        
2. Bolts - Come in two varieties - carbon steel and high-strength. The carbon steel bolts obtain their strength through shear (or tension) along the shaft of the bolt only. The most common ASTM designation for carbon steel bolts used in structures is A307. These bolts cannot carry the same load that high-strength bolts can and are used for light-duty applications such as anchor bolts. High-strength bolts obtain their strength not only through the shear and tension along the shaft, but also from the friction forces that are generated from tensioning the nut to prescribed levels. The most common ASTM designations for these high-strength bolts used in structures are A325 and A490.
           
3. Welding - Welding is accomplished by mechanically joining steel together by heating electrodes into a molten state, which forms one piece from two. Creates extremely rigid connections. The most common type of weld used for building structures is the "fillet" weld, which joins pieces at right angles. It is common to shop-weld connection angles to beams and columns, then field-bolt them together at the job site.

                   

7. Steel Decking:

Steel deck falls into one of several major types. All deck is cold-rolled and is sold in gage thicknesses typically ranging from 16 (heaviest) to 28 gage (lightest).
     

1. Roof Deck - Generally available in heights of 1½" and 3" and gage thicknesses of 16 thru 22 gage. Roof deck is characterized by having a much wider top flute than bottom flute, for purposes of providing the largest possible flat surface for carrying non-structural building components such as rigid insulation. The bottom flute width varies, and different profiles are available such as "Narrow Rib", "Intermediate Rib" and "Wide Rib".
2. Floor Deck (Non-composite) - Used for floors, this type of deck has approximately equal length top and bottom flutes and acts as a form for concrete. Typical available heights are 9/16", 1", 1 5/16", 1½", 2" and 3". Concrete placed in the deck is generally reinforced with Welded Wire Fabric.
3. Floor Deck (Composite) - Similar to non-composite floor deck, this deck usually has additional perforations in the decking to "grip" the concrete. This deck acts as part of a structural system with steel beams. The deck is placed on top of the steel beam and steel "shear studs" are welded through the deck and onto the top flange of the beam. After the concrete is placed and cured, it grips the shear studs and acts compositely with the steel beam and greatly increases the load-carrying capacity of the steel beam alone.

         

8. Steel Joists (Bar Joists):

These lightweight, open-web "trusses" are manufactured by various companies (such as Vulcraft), to provide the lightest weight possible alternative to beams. The are used most often for roof assemblies. A typical steel joist is shown below:

An example of a typical steel joist (as manufactured by Vulcraft) is "18K6" where 18 = joist depth in inches, "K" = series of joist and 6 = relative chord size.

9. Light Gauge Steel Framing:

Light gauge steel framing members are used to frame walls, headers, joists, and lintels - everything that conventional wood-framing members would be used for. Most state building codes dictate that non-combustible construction be used for certain type facilities and wood framing members may NOT be used. These light gauge members are cold-rolled (similar to steel deck) sheet metal products that are available from various manufacturers in "C" shaped members in gages typically ranging from 12 gage up to 26 gage. They are fastened and assembled by use of self-tapping screws and spot-welding.

         

10.  Fire Protection:

All steel structures must be fire protected as per state and local building codes. Although it takes a very substantial amount of heat to actually melt steel, it will lose most of its strength at temperatures above 700⁰ F. There are generally two major categories of fire proofing - thermal and absorptive.

1. Thermal Fire Protection - Slows heat passage through the steel. Methods used include providing insulation and intumescent paint.
2. Absorptive Fire Protection - Absorbs heat. Methods used include covering steel members with concrete, gypsum (spray-on), and elaborate methods such as liquid-filled chambers (generally wrapped around columns).