On August 13, 2009, a destructive structural test was conducted at the University of Waterloo Ontario, Canada (UW) Structures lab to determine the maximum load capacity of a Make-A-Bridge® modular bridge system. The testing toke place under the supervision of Dr. Scott Walbridge, P.Eng. Alberta, Assistant Professor at Department of Civil and Environmental (UW). Only part of the report is shown.
The specimen consisted of a 20’ long aluminum pony-truss, fabricated using extruded sections and cast joints developed by MAADI Group Inc. The width was reduced to 2’ – smaller than would be used in an actual pedestrian structure – so that the specimen would fit in the UW test frame with sufficient clearance for top chord deflection during testing.
The load was introduced via a “load tree” which consisted of two simply-supported steel I-beams loaded in such a way as to facilitate the splitting of the jack load (i.e. the total load) into three equal point loads to be introduced at each of the three interior panel points of the truss. The three point loads were introduced via machined aluminum bearing pads. On these pads, Teflon® plates, and plastic sheets wiped with grease were placed, to allow rotation and lateral slippage between the load tree and the cross-tubes during testing.
The specimen sat on two end supports one pin and one roller. Above the “knife edges” at each support, rested two notched steel bearing plates, which were attached to the specimen via a welded aluminum plate detail that was bolted to the bearing plate and to the cast nodes at the ends of the specimen.
While viewing the DESTRUCTIVE TEST VIDEO a total load verses mid-span displacement curve is presented during the accelerated observation. This curve shows linear behaviour initially. The softening of the specimen occurs very gradually, and thus, it is difficult to identify a load coinciding with the onset of non-linear behaviour. The peak total load obtained was 183 kN (41.0 kips). At this load, the vertical deflection at the mid-span was approximately 61 mm. The specimen exhibited some ductility beyond this peak load, however, with the final failure occurring at a total load of 157 kN (35.2 kips) and a mid-span deflection of approximately 80 mm.
The load was introduced into each cross-tube through an 8” patch in the middle of the tube (i.e. it was not distributed over the full tube width). This resulted in significant bending stresses in the cross-tubes. In an actual pedestrian structure, it is believed that the solicitation of the cross tubes would less severe, as the load would be distributed differently. This would be offset to some extent, however, by the fact that the cross tubes would be longer.
This testing specimen was built using 6063-T54 aluminum alloy. Since then, the MAADI Group brought several modifications to the metal distribution into each cross section in order to lower high stress regions. The 2010 MakeABridge® version is now made of 6005A-T6. Aluminum alloy 6005A-T6 shows an ultimate’ strength of 260 MPa (37.7 ksi) with a yield strength of 240 MPa (34.8 ksi) while 6063-T54 ultimate’ strength is 230 MPa (33.3 ksi) with a yield strength of 205 MPa (29.7 ksi).