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Advisability of higher steam pressures
156
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tension has been reduced. During the following summer, roving t,, the higher temperatures, the wires will sag still further. Iron wire supported on poles placed 25o feet apart to give (}-foot sags is much stronger and much more likely to retain its original length and position than copper wire. usually erected on more closely spaced poles. In the opinion of the commission, there seemed to be no questi~m as to the strength of galvanized-iron wire when first installed, hut the liability to rusting was viewed with some alarm. The petitioucr held that {ton wire can rust considerably before its cross-section is reduced to the point where it would 1)¢2 seriously weakened. If ,~ small iron telephone wire is safe for ten or twenty years, the larger wires proposed for transmission purposes will. of course, have a longer life, since the larger the diameter the less the relative surface exposed to rust compared with the weight. Furthermore, although practically all 6oo-volt trolley wires suspended oxer the beads of pedestrians in bus)" city streets are supported by stranded galvanizediron wire, casualties due to the rusting away of these wires sel&,m occur. While the commission was aplmrently satisfied with tlw initial strength of the proposed iron wire. it demanded, in grantinx l)ermission for its use, that the operating company use extra \i,~ilance during the semi-annual line inspection.
Advisability of Higher Steam Pressures. J.T. l;osTV.k. (Eh'ct;'ic lUorht, vol. 7 o, No. 23, l ). to9o. December 8, 19I 7. ) - - A t tile time the subject of higher steam pressures was first brought prominently before the engineering world in ~9 ~5 there seems to have been some doubt as to whether it would be practicable to attemln a gain in thermal efficiency by increasing pressures much above those now in use. Since that tinge, however, it appears that considerable thought has been given to the subject, and a number of prominent engineers have exl)ressed the view that high pressures are not only feasible, but inevitable, l~revious discussions have been confined to thermal efficiencies, hut a consideration of water rates is of greater practical interest. Calculations from (ioodenough's fornmlas, puhlished ill Bulletin 75 of the Engineering Experiment Station of the University of Illinois, show how thermal efficiencies and water rates vary a', boiler teml)eratures of 5oo . , 0 o o ' , and 7oo , F. with a vacuum of 28 ~~, inches at the exhaust. From these results the lowest water rate of 6.95 pounds was found to occur at a pressure of r Ioo pounds absolute and at a temperature of 7oo ~ b'. Practical considerations seem to offer no insuperable difficulties. Engineers experienced in the design of boilers and turbines have expressed the view that, while a certain amount of re-design will be necessary, the changes in the case of 5oo-pound or 6oo-pound operating pressures are not radical, and that as soon as there is sufficient demand the work of developing high-pressure apparatus will be undertaken. No reasons appear why large gains should not be realized. The change from 20o pounds to 5oo pounds and (),:~
Jan., 19~8.1
CURRENT TOPICS.
~57
pounds is no more revolutionary than the change which was made at the time standard practice raised the pressure from 80 pounds to 2oo pounds or more. In view of the fact that the earlier change netted a gain of approximately 2o per cent., the expected gain in economy from the change to 5oo pounds or 6oo pounds does not seem excessive. A New Method of Constructing Difficult Foundations.
E.A.
PRENTIS, JR. (Engineerin 9 A'e.u,s-Record, vol..79, No. 23, p. Io6I, December 6, I9I 7 . ) - - D u r i n g the construction of the subways in New York Citv, particularly in the skyscraper district of Manhattan, a great deal of underpinning has been done. From experience in carrymg on this work, a new method of constructing foundations for new buildings has been developed. The method may be said to consist in underpinning the building during its erection, so that the construction of the foundation goes hand in hand with that of the building itself. It is evident that with such a method a great saving of time can be effected--where, for instance, caissons would be needed to carry the cohnnn loads and a spread foundation is out of the question. in the new method the foundations are designed in the usual way as pile foundations. Steel shells for the piles are driven a small distance into the ground, from 5 to IO feet, by means of a hammer, and are cleaned out and concreted. Wooden blocks 3 or 4 feet long are placed on top of the piles, and the space around the blocks and the tops of the piles is back-filled with carefully tamped earth. A reinforced concrete girder with the necessary grout pipes is then cast on the back-fll and posts, thus supplying a preliminary support for the colunm loads. The footing so constructed is ready for the beginning of the construction of the building. When the building has reached three or four stories--of perhaps twenty to be built--the second stage of the construction proceeds. The colunm by this time supports sufficient weight for jacking purposes. Each wooden post is then removed and the pile jacked until the desired reaction is obtained. The load is then transferred to two I-beam posts or their equivalent on each side of the jack, the jack removed, and the I-beams concreted in as well as space allows. Final contact is made by means of the grout pipes. W o o d Trusses Incased in Concrete. ANON. (The Contract Record, vol. 3 I, No. 48, p. 983, November 28, 19/7.)--Casing timber roof trusses in cement to protect them against fire and acid fumes is a novel method employed recently in the buildings of a large oil refinery in Texas. Wood construction was adopted on account of the shortage of structural steel. After the trusses had been erected each member was wrapped with a single layer of light water-proof building paper, placed close against the surface and fastened with carpet tacks. The object of this covering is to prevent any bond be-
CURREXT "l'(~t'i~S.
tJ. 1, t,
tension has been reduced. During the following summer, roving t,, the higher temperatures, the wires will sag still further. Iron wire supported on poles placed 25o feet apart to give (}-foot sags is much stronger and much more likely to retain its original length and position than copper wire. usually erected on more closely spaced poles. In the opinion of the commission, there seemed to be no questi~m as to the strength of galvanized-iron wire when first installed, hut the liability to rusting was viewed with some alarm. The petitioucr held that {ton wire can rust considerably before its cross-section is reduced to the point where it would 1)¢2 seriously weakened. If ,~ small iron telephone wire is safe for ten or twenty years, the larger wires proposed for transmission purposes will. of course, have a longer life, since the larger the diameter the less the relative surface exposed to rust compared with the weight. Furthermore, although practically all 6oo-volt trolley wires suspended oxer the beads of pedestrians in bus)" city streets are supported by stranded galvanizediron wire, casualties due to the rusting away of these wires sel&,m occur. While the commission was aplmrently satisfied with tlw initial strength of the proposed iron wire. it demanded, in grantinx l)ermission for its use, that the operating company use extra \i,~ilance during the semi-annual line inspection.
Advisability of Higher Steam Pressures. J.T. l;osTV.k. (Eh'ct;'ic lUorht, vol. 7 o, No. 23, l ). to9o. December 8, 19I 7. ) - - A t tile time the subject of higher steam pressures was first brought prominently before the engineering world in ~9 ~5 there seems to have been some doubt as to whether it would be practicable to attemln a gain in thermal efficiency by increasing pressures much above those now in use. Since that tinge, however, it appears that considerable thought has been given to the subject, and a number of prominent engineers have exl)ressed the view that high pressures are not only feasible, but inevitable, l~revious discussions have been confined to thermal efficiencies, hut a consideration of water rates is of greater practical interest. Calculations from (ioodenough's fornmlas, puhlished ill Bulletin 75 of the Engineering Experiment Station of the University of Illinois, show how thermal efficiencies and water rates vary a', boiler teml)eratures of 5oo . , 0 o o ' , and 7oo , F. with a vacuum of 28 ~~, inches at the exhaust. From these results the lowest water rate of 6.95 pounds was found to occur at a pressure of r Ioo pounds absolute and at a temperature of 7oo ~ b'. Practical considerations seem to offer no insuperable difficulties. Engineers experienced in the design of boilers and turbines have expressed the view that, while a certain amount of re-design will be necessary, the changes in the case of 5oo-pound or 6oo-pound operating pressures are not radical, and that as soon as there is sufficient demand the work of developing high-pressure apparatus will be undertaken. No reasons appear why large gains should not be realized. The change from 20o pounds to 5oo pounds and (),:~
Jan., 19~8.1
CURRENT TOPICS.
~57
pounds is no more revolutionary than the change which was made at the time standard practice raised the pressure from 80 pounds to 2oo pounds or more. In view of the fact that the earlier change netted a gain of approximately 2o per cent., the expected gain in economy from the change to 5oo pounds or 6oo pounds does not seem excessive. A New Method of Constructing Difficult Foundations.
E.A.
PRENTIS, JR. (Engineerin 9 A'e.u,s-Record, vol..79, No. 23, p. Io6I, December 6, I9I 7 . ) - - D u r i n g the construction of the subways in New York Citv, particularly in the skyscraper district of Manhattan, a great deal of underpinning has been done. From experience in carrymg on this work, a new method of constructing foundations for new buildings has been developed. The method may be said to consist in underpinning the building during its erection, so that the construction of the foundation goes hand in hand with that of the building itself. It is evident that with such a method a great saving of time can be effected--where, for instance, caissons would be needed to carry the cohnnn loads and a spread foundation is out of the question. in the new method the foundations are designed in the usual way as pile foundations. Steel shells for the piles are driven a small distance into the ground, from 5 to IO feet, by means of a hammer, and are cleaned out and concreted. Wooden blocks 3 or 4 feet long are placed on top of the piles, and the space around the blocks and the tops of the piles is back-filled with carefully tamped earth. A reinforced concrete girder with the necessary grout pipes is then cast on the back-fll and posts, thus supplying a preliminary support for the colunm loads. The footing so constructed is ready for the beginning of the construction of the building. When the building has reached three or four stories--of perhaps twenty to be built--the second stage of the construction proceeds. The colunm by this time supports sufficient weight for jacking purposes. Each wooden post is then removed and the pile jacked until the desired reaction is obtained. The load is then transferred to two I-beam posts or their equivalent on each side of the jack, the jack removed, and the I-beams concreted in as well as space allows. Final contact is made by means of the grout pipes. W o o d Trusses Incased in Concrete. ANON. (The Contract Record, vol. 3 I, No. 48, p. 983, November 28, 19/7.)--Casing timber roof trusses in cement to protect them against fire and acid fumes is a novel method employed recently in the buildings of a large oil refinery in Texas. Wood construction was adopted on account of the shortage of structural steel. After the trusses had been erected each member was wrapped with a single layer of light water-proof building paper, placed close against the surface and fastened with carpet tacks. The object of this covering is to prevent any bond be-