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-t
where (b) the average data value of the bottom four sensors; (t) 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.