Single-bearing cylinders (Fig. 16.16a) are a simple type that operate like a hydraulic jack. They are suitable for elevator and sidewalk lifts where the car is guided at top and bottom, preventing eccentric loading from exerting side thrust on the cylinder bearing. A cylinder of heavy steel usually is sunk in the ground as FIGURE 16.15 Hydraulic elevator. far as the load rises. The plunger, of thickwalled steel tubing polished to a mirror finish, is sealed at the top of the cylinder with compression packing. Oil is admitted under pressure near the top of the cylinder, while air is removed through a bleeder. A different cylinder design should be used where the car or platform does not operate in guides. One type capable of taking off-balance loads employs a twobearing plunger (Fig. 16.16b). The bearings are kept immersed in oil. Another type, suitable for general industrial applications, has a movable bearing at the lower end of the plunger to give support against heavy eccentric loads (Fig. 16.16c). At the top of the cylinder, the plunger is supported by another

1.397E/F , u P
1.40td F (8.28) n au where da
average diameter, in, of the arc spot weld at midthickness of sheet
d  t for a single sheet
d  2t for multiple sheets (not more than four lapped sheets over a supporting member) d
visible diameter of outer surface of arc spot weld, in de
effective diameter of fused area, in
0.7d  1.5t  0.55d t
total combined base steel thickness, in (exclusive of coatings) of sheets involved in shear transfer Fxx
stress-level designation in AWS electrode classification, ksi Fu
tensile strength of the base steel as specified, ksi The distance measured in the line of force from the centerline of a weld to the nearest edge of an adjacent weld or to the end of the connected part toward which the force is directed should be at least e , in, as given by min e
e (8.29) min e where e
P/ (Fu t) e
factor of safety for sheet tearing
2.0 when Fu /F 1.08 sy
2.22 when Fu /Fsy  1.08 P
force transmitted by weld, kips Fsy
yield strength of sheet steel, ksi, as specified t
thickness of thinnest connected sheet, in In addition, the distance from the centerline of any weld to the end or boundary of the connected member should be at least 1.5d. In no case should the clear distance between welds and the end of the member be less than d. The nominal tension load Pn, kips, on an arc spot weld between a sheet and a supporting member should be computed as the smaller of either: P
0.785d2F (8.30a) n exx

Waterproofing. A number of substances, such as stearates and oils, have been used as masonry-mortar and concrete admixtures for waterproofing. Indiscriminate use of such materials in concrete without extremely good quality control usually results in disappointment. The various water-repellent admixtures are intended to prevent capillarity, but most severe leakage in concrete occurs at honeycombs, cold joints, cracks, and other noncapillary defects. Concrete containing waterrepellent admixtures also requires extremely careful continuous curing, since it will be difficult to rewet after initial drying. Waterproof concrete can be achieved by use of high-strength concrete with a low water-cementitious materials ratio to reduce segregation and an air-entraining agent to minimize crack width. Also, good quality control and inspection is essential during the mixing, placing, and curing operations. Surface coatings can be used to improve resistance to water penetration of vertical or horizontal surfaces. For detailed information on surface treatments, see Guide to Durable Concrete, ACI Cement Replacement. The term cement replacement is frequently misused in reference to chemical admixtures intended as accelerators or water reducers. Strictly, a cement replacement is a finely ground material, usually weakly cementitious (Art. 9.1), which combines into a cementlike paste replacing some of the cement paste to fill voids between the aggregates. The most common applications of these admixtures are for low-heat, low-strength mass concrete or for concrete masonry. In the former, they fill voids and reduce the heat of hydration; in the latter, they fill voids and help to develop the proper consistency to be self-standing as the machine head is lifted in the forming process. Materials commonly used are fly ash, silica fume, ground granulated blast-furnace slag, hydraulic lime, natural cement,