Author: Cramer, W
First published: N/A
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SIR,-The publication of Mr. Roberts’s paper* read before the Institution last March gives an opportunity for reconsideration. Coming to it with fresh interest one is able to grasp points missed upon the first encounter. One of the most interesting parts to me was the description of tests made on mild steel and Chromador pillars and followed by proposed pillar stresses for the new steel. But Professor Robertson’s pillar formula contains within itself the necessary adjustment for steels of various yields.
0WING to the traffic and to the parapet being in the way, the problem of relating
the survey line over a bridge with the line on the road below it cannot usually be solved by setting up a theodolite on the bridge itself. The author has therefore developed the following method and used it successfully at many bridges ranging in span from 40 feet to 100 feet in the course of an extensive survey of electrified railway having a very dense train service.
When the Roads Department of the Ministry of Transport published their Chart of "Equivalent Distributed Loads for Highway Bridges," they also issued a leaflet showing the permissible working stresses they were willing to allow for various qualities of reinforced concrete. The Loads Chart also made various provisions for
bridge deckings constructed in concrete. In the table of stresses an increase was allowed of 25 per cent. over what had been previously allowed by Public Authorities previous to that date, September, 1933. It was evident that, if such higher stresses were recognised as safe for bridges, other Authorities could not maintain their old conservative limits much longer; recently the London County Council, pending the revision of their Regulations, have recognised (and accepted under their Waiver Clause)
stresses of about the same intensity as those allowed by the Ministry of Transport. Since the Recommendations of the Reinforced Concrete Structures Committee of the Building Research Board have been published and the new standard for concrete stresses are thereby confirmed it is essential for designers to accustom themselves to the higher resistances of reinforced concrete members which they are now able to employ.
Ernest A. Scott