Author: Andrews, Ewart S
First published: N/A
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Andrews, Ewart S
The new system of reinforced concrete construction to be described has a wider range of application than is suggested by the title of this paper. It has a very economical application in the case of wide span floors for factories, warehouses, hospitals, and similar structures. For heavy floor foundations for engineering and architectural works to be carried by poor-bearing strata of a semi-liquid and plastic nature or of the nature of running sand, the system is markedly efficient and economical. When to these conditions of construction and foundation strata is added the problem of alterations of upward and downward pressure, actual experience has proved that an economy of at least 50 per cent. is effected by the application of the new system. Under the conditions mentioned, tmhe application of the system to the floors and foundations of the new sewage settlement tanks at the Low Level Sewage Disposal Works of the Leeds Corporation showed that the saving on ordinary methods of construction amounted to 53 1/2 per cent. The most economical of the ordinary methods of construction gave a cost of £215,000, whereas th6 new system qave a cost of less than £1OO,OOO, thus representing a saving of over £115,000. The various alternative designs were submitted by the Sewerage Engineer, Mr. George A. Hart, M.Inst.C.E., M.I.Rlech.E.-a member of this institution-to Mr. Basil Mott, C.B., M.Inst.C.E., for expert criticism. Mr. Mott, after prolonged and careful consideration and investigation, reported in favour of the new system on grounds of both efficiency and economy.
The introduction of special braking girders into the floor system of large span bridges is of comparatively recent practice. A brief notice of this detail as applied in the Hell Gate Bridge may be of interest. The Hell Gate Bridge is a two-hinged steel arch of 977 ft. 6 in. span between bearings. The floor carries four railway tracks between the main ribs and two bracketed side walks projecting 16 ft. 6 ins. on the outsides of the main ribs, which are 60 ft. apart, centre to centre. The total width is thus 93 ft. between centres of parapet girders. The spacing of cross-beams is 42 ft. 6 ins., and their weight upwards of 80 tons each. The four cross-beams adjacent to the abutments, that is, up to the points G and G, Fig. 20, where the lower chords of the arch rise above the roadway, are framed in between the vertical struts of the arch ribs. The remaining cross-beams over the middle portion of the span are suspended from the arch ribs by suspenders hung to pin joints so arranged as to permit freedom of transverse movement, Fig. 21. The suspended portion of the floor, therefore, would possess no inherent lateral stability in the absence of a lateral bracing system. This method of suspension was adopted to avoid bending stresses in the suspenders due to vertical deflection of the floorbeams and horizontal deflection of the floor lateral system. The suspenders are further made slender in elevation in order to minimise bending stresses due to the longitudinal expansion and contraction of the floor. The floor lateral bracing consists of stiff diagonals in the plane of the bottom flanges of the railroad stringers, as indicated by the heavy lines in the plan, Fig. 20. These diagonals are riveted to the stringers and cross-beams at the intersection points. The lateral system thus forms a rigid horizontal girder between panel points 3 and 6, but owing to the transverse flexibility of the suspenders, the portion of the lateral system between panel points 3 and 6 is called upon to act as a horizontal cantilever for restraining lateral deflection of the suspended portion of the floor. At panel point 6 an expansion joint is introduced in all longitudinal floor members and the diagonal lateral system coming from the right terminates at P in a pocket on floor-beam No. 6, so arranged as to prevent lateral movement whilst allowing longitudinal sliding to take place. The point P therefore receives the horizontal reaction from the central portion of the lateral system and transfers it as a concentrated load to the cantilever portion between panel points 3 and 6. At the corresponding panel point No. 17, on the opposite side of the centre of span, only partial freedom of movement of the lateral system is provided for. Expansion joints are introduced in the parapet girders at E, E, and in the side-walk stringers, but the diagonal joint at P1, and the railrotad stringers at cross-beam No. 17 are riveted. The floor lateral system between P and P, therefore, acts very sen
MR. G. V. GRIFFITHS (Keighley) said he was very interested in “ mushroom ” construction, and asked Mr. Davidson to explain what, special provisions had been made in the reinforcement for end spans.