Four LVDTs (1,2,3 and 4) were placed at bottom and two (5 and 6 in parenthesis)were at top (Fig. 3(c)), so relative vertical displacement (u) after the specimen and devises were close-contact, is calculated with the following equation:
u=b-u
where (b) is the average data value of the bottom four sensors; (u)is the average data value of the top two sensors.
The data from the load cell and LVDTs were input into the Beijing spectrum instrument so that the axial load -displacement curves were real-time monitored.
Four dial gauges were symmetrically arranged with starting gage length 20 mm in the middle of each steel plate (in Fig. 3(c)), for measuring displacement increment prior to local buckling.
Vertical and transverse strain gauges were also employed on the tubes for monitoring the strain and stress development of non-stiffened specimens and stiffened specimens.
To assure uniform compression, preliminary tests within the elastic range were conducted by carefully adjusting the position of the specimen, based on the data of the dial gauges and strain gauges.
The dial gauges and strain gauges were used for displacement and strain measurement, and for physical centering adjustment before actual loading.
A load increment of less than one tenth of the estimated peak resistance was employed in the loading scheme.
Each load interval was maintained for about 2 min.