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3.2mm SHS - both SHS and CHS specimens were used in the preliminary tests - with encastre conditions being established using end caps bolted to the upper and lower plattens, bond between these caps and the ends of the strut being achieved by the use of proprietary resin. This arrangement was effective in restricting rotation of the ends of the strut but inconvenient to implement. Plate 1 also shows the dial gauges which were initially employed to measure specimen end-shortening whilst the centrally mounted strain gauges set a pattern for future tests. It is to be noted that the deflected shape conforms to that expected in the post-buckling range. Further discussion on the preliminary compression tests follows delineation of the actual testing system enhancement undertaken. It is to be understood that Sections 2.3 and 2.4 relate to the testing system and procedures as finally established in the enhanced configuration, ie the system and procedures employed in the formal testing programme described in Chapter 3. Section 2.5 deals with tests undertaken concurrently with this enhancement, these preliminary tests being integral with the enhancement process itself.

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2.3 SCHENCK ENHANCEMENT - PERIPHERAL HARDWARE

2.3.1 Strut Facility

Provision of an outboard 225kN Strainsert (low profile) load cell and the associated load cell amplifier, constructed in-house, ensured that the testing system complied with the highest grade, 0.5, of BS 1610(45); ie load measurement was accurate to ± 0 . 5 % of full scale. Plate 2 depicts the load cell located between purpose-built hardened steel housing units. The lower housing unit was located on the machine actuator by a central locating spiggot, all relevant surfaces being milled flat to obviate alignment errors. As illustrated in Plate 2, the upper housing unit was recessed in the centre to accept a mechanical split taper lock collett which is displayed alongside. Colletts were employed to provide for end fixity.

Considerable thought was given to the means by which effective clamping action, to be maintained throughout the loading procedure, could be provided. Several mechanical devices were considered and, after trials, the use of taper lock colletts, a familiar item in mechanical engineering systems, was accepted as providing the best means by which the desired effect could be achieved. As shown in Plate 2, each collett accepted a 50mm length of section which was then gripped circumferentially upon mounting the collett in the appropriately recessed housing and fitting the two grub screws provided. Further, and importantly, the grip increases upon application of axial compression. End

bearing on the extreme cross-sections completed the requisite end fixity definition. In addition to the lower mounting unit displayed in Plate 2, a similar but simplified - no load cell being required - unit was employed at the upper end of the strut where connection to the cross-head plat ten was effected. Initially, struts were turned down to fit the colletts. This was unsatisfactory as it introduced unnecessary effort and additional cross-sectional geometric imperfections. Accordingly, a set of colletts was obtained, these being turned to a variety of internal diameters to provide for a range of tolerances appropriate to the nominal section diameter - formal testing was to be made using 48.3mm by 3.2mm CHS as noted in Section 1.5.

The necessary slack on the Schenck threaded column permitted movement of the cross-head under strut loading. It was considered that this could be problematical, particularly in the cyclic tests. A tie-rod arrangement was therefore implemented to pull the cross-head up against the threads of the columns using the top plate of the Schenck as an anchor point. The cross-head was forced back onto the column threads, under a load in excess of the maximum anticipated compressive test load; the tie-rod was then pre-tensioned to secure the cross-head in position.

A further development saw the provision of transducer monitoring of the primary kinematic response parameters - that is end-shortening and central transverse displacement. In addition, a more precise level of actuator control would be made available. This development consisted of a transducer network made up of a

ten +50mm stroke transducer monitoring array located on a purpose built framework independent of specimen and machine strain. The stroke range employed was dictated by the prescribed limiting magnitude of the net central transverse displacement wcl-wocl. This prescribed magnitude was determined from the concurrent preliminary strut tests - cf Section 2.5. Six transducers were mounted to monitor the upper and lower end displacements and thereby provide definitive net axial strut movement. At each end, the respective transducers were located, radially, at

120

(2.1)

= u

The algorithm was implemented using an in-house constructed, hard­ wired analogue device. The lower three readings were themselves averaged, this similarly hard-wired reading supplanting the cruder inboard transducer in providing for a more accurate feedback signal regarding actuator movement. The remaining four transducers, denoted by G, H, I and J were mounted to record net central transverse displacement w c l“Wo c l. These were orthogonally located so as to bear onto the facets of a square target, itself centralised using knife-edges on the strut. Average values, taking account of the vectors provided by the respective transducers, of opposing pairs afforded a non-prejudicial net