1 and 2 way Structures

Question 1

Long span systems

Answer 1

greater than 60’

don’t have much redundancy, bc too costly

must primarily resist loads in bending, via beam action

thermal size changes more pronounced, need room for, and beware when attaching things to it

tolerances of greater than an inch can occur bc of long distances/depths

connections are more crucial bc of lack of redundancy, overconnect, even past code

high cost, need skilled labor, hard to transport if factory made (60’ max by truck, 80’ max by rail)

choose cheapest material closest to hand

trusses for shorter long spans, arches for longest long spans

Question 2

Steel one way members

Answer 2

steel girders: rolled steel/I beams, max span length is 72’, add plates and top and bottom to strengthen

steel plate girders: plates at top/bottom w angles in between, and stiffeners, a built up system, can get deeper, span longer than I beam, deeper means higher moment of inertia, often used as transfer beams

rigid/moment frames: 3 pinned arch, eg, rigid connections make it an indeterminate structure, pinned makes it a determinate structure, they do bending, compression and flexure

trusses: built of triangles, higher strength to wt ratio, good in both tension and compression, good for lt wt, offsite production, quick erection, easy to penetrate w MEP, efficient material use, but long fabr time makes them costly, purlins on triangle pts span btwn them

open web steel joists: essentially trusses from steel parts, K goes to 60’, LH goes to 100’, DLH goes to 144’, most often parallel chord, underslung, but can have square ends, be pitched one or two ways, have camber, are flexible in configuration, are usu supported by joist girders

vierendeel trusses: not actually trusses, more like holes in web of I beam, the cut out shape helps resist bending

Question 3

Wood one way members

Answer 3

glulams rarely make it past 60’

wood trusses or arches ok

Question 4

Concrete one way members

Answer 4

prestressed conc. can span to above 60’

single or double Ts act like girders

AASHTO girders: for highway bridges, like bulky I beam in conc.

funicular or parabolic conrete arches, A-frame, gable frame, radial, 3pt centered, all have thrust which must be countered, but go waaay further than beams, 50-500’

Question 5

Steel two way members

Answer 5

space frames: 3D trusses, most common is the offset grid

steel framed domes, if lateral hoops check bending well enough, no thrust is generated, no tie to foundation needed, show little deflection, v stiff

Schwendler dome is trussed, geodesic dome is built up triangles, both have so many jts, are tough to waterproof

Question 6

Roof two way members

Answer 6

steel or steel-reinforced concrete, but may be made from wood members, too

thin-shell structures: barell vaults (an arch, extruded, when formed from triangles, is lamella roof, often wood sports area roofs), synclastic shell (looks like picnic tent), anticlastic shell (hyperbolic parabaloid, is flared at ends), very rigid and efficient, but can’t get thick to be struc sound, and are labor intensive, in US we ignore efficieny benefits of them

membrane structures: resist loads in tension, prone to changing shape, are tents, if air supported, means positive pressure inside

folded plates, are stronger than non-folded planes

suspension: resist loads in tension, utilizes fullest stress capacity, so very efficient, spans the longest of all, there is an optimum sag, bc more sag means more length, but less sag means greater cable size

catenary curve: uniform load on a cable