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)
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