(c) The mixing must be done with a power-driven batch mixer except when the local or State Building Commission issues a permit for the use of a continuous mixer or a permit for mixing by hand. This provision shall not prohibit the mixing by hand of small quantities for repair work or for the construction of small piers or footings remote from the machine mixer. (d) Partly set concrete shall not be re-tempered and used. Any concrete that has taken an initial set before placing in the work shall be discarded. (e) Partly set concrete shall not be subjected to shocks from men walking or wheeling over it, or laying material upon it. 316. CONCRETE PLACED IN FREEZING WEATHER.] When concreting is carried on in freezing weather, the ingredients must be heated, and such provisions made that the concrete can be put in place without freezing. The use of frozen lumpy sand or stone, depending on the hot water used in mixing to thaw it out, will not be permitted. All forms for concrete laid in freezing weather shall remain in place until all evidences of frost are absent from the concrete, and until the natural hardening of the concrete has proceeded to the point of safety. 317. ALLOWABLE STRESS ON CONCRETE.] The allowable stress upon plain concrete shall not exceed the following: Portland cement concrete, 1: 2: 4 mixture. Portland cement concrete, 1: 212: 5 mixture. Portland cement concrete, 1 3 6 mixture.. E. REINFORCED CONCRETE. .500 lbs. per sq. in. 450 lbs. per sq. in. 400 lbs. per sq. in. 318. REINFORCED CONCRETE DEFINED.] The term "reinforced concrete" shall be considered to mean any combination of metal imbedded in concrete to form a structure so that the two materials assist each other to sustain the stresses imposed. 319. PERMIT.] The permit to erect any reinforced concrete structure does not in any manner approve the construction until after tests have been made of the actual construction to the satisfaction of the local or State Building Commissioner. 320. CEMENT AND SAND.] The cement and sand shall be of the quality specified in sections, 311 and 312 of this Act. 321. STONE.] The stone shall be of the size and quality specified in section 313 of this Act, except that it shall be screened through a one-inch square mesh. 322. MIXING.] The mixing of the concrete shall be in accordance with section 315 of this Act. 323. PLACING.] (a) The placing of concrete shall be in accordance with section 315 of this Act, and also in accordance with the following provisions: (b) In placing concrete around reinforcing steel, the concrete must be worked continuously with suitable tools as it is put into place, to expel air bubbles and surplus water. Filling the forms completely and puddling afterward will not be permitted. (c) All reinforced concrete shall be kept at a temperature above freezing for at least forty-eight hours after being put in place. 324. COMPRESSIVE STRENGTH AND MODULUS OF ELASTICITY OF CONCRETE.] (a) The calculations for the strength of reinforced concrete shall be based on the assumed ultimate compressive strength stated under the letter U in the table below. (b) The ratio of the modulus of elasticity of steel to that of concrete shall be taken in accordance with the value stated under R in the following table: Mixture. 1 cement; 1 sand; 2 broken stone or slag... 1 cement; 11⁄2 sand; 3 broken stone, gravel or slag. 325. UNIT STRESS FOR STEEL.] (a) The tensile stress in steel shall not exceed one third of its elastic limit, nor 18,000 pounds per square inch. (b) The shearing stress in steel shall not exceed 12,000 pounds per square inch. (c) The compressive stress in steel shall not exceed the product of the compressive stress in the concrete multiplied by the value of R in the above table. 326. UNIT STRESSES FOR CONCRETE.] (a) The direct compression in concrete shall not exceed one fifth of its ultimate strength. (b) The compression in the extreme fibre of concrete due to bending shall not exceed thirty-five one-hundredths of the ultimate strength. (c) The tension in concrete on a diagonal plane shall not exceed one fiftieth of the ultimate compressive strength. (d) The adhesion or bond of concrete composed of one part of cement, two parts of sand, and four parts of broken stone shall not exceed the values in the table below; and the adhesion or bond for concrete of other proportions shall be in proportion to the values of U in section 324. Adhesion or bond, pounds Kind of bar. Plain round or square bars of structural steel. Twisted bars, when the twisting is not less than one complete per sq. inch. 70 50 55 100 (e) For specially deformed bars, the allowable unit stress for adhesion or bond shall not exceed one fourth of the ultimate bond strength of such bars without appreciable slip, which shall be determined by tests made by the person, firm or corporation selling or proposing to use such bars, to the satisfaction of the local or State Building Commissioner; provided, that in no case such allowable unit stress shall exceed 100 pounds per square inch. 327. BENDING AND ELONGATION OF REINFORCING STEEL.] The bending and elongation of steel used in reinforced concrete construction shall conform to the following requirements: (a) Steel having a diameter of three fourths of an inch or less shall be capable of bending cold 90 degrees over a diameter equal to twice the thickness of the piece without fracture; and steel over three fourths of an inch in diameter shall be capable of bending cold 90 degrees over a diameter equal to three times the thickness of the piece. (b) The material of the reinforcement shall be of such form that it will not elongate under working stress to exceed one fifteenth-hundredth of its length. 328. PAINTING STEEL.] Reinforcing steel shall not be painted; but shall be free from all mill scale and loose rust when the concrete is deposited around it. 329. DESIGN OF SLABS, BEAMS AND GIRDERS.] Reinforced concrete slabs, beams and girders shall be designed in accordance with the following assumptions and requirements: (a) The common theory of flexure is applicable to reinforced concrete beams and members resisting bending. (b) The adhesion between the concrete and the steel is sufficient to make the two materials act together. (c) The steel takes all the direct tensile stresses. (d) The stress-strain curve of concrete in compression is a straight line. (e) The ratio of the modulus of the elasticity of concrete to that of steel shall be as specified in the table in section 324. 330. CEMENT FINISH.] Cement finish added to the top of slabs, beams or girders shall not be included in calculating the strength of the member, unless it is laid integrally with the concrete. No greater unit stress shall be allowed on such cement finish than on the concrete. 331. SHRINKAGE AND THERMAL STRESSES.] Shrinkage and thermal stresses shall be provided for by the introduction of reinforcing steel. 332. MOMENTS OF EXTERNAL FORCES.] (a) Beams, girders, floor or roof slabs and joists shall be calculated as having supported ends or fixed ends, or partly fixed ends, according to the actual end conditions, the number of spans, and the design. (b) When an intermediate span having an equally distributed load, where the adjacent spans are of approximately equal lengths, is calculated as having ends partly fixed, the bending moment at the center of the span WL2 WL2 12 10 shall not be taken at less than for intermediate spans, and for end spans, in which W is the load in pounds per lineal foot and L the ·length of span in feet. WL2 ; (c) The moment over supports shall not be taken at less than 18 and the sum of the moments over one support and at the center of span WL2 6 shall not be taken at less than (d) In case of concentrated or special loads the calculations shall be based on the actual condition of loading. (e) For fully supported slabs, the span is to be taken as the free opening plus the depth; and for continuous slabs, the distance between centers of supports is to be taken as the span. (f) Where the vertical shearing stress measured on the section of a beam or girder between the centers of action of the horizontal stresses, exceeds one fiftieth of the ultimate direct compressive stress, web reinforcement shall be supplied sufficient to carry the excess. The web reinforcement shall extend from top to bottom of beam, and also loop around or connect to the horizontal reinforcement. The horizontal reinforcement carrying the direct stresses shall not be considered as web reinforcement. (g) In no case shall the vertical shear, measured as stated above, exceed one fifteenth of the ultimate compressive strength of the concrete. (h) For "T" beams the width of the stem only shall be used in calculating the vertical shearing stress. (i) When steel is used in the compression side of beams and girders, the rods shall be tied in accordance with requirements of vertical reinforced columns, and have stirrups connecting with the tension rods of the beams or girders. (j) All reinforcing steel shall be accurately located in the forms, and be secured against displacement; and shall be inspected by the representative of the architect or engineer in charge before the inclosing concrete is put in place. The steel shall be completely enclosed by the concrete. No steel shall be nearer the surface of the concrete than one and one half inches for columns; one and one half inches for beams and girders, and one half inch, but not less than the diameter of the bar, for slabs. (k) The longitudinal steel in beams and girders shall be so disposed that there shall be a thickness of concrete between the separate pieces of steel of not less than one and one half times the maximum cross-sectional dimension of the steel. (1) For square slabs with two-way reinforcements, the bending moment at the center of the slab shall be not less than that expressed in the formula WL2 WL2 for end spans. for intermediate spans; and 24 20 WL2 (m) The moment over the supports shall not be taken at less than 36 and the sum of the moments over one support and at the center of the span WL3 shall not be taken at less than in which formula W is the load per 12 lineal foot and L the length of the span in feet. (n) For square or rectangular slabs, the distribution of the loads in the two directions shall be inversely as the cubes of the two dimensions. (0) Exposed metal of any kind will not be considered a factor in the strength of any part of any concrete structure; and the plaster finish applied over the metal shall not be deemed sufficient protection unless applied of sufficient thickness and so secured as to meet the approval of the local or State Building Commissioner. 333. LIMITING WIDTH OF FLANGE IN "T" BEAMS.] (a) In the calculation of ribs, a portion of the floor slab may be assumed as acting in flexure in combination with the ribs. The width of the slab so acting in flexure is to be governed by the shearing resistance between rib and slab; but is limited to a width equal to one third of the span length of the ribs between supports, and also to a width of three quarters of the distance from center to center between ribs. (b) No part of the slab shall be considered as a portion of the rib unless the slab and rib are cast at the same time. (c) Where reinforced concrete girders support reinforced concrete beams, the portion of floor slab acting as flanges to the girders must be reinforced with rods near the top, at right angles to the girder, to enable it to transmit local loads directly to the girder and not through the beams. 334. REINFORCED-CONCRETE COLUMNS.] (a) Reinforced concrete may be used for columns, provided the concrete shall not be a leaner than a 1: 2:4 mixture; and provided, further, the ratio of length to least side or diameter does not exceed 12: and provided, still further, the cross section of the column shall not be less than 64 square inches. Longitudinal reinforcing rods must be tied together to effectively resist outward flexure, at intervals of not more than 12 times the least diameter of rod and not more than 18 inches. When compression rods are not required, reinforcing rods shall be used equivalent to not less than one half of 1 per cent of the cross sectional area of the column; provided, the total section area of the reinforcing steel shall not be less than one square inch; and provided, further, no rod or bar shall have a smaller cross sectional dimension than one half inch. The area of reinforcing compression rods shall be limited to 3 per cent of the cross sectional area of the column. Vertical reinforcing rods shall extend upward or downward into the column, lapping the reinforcement above or below enough to develop the stress in rod by the allowable unit for adhesion. When beams or girders are made monolithic with, or are rigidly attached to, reinforced concrete columns, the latter shall be designed to resist a bending moment equal to the greatest possible unbalanced moment in the beams or girders at the columns, in addition to the direct loads for which the columns are designed. (b) When the reinforcement consists of vertical bars and spiral hooping the concrete may be stressed to one fourth of its ultimate strength as given in section 324, provided the following conditions are met: (1) The amount of vertical reinforcement shall not be less than the amount of the spiral reinforcement, nor greater than 8 per cent of the area within the hooping; (2) the percentage of spiral hooping shall be not less than one half of 1 per cent, nor greater than one and one half per cent; (3) the pitch of the spiral hooping shall be uniform and not greater than one tenth of the diameter of the column nor greater than three inches; (4) the spirals shall be secured to the verticals at every intersection in such a manner as to insure maintenance of its form and position; (5) the vertical rods shall be spaced so that their distance apart, measured on the circumference, is not greater than 9 inches, nor one eighth of the circumference of the column within the hooping. In columns in which all of the preceding conditions are met, the action of the hooping may be assumed to increase the resistance of the concrete equivalent to two and one half times the amount of the spiral hooping figured as vertical reinforcement. No part of the concrete outside of the hooping shall be considered as a part of the effective column section. 335. STRESS IN CONCRETE WITH STRUCTURAL STEEL-COLUMNS.] When the vertical reinforcement consists of a structural steel-column or box shape with lattice or batten plates, having such a form as to permit its being filled with concrete, the concrete may be stressed to one fourth of the ultimate strength given in the table in section 324, provided no shape of less than one square inch cross section is to be used, and provided further the spacing of the lacing or battens be not greater than the least width of the column. 336. REINFORCED-CONCRETE CERTAIN WALLS.] Buildings having a complete skeleton of steel, or of reinforced concrete, or a combination of both, may have exterior walls of reinforced concrete 8 inches thick: Provided, such walls shall support only their own weight: And, provided, further, that such walls shall have reinforcement in accordance with the following conditions: (1) Such walls shall have vertical and horizontal steel reinforcement of not less than three tenths of 1 per cent in each direction. (2) The rods shall be spaced not more than 12 inches center to center; and shall be wired to each other at each intersection.. (3) All reinforcing bars shall be lapped sufficiently to develop their full strength at the allowable unit stresses for adhesion. (4) Additional bars shall be set around all openings in said wall; and such vertical rods shall be wired to the horizontal bars, and the horizontal bars at the top and bottom of said openings shall be wired to the vertical bars. (5) All reinforcing rods shall be so placed as to develop the greatest strength in the combination of concrete and steel. (6) The reinforcement shall be so placed and secured that the wall will resist a pressure, from either the exterior or the interior, of 30 pounds on each and every square foot of said wall. 337. REINFORCED HOLLOW-TILE CONSTRUCTION.] (a) The term reinforced hollow-tile construction is hereby defined to mean a system of hollow burnedclay tile in combination with reinforced concrete, in which combination the hollow-tile may be assumed to resist compressive and shearing stresses subject to the following provisions: (b) The provisions relating to reinforced concrete construction shall hold as far as applicable to the reinforced hollow-tile system. (c) All tile shall be hard-burned terra-cotta of uniform quality, free from shrinkage cracks, with true beds; and shall have an ultimate compressive strength of not less than 4,000 pounds per square inch of net area of surface tested. (d) The following stresses and values shall not be exceeded: Compressive stress in extreme fiber of hollow tile, 500 pounds per square inch. Shearing stress on hollow tile, 200 pounds per square inch. Adhesion between tile and 1:2:4 concrete or 1:3 cement mortar, 40 pounds per square inch. Ratio of modulus of elasticity of steel to that of tile with cement mortar joints, 10. (e) The hollow tile shall be thoroughly soaked with water at the time the concrete is poured; and shall be kept drenched for at least thirty-six hours afterwards. The joints between tiles shall be staggered, buttered and slushed full of mortar, consisting of one part of Portland cement and three parts of clean sharp sand, thoroughly mixed. 338. REMOVAL OF FORMS.] In no case shall the forms be removed until the concrete had developed sufficient strength to sustain its own weight. No load or weight shall be placed on any portion of the construction until the concrete has fully set. 339. TESTS OF COMPLETED STRUCTURE.] The contractor for the reinforced concrete construction shall make such tests on any portion of the work within a reasonable time after erection as may be required by the local or State Building Commissioner. Such tests must be made under the direction of the Commissioner, in his presence or in that of his representative, and must show that the construction will sustain, without any sign of failure, a load twice the sum of the live and dead loads for which it was designed. The construction may be considered a part of the test load. Each test load shall cover two or more panels, and the load shall remain in place at least twenty-four hours. The deflection under the full test load at the expiration of twenty-four hours shall not exceed one eight-hundredth of the span. These tests shall be considered as tests of workmanship only. 340. OTHER TYPES OF REINFORCED-CONCRETE CONSTRUCTION.] If other types of reinforced-concrete construction than those described or implied above are proposed for any building, a Board of Arbitration shall be appointed as provided for by section 11 of this Act, to determine the advisability of permitting the use of such type of construction. |