THE IDEA BEHIND FR AMING Steel evolved as an elemental system of construction derived from early industrialized practices that were developed for cast- and wrought-iron buildings. Discrete members are either bolted or welded together. Buildings are typically created from a series of prefabricated pieces that are sub-assembled in the fabrication shop, with final assembly and erection taking place on site. Maximized shop fabrication is preferred, as it is more expedient to cut, shape, weld and finish elements in controlled conditions. Lifting is simply done by an overhead crane. Quality is improved. Economies are possible through modularity and the production of larger quantities of identical elements. Transportation from the shop to the site limits the sizes of members that can be shipped. Elements must be designed to fit on the flatbed of a truck. Larger pieces may require a police escort or pose difficulties navigating narrow streets. Sub-assembly of smaller elements into larger ones on site will be limited by the lifting capacity of the crane as well as the size of the staging area. Framing simplifies fabrication, erection and structural analysis. Basic steel framing is based upon a rectilinear arrangement of straight members that are connected at framed joints. Regular geometry and even grid-based arrangements of columns work to minimize eccentric loading on the structure. Orthogonal geometry, although good for spatial planning, is inherently unstable. A language of reinforcement and bracing provides lateral stability either by using solid panels, moment-resisting connections or triangulation. Framing also allows for a simpler method of structural analysis, as most steel systems can be broken down into two-dimensional segments and determinate structures – unlike concrete systems, which use continuous members and monolithic construction methods.
BASIC CONNECTION STR ATEGIES All steel framing, no matter how complicated, is based upon standard methods of connections and means of satisfying load path requirements. The majority of connections are designed to function as “hinges”, transferring vertical and horizontal shear forces. They are not intended to resist moment, bending or torsional forces. This permits simple bolted or welded methods of fastening for the connections. In cases where moment or bending forces are high, connections can be reinforced to become stiff. This may be achieved by adding material in the form of plates or angles to the connection by additional welding or bolting in order to resist moment forces. Lateral loads can be resisted through the addition of bracing systems that introduce triangles into the frame, triangular forms being inherently rigid. The additional requirement of seismic stability builds upon the same connection strategies and methods of jointing of the frame. Connections between steel pieces are either bolted or welded. Bolts can vary in terms of their strength and head type. If the steel is concealed then the choice of bolt type is purely a structural consideration, ensuring that the bolts are adequate in number to resist the shear forces and that there is sufficient plate area to accommodate the bolting pattern. The design of the framing systems and connections feeds directly into practical considerations of construction methods. It is faster to erect using bolted connections, but this does not preclude welding if this is a design requirement, be it for aesthetic or structural reasons. The two types of bolts typically used are Hex Head and Tension Control (TC) bolts. Both types of bolts are fabricated from high-strength steel and both serve the same purpose. The Hex Head bolts need access from both sides for tightening, but no special equipment. The TC bolts need a special type of equipment to install and snap off the end, but only one side needs access for tightening.
– STEEL CONNECTIONS AND FRAMING TECHNIQUES
The Fair Store in Chicago, IL, USA, designed by William Le Baron Jenney in 1890, was one of the multi-story buildings which began to generate a language of standardized framing. At the time, structural member types were limited to I-beams, angles and plates. These were connected for the most part using hot rivets. The framing language of today is derived from these early structures.