Prefabrication experiments - 368 - Modern structural archetypes - 08 - Moment frames and connections

 

In skeletal construction, post and beam, box, balloon, platform or regular braced frames, joints and connections between vertical bearing elements (columns/posts) and horizontal spanning elements (beams / girders) are either nailed, screwed, bolted or riveted. These pin connections resist vertical gravitational loads and transfer constraints from one element to another taking the path of least resistance. Generally flexible, these joints are located where moment and shear forces are at their summum. Flexible frame connections require some form of bracing, diaphragm planes, or diagonals to lock connections in position to resist lateral loads. Moment Frames are skeletal constructions where conventional flexible fixations are replaced with more complex rigid or moment connections to keep the frames stable. 

 

Portal moment frames like the infamous Butler steel building frames, invented in 1940 by Wilbur and Kenneth Larkin of the Butler Manufacturing Co., express compression constraints by tapering elements to mid span where bending moment is at its minimum and enlarging elements at points of maximum stress. The connections are rigid; instead of standard bolted beam to column joinery, the constituent parts are either welded together or bolted with larger stiffening plates. The connection is further reinforced by welding plates in webs to ensure a complete transfer of bending moment and shear stresses from the horizontal to the vertical. Deformation or displacement of the moment connection implies bending or distortion of the entire frame which makes the whole system behave with monolithic geometry. Tapering composing parts reduces material use but connection complexity makes this frame's use appropriate only where completely open spans are required, arenas, factories, etc. 

 

A beautiful example of a moment frame designed by Camenzind Evolution in 1998, the Sports centre in the city of Uster in Switzerland employs hinged rigid frames to reduce effective spans and present an elegant structural solution derived from structural analysis; wide flange beam are shaped according to stresses. In these types of portal frame structures, rigid joints resist lateral constraints in their traverse direction while roof surfaces play a structural role bracing frames in their longitudinal direction.

Butler Frame (left), Sports centre frame by Camenzind Evolution (right)

Prefabrication experiments - 367 - Modern structural archetypes - 07 - Harmonized tension and compression

Since industrialization reformed engineering and its education, normalized structural frameworks’ materials, sizing and detailing have orchestrated building construction. In post and beam braced frames, mathematical calculations and catalogued precedents became the way forward to inform component specification. For all load-bearing elements, engineers evaluate and size according to constraints to shape a stable structure with manufactured parts: beams, struts, posts. Before industrialized frames, in masonry construction, geometry, namely arch effect was deployed more intuitively in domes, arches, and vaults to resist vertical loading in a shape inversely proportional to gravitational loads. Catenary arches and funicular figures represent idealized lines of stress that minimize tension, important for masonry structures. Modern engineering principles and longstanding arch effect have also been combined to optimize the link between structural form and a material’s tendencies. Freyssinet's prestressed concrete using tended cables to compress concrete is perhaps the greatest expression of harmonized tension and compression in structures to produce a superlative material. 

 

Another structural genius, Robert le Ricolais, professor at Penn State in the 1950s explored potential lightweight spatial structures, their geometry and the interplay of compression and tension to propose some of the most unique structural spanning elements of the 20th century.  His Polyten Bridge developed in 1968-69 while still at Penn is a notable example of using the prestressing principle that is normally applied to concrete to compose a resistant geometry. The unit leverages, the bowstring truss principle to tie and arch a superior cord. Symmetrical king posts expand the shape at its center to resist the greatest loads.  Together, top cord and bottom cables connected by a web of interrelated struts (short columns suspended within the framework) create a thick wing-type space frame. Akin to tensegrity (compression suspended in tension) the frame could be used for roofs reducing the amount of material that would normally be used by a monolithic element to cross the same distance. The Polyten bridge's geometry and structural effect accord compression and tension to arrange a robust structural framework that literally hangs arched and linear struts within a web of stretched cables in the service of an idealized structural efficiency.

Polyten Bridge - Robert le Ricolais

Prefabrication experiments - 366 - Modern structural archetypes - 06 - Outrigger Frame

 

Structural frameworks are explored and optimized by fine tuning geometry to increase spans while reducing material use. As a structure's dead weight increases, structural capacity is diminished; Achieving great spans with minimal material is the basis of imaginative structural form. Anthony Hunt, a famous structural engineer who worked with Norman Foster and Richard Rogers also taught structures to architects. He defined structural engineering as maximum result (span) with minimal weight (material). This ideal relationship is particularly important in buildings that require large open spaces: stadiums, auditoriums. 

 

Strategies for large spanning roof structures are often based on truss effect or shell / membrane effect to define form actively increasing spanning capacity. Tall buildings also demand an efficient structural ratio as each stacked floor multiplies weight on foundations and imposes greater rotational and moment forces as the edifice rises much like a long vertical cantilevered beam. Reducing these moment forces lessens the lateral forces on a building's framework. The outrigger frame designed for tall buildings uses a solid core and perimeter column organisation with outrigger beams that connect core and columns at certain levels creating a rigid bond between the center and the perimeter of the building. The outrigger beams can be monolithic or trussed but must rigidly connect center and periphery. This equilibrium force is analogous to how outriggers are used on watercraft to increase lateral stability. Floors where outriggers are positioned are usually less flexible than floors where only cores and columns are present freeing up the floor plate from of any structural obstacles. 

 

The Montreal Stock Exchange Tower  (Place Victoria Tower today) designed by architect Luigi Moretti and engineer Pier Luigi Nervi in the 1960s is an elegant example of the outrigger frame principle. Well-known for its four towering external structural pillars, it was once the tallest reinforced concrete tower in the world. Mechanical floors 5, 19 and 32 are crossed by diagonal full floor height reinforced concrete truss beams that stabilise core and perimeter.  The full height trusses rigidly connect the central stabilized core with floor slabs and corner columns strengthening the entire structural tube. The reinforced concrete prism is not only stabilized vertically by the outrigger beams, but the criss-crossing beams also buttress against rotational constraints.

Place Victoria Tower - stabilized outrigger core in red

Prefabrication experiments - 365 - Modern structural archetypes - 05 - J.H. Gray Column

 

The construction of tall buildings implies the efficient and economical use of materials to minimize dead loads for compounded stories. Weight and functional loads from each floor plate are transferred to columns that transmit them down to foundations. Steel works particularly well for tall buildings as structural elements can be profiled to reduce material use and increase span to weight ratio both horizontally and vertically. The iconic H shape of columns and beams depicts this type of material efficacy. The assembly of rolled posts and beams in platform structures became an iconic representation of the early Chicago or New York City style skyscrapers. 

 

The Reliance Building designed by Burnham and Root with Charles Atwood in the early 1890s is often cited as the archetype of the towering brace framed steel skeleton. The fourteen-story structure built in (1894-1895) with a floor plate of 56 by 85 feet exemplified the modern canon of separate structure and skin; glass and glazed terracotta panels were hung or even cantilevered from the steel grid foreshadowing the development of lightweight modular curtain walls. Recognized for its use of projected bays, the building's simple façade demonstrated what would become the commercial urban glass building of the twentieth century. 

 

An expression of industrialization's advances in mechanization (elevators) and pig iron's refinement into steel, the tall building skeletons were devised as large-scale kits-of-parts. The Reliance Building's structure is a basic assembly of a type of balloon frame where continuous pillars carry floor plates composed of primary and secondary beams. The chief innovation in terms of structure was the use of an open web column invented by civil engineer J.H. Gray. Contrary to the Z-bar riveted closed iron columns that had become common in steel assemblies, Gray invented an open trellis framework, a type of rising chase, that would allow for electrical and piping distribution. The posts were made by riveting plates at 30-inch intervals to 12-foot long continuous angles; columns were spliced at every 12 feet. The open web concept reduced column weight, increased accessibility for fireproofing and straight edges made standardized assemblies for beams and columns more efficient. 

Reliance building sketch and J.H. Gray column details