Some Mooted Questions in Reinforced Concrete Design Part 4

TEST NO. 1.[I]

Marks on column--none.

Reinforcement--eight 1-1/8-in. round bars vertically.

Band s.p.a.cing--- 9 in. vertically.

Hooped with seven 32-in. wire spirals about 2-in. raise.

Outside diameter of hoops--14-1/2 in.

Total load at failure--1,360,000 lb.

Remarks.--Point of failure was about 22 in. from the top. Little indication of failure until ultimate load was reached.

Some slight breaking off of concrete near the top cap, due possibly to the cap not being well seated in the column itself.

TEST NO. 2.

Marks on column--Box 4.

Reinforcement--eight 1-1/8-in. round bars vertically.

Band s.p.a.cing about 13 in. vertically.

Wire spiral about 3-in. pitch.

Point of failure about 18 in. from top.

Top of cast-iron cap cracked at four corners.

Ultimate load--1,260,000 lb.

Remarks.--Both caps apparently well seated, as was the case with all the subsequent tests.

TEST NO. 3.

Marks on column--4-B.

Reinforcement--eight 7/8-in. round bars vertically.

Hoops--1-3/4 in. 3/16 in. 14 in. outside diameter.

Band s.p.a.cing--13 in. vertically.

Ultimate load--900,000 lb.

Point of failure about 2 ft. from top.

Remarks.--Concrete, at failure, considerably disintegrated, probably due to continuance of movement of machine after failure.

TEST NO. 4.

Marks on column--Box 4.

Reinforcement--eight 1-in. round bars vertically.

Hoops s.p.a.ced 8 in. vertically.

Wire spirals as on other columns.

Total load at failure--1,260,000 lb.

Remarks.--First indications of failure were nearest the bottom end of the column, but the total failure was, as in all other columns, within 2 ft. of the top. Large cracks in the sh.e.l.l of the column extended from both ends to very near the middle. This was the most satisfactory showing of all the columns, as the failure was extended over nearly the full length of the column.

TEST NO. 5.

Marks on column--none.

Reinforcement--eight 7/8-in. bars vertically.

Hoops s.p.a.ced 10 in. vertically.

Outside diameter of hoops--14-1/2 in.

Wire spiral as before.

Load at failure--1,100,000 lb.

Ultimate load--1,130,000 lb.

Remarks.--The main point of failure in this, as in all other columns, was within 2 ft. of the top, although this column showed some scaling off at the lower end.

In these tests it will be noted that the concrete outside of the hooped area seems to have had very little value in determining the ultimate strength; that, figuring the compression on the core area and deducting the probable value of the vertical steel, these columns exhibited from 5,000 to 7,000 lb. per sq. in. as the ultimate strength of the hooped area, not considering the vertical steel. Some of them run over 8,000 lb.

The concrete mixture was 1 part Alpena Portland cement, 1 part sand, 1-1/2 parts buckwheat gravel and 3-1/2 parts gravel ranging from 1/4 to 3/4 in. in size.

The columns were cast in the early part of December, and tested in April. The conditions under which they hardened were not particularly favorable, owing to the season of the year.

The bands used were 1-3/4 by 1/4 in., except in the light column, where they were 1-3/4 by 3/16 in.

In his remarks regarding the tests at Minneapolis, Minn., Mr. G.o.dfrey has failed to note that these tests, faulty as they undoubtedly were, both in the execution of the work, and in the placing of the reinforcement, as well as in the character of the hooping used, were sufficient to satisfy the Department of Buildings that rational design took into consideration the amount of hooping and the amount of vertical steel, and on a basis not far from that which the writer considers reasonable practice.

Again, Mr. G.o.dfrey seems to misunderstand the influence of Poisson's ratio in multiple-way reinforcement. If Mr. G.o.dfrey's ideas are correct, it will be found that a slab supported on two sides, and reinforced with rods running directly from support to support, is stronger than a similar slab reinforced with similar rods crossing it diagonally in pairs. Tests of these two kinds of slabs show that those with the diagonal reinforcement develop much greater strength than those reinforced directly from support to support. Records of small test slabs of this kind will be found in the library of the Society.