While city building regulations are not always criteria of good practice, yet the Regulations of the Bureau of Buildings of the Borough of Manhattan concerning the use of concrete-steel construction are exceptional. Emanating from a bureau that has been distinctly hostile to concrete-steel, they are naturally conservative, but are, on the whole, excellent, and work conscientiously done in accordance with them will not bring discredit on concrete construction.
It is specified that the cement shall be only high grade Portland standing certain tests, that the sand shall be clean and sharp, aggregate, broken trap, or gravel of a size that will pass a three-quarter inch ring, and that the proportions used shall be one cement, two sand and four of stone or gravel, or that the concrete shall have a crushing strength of two thousand pounds per square inch in twenty-eight days. Only the best quality of concrete is thus permitted.
629. The Regulations concerning the design are then stated as follows: —
"Concrete-steel shall be so designed that the stresses in the concrete and the steel shall not exceed the following limits: —
Extreme fiber stress on concrete in compression, 500 lbs. per sq. in.
Shearing stress in concrete......... 50 "
Concrete in direct compression....... 350 "
Tensile stress in steel........... 16,000 "
Shearing stress in steel.......... 10,000 "
"The ratio of the moduli of elasticity of concrete and steel shall be taken as one to twelve.
"The following assumption shall guide in the determination of the bending-moments due to the external forces: Beams and girders shall be considered as simply supported at the ends, no allowance being made for the continuous construction over supports. floor plates when constructed continuous and when provided with reinforcement at top of plate over the supports, may be treated as continuous beams, the bending-moment for uniformly distributed loads being taken at not less than WL/10 ; the bending-moment may be taken as WL/20 in the case of square floor plates which are reinforced in both directions and supported on all sides. The floor plate to the extent of not more than ten times the width of any beam or girder may be taken as part of that beam or girder in computing its moment of resistance.
"The moment of resistance of any concrete-steel construction under transverse loads shall be determined by formulas based on the following assumptions: —
"(a) The bond between the concrete and steel is sufficient to make the two materials act together as a homogeneous solid.
"(6) The strain in any fiber is directly proportionate to the distance of that fiber from the neutral axis.
"(c) The modulus of elasticity of the concrete remains constant within the limits of the working stresses fixed in these Regulations.
"From these assumptions it follows that the stress in any fiber is directly proportionate to the distance of that fiber from the neutral axis.
"The tensile strength of the concrete shall not be considered.
"When the shearing stresses developed in any part of a construction exceed the safe working strength of concrete, as fixed in these Regulations, a sufficient amount of steel shall be introduced in such a position that the deficiency in the resistance to shear is overcome.
"When the safe limit of adhesion between the concrete and steel is exceeded, some provision must be made for transmitting the strength of the steel to the concrete.
" Concrete-steel may be used for columns in which the ratio of length to least side or diameter does not exceed twelve. The reinforcing rods must be tied together at intervals of not more than the least side or diameter of the column.
"The contractor must be prepared to make load tests on any portion of a concrete-steel construction, within a reasonable time after erection, as often as may be required by the Superintendent of Buildings. The tests must show that the construction will sustain a load of three times that for which it is designed, without any sign of failure".