5.3. Prisms Test results on eccentrically loaded 2-block high and 3-block high prisms made of 6-in and 8-in block are given in tables 5.5 and 5.6, respectively. These tables note the age, as well as the date of fabrication of prisms. This is noted since the strength of prisms fabricated on certain dates sometimes deviated markedly from the generally observed trend. 6. Interpretation of Results 6.1. Stress-Strain Relationships Stress-strain relationships, measured on unreinforced as well as reinforced walls, are shown in figure 6.1. Curve A shows a stress-strain curve computed from the longitudinal deformation of an axially loaded 16-ft unreinforced wall (specimen 2). This is the only case where deflections were measured to the point of ultimate load. In all other cases instru mentation was removed prior to failure. Curves B and C are stress-strain curves for axially loaded 10-ft unreinforced walls, computed from the average of measurements of 4 linear variable differential transformers having a gage length of 24 in, which were specially installed for that purpose. (Curve B is for specimen 14 and curve C for specimen 4.) Curve C was obtained from one of the specimens with exces sive joint thickness and may therefore represent a wall of lower than normal strength. The ultimate compressive strength of the specimens from which curves B and C were derived is also shown in the figure. Stress was computed on the basis of the average net area, determined in accordance with ASTM Standard C140-65T. There is good agreement between the three In general, the measured stress-strain curves. Description of Failure Wall split through center vertically to bottom 1/4h where it broke down and out to bottom corners. Typical compression failure of block. Compression failure in 2nd course from bottom. Vertical splitting and cracking in the shape of an inverted V at top 1/4h point of wall. Bending with subsequent compression failure in mortar joint at yoke location (top of 9th course). Bending in the wall, compressive failure in the mortar joint on inside of bend and sudden rupture. Bending and subsequent failure at the mortar joint between 3rd and 4th courses from the top. Splitting of 1st and 2nd courses from the bottom. Wall broke about 1/4h from top along a horizontal joint and blew out of frame. Compression failure in 2nd course from bottom, Compression failure in 1st and 2nd courses from bottom, wall broke along horizontal joint 1/4h from bottom. Wall broke along horizontal joint between 1/3h Wall broke along horizontal joint 1/4h from the top. Failure originated at 2nd course from top and wall fell sideways. Failed at 6th joint from top. 39-44 Failure 5th course from the top - 2/3 way through test vertical cracking in 2nd and 3rd courses from bottom. curves can be approximated by a straight line. Curve A, which is the only curve that covers stress-strain relations to the point of failure, was derived from a specimen of higher-than-average strength. This curve therefore covers a range of stresses not normally developed by a typical specimen. The following values of moduli of elasticity were experimentally derived for the unreinforced walls: Initial modulus of elasticity, E;= 1.4 x 106 psi. Approximate final tangent modulus of elasticity at the stress level of most wall failures, E1 = 0.4 × 106 psi. Curve D in figure 6.1 shows stress-strain relationships for an axially loaded reinforced 10-ft wall. In this case, stresses were determined on the basis of the transformed section shown in figure 6.2. This transformed section was developed using a "net" cross-sectional area for the block which is based on minimum face-shell and web thicknesses, the are of the grout and a transformed steel area based on "n" of 29 assuming that the average modulus elasticity of masonry is approximately 1 x 10 p and that of steel is 29 × 106 psi. It can be seen fr a comparison of curve D with curves A, B and C ta the area transformation which conservatively a sumed equal stiffness for grout and masonry and also was based on minimum, rather than avera net area of masonry probably resulted in overest: mating the stresses in the masonry. The value of the initial modulus of elastic derived from curve D is 2.8 × 106 psi. A tange modulus at failure could not be obtained since strumentation was removed before the masin developed its ultimate strength. Note that the stre strain curve is essentially linear up to a stress about 80 percent of the failure stress. A linear stress strain relationship probably approximates the stres distribution up to failure reasonably well. |