Substituting in Equations 2 to 8, inclusive, gives the following factors: On account of the irregularity of both abutments, this amount has 65 The ratios of heights of abutments and average pier are = 0.87 75 and 25 a ratio of 2.1 and for the small one a ratio of 0.44, making a total of 2.54 for the two abutments; hence their combined volume is 1,580 X 2.54 = 4,010 cu. yds. Adding this to the 17,300 cubic yards found for the eleven piers makes a grand total of 21,310 cu. yds. This chapter was the last one of the book to be completed, because the quantities of materials for reinforced-concrete bridges were not figured until after the MS. of all the other chapters had gone to press; and this question of quantities for piers and abutments was the last one of all to be solved. It had been considered not only by all of his assistants, but also by the author himself to be absolutely impossible to prepare any diagram, combination of diagrams, table, rule, or formula for determining the said quantities, even with an exceedingly liberal allowance for variation from correctness; but at the last moment he evolved the method herein given. It is specially hoped that the diagrams of Figs. 56t to 56ff, inclusive, and the directions for determining approximately the quantities in the piers and abutments of reinforced-concrete bridges will be found truly useful by the engineering profession. In the author's opinion, they are sufficiently accurate for making preliminary estimates of cost; but bidding figures on concrete bridges should never be considered safe unless they are prepared from thoroughly made special computations and sketch drawings. The curves and formula, however, should be found serviceable. in obtaining an approximate check on the accuracy of all the quantities that have been computed in detail for contractors' bidding figures. Considering the fact that the establishing of fairly accurate data for making quick estimates for reinforced concrete bridges has, during the last decade, been a dream of the author's (occasionally claimed by some of his friends to be merely a pipe dream), and that, almost without exception, every engineer whom he has consulted about the practicability of preparing such data has declared the task to be impossible of accomplishment, in thus completing the MS. of his book he experiences deep satisfaction in having finally solved the problem (and especially that portion of it relating to the piers and abutments)-at least to his own contentment. In concluding this chapter, the author desires to tender to Messrs. Harrington, Howard, and Ash, his former partner and associated engineers, his thanks for their courtesy in furnishing him the data for the Tulsa Bridge and the two Dayton bridges recorded in Table 56a. CHAPTER LVII ESTIMATES THE making of estimates is one of the most important functions of the bridge engineer, for it is generally the first step that he has to take in connection with any engineering project. Upon his ability to prepare a correct estimate will often depend the important question of whether the projected work is to materialize; and unless he have an established reputation for accuracy, he will not often be entrusted with the making of preliminary estimates for important projects. The requisites for preparing accurate estimates are as follows: First. A wide experience in construction and in the actual supervision thereof. Second. The habit of keeping in touch, through the technical press and otherwise, with the current prices of all materials and labor that are used on engineering works. Third. The ability to grasp great problems, to follow mentally in advance their entire development and every probable detail of the construction, and to foresee eventualities. Fourth. The habit of general accuracy and of checking and counterchecking one's computations so as to avoid all errors of magnitude. Fifth. The faculty of systemization, so as to avoid the possibility of omission of important items of expense by the preparation of lists and the making of records. Sixth. Absolute honesty, developed to such an extent that the desire to materialize the project will in no way influence the mind to minimize the estimated expense or to omit any probable item thereof. Seventh. Good judgment to prevent a too honest intention or timidity from overloading the estimate and thus killing the enterprise at the outset. Eighth. The courage of one's convictions, in order to be able to endorse every estimate unhesitatingly and unequivocally and thus to compel clients to have confidence in the ability of their engineer. A good fundamental rule for the preparation of any estimate is not to try to round out to too great an extent each item of expense, or to increase it for contingencies, but to add a general item of contingencies at the end. Of course, one should not record the result of the calculations for any item with ridiculous accuracy, because that would shake the client's confidence in the business ability of his engineer; but it is easy enough to use round figures for each item without making it include any contingent allowance. This can be accomplished by diminishing as well as by augmenting the computed figures for the various items, striking an average for plusses and minuses so that the summation will reduce the resultant error to very small dimensions. If one adds a contingent amount to each item, he is liable to deceive himself in the summation by overloading the estimate; moreover, laymen like to see something added to an estimate to cover contingencies, and they would very properly look askance at any estimate not containing such an item of expense. However, if the contingency amount be made too great, the reader of the estimate will think that the engineer did not know his business, and that he was trying to cover his ignorance by a large allowance for the cost of the unknown. What percentage should properly be added to an estimate for contingencies will depend entirely upon the character of the construction and the probable difficulties to be encountered. For instance, in the case of a viaduct over a dry gorge in a well populated district where there are ample facilities for transportation and where the labor problem cuts no figure, the contingency allowance should be small-perhaps as low as two or three per cent; but in the case of a bridge over a deep and rapid river, with foundations far below the riverbed and at a place distant from civilization, it should be high, say from five to ten per cent. The author considers the latter figure to be the extreme limit for contingencies in good engineering practice; for any larger amount would indicate either that the engineer had not the proper data or that he was timid or inexperienced. The experienced engineer will not determine the amount to add for contingencies by either guess-work or snap-judgment; but will go through his entire list of items of cost and will consider each item separately, so as to decide whether it contains an element of uncertainty, and, if so, about how much should be allowed therefor, summing up all such allowances and perhaps adding a trifle for the absolutely unknown in order to obtain the general item. The following is a list of items of expense that will aid one in figuring the total cost of any bridge project. It is as complete as the author can make it, nevertheless he would be loth to guarantee that it contains every possible item for any case that may arise. It is understood that no particular project will require all of these items. Preliminary Expenses 1. Organization of the company, including lawyers' fees, state charges, and all small expenses such as typewriting. 2. Preliminary surveys and the plotting of the data accumulated therefrom. 3. All other preliminary engineering work. 4. Obtaining approval of plans by the War Department. 5. Drafting of complete detail plans and the specifications preparatory to construction. 6. All expenses connected with raising the money to build the proposed structure. Substructure Construction 1. Mass of cribs and caissons in place. 2. Mass of pedestals in place. 3. Foundation piles in place. 4. Concrete or masonry in shafts of piers, pedestals, and abutments. 5. Coping stones for piers, pedestals, and abutments. 6. Steel or stone protection for piers against ice. 7. Steel shells for piers. 8. Rip-rap for piers and abutments. 9. Mattress work for pier protection. 10. Earth and rock excavation. 11. Back filling. 12. Reinforcing metal for concrete. 13. Removal of old bridge. Superstructure Construction 1. Superstructure metal delivered at site. 2. Floor timber delivered at site. 3. Rails and their attachments delivered at site. 4. Hand-rails delivered at site. 5. Falsework. 6. Maintenace of traffic. 7. Erection of metalwork. 8. Painting of metalwork. 9. Framing and placing of timber. 10. Laying of rails. 11. Pavement, including base therefor. 12. Operating machinery of all kinds. 13. Machinery house and shelter house. 14. Electric lighting. 15. Counter-weights. 16. Toll house. 17. Concrete. 18. Reinforcing metal for concrete. Approaches 1. Clearing and grubbing of right of way. 2. Earthwork, including ditches and off-take drains. 3. Track on embankment, including ballast. 4. Frogs, crossings, switches, and signals. 5. Interlocking apparatus. 6. Culverts and tile drains. |