De Pontibus: A Pocket-Book for Bridge Engineers – Chapter II

De Pontibus – Waddell

First Principles of Designing


Principle I. Simplicity is one of the highest attributes of good designing.


Principle II. “the easiest way’s the best.”


Principle III. The systemization of all that one does in connection with his professional work is one of the most important steps that can be taken towards the attainment of success.


Principle IV. There is an inherent sense of fitness in the mind of a well-trained and well-balanced metal-work designer, which sense of fitness is of the greatest importance in all that he does.


Principle V. There are no bridge specifications yet written, and there probably never will be any, which will enable an engineer to make a complete design for an important bridge without using his judgment to settle many points which the specifications do not properly cover; or as Mr. Theodore Cooper puts it: “The most perfect system of rules to insure success must be interpreted upon the broad grounds of professional intelligence and common sense.”


Principle VI. In every detail of bridge-designing the principles of true economy must be applied by every one who desires to be a successful bridge engineer.


Principle VII. In bridge-designing rigidity is quite as important as element as is mere strength.


Principle VIII. The strength of a structure is measured by the strength of its weakest part.


Principle IX. In bridge-designing provision must always be made for the effect of impact, either by increasing the calculate total stresses by a varying percentage of the live-load stresses, or by decreasing the intensities of working stresses below those allowed for statically applied loads.


Principle X. In making the general layout of any structure, due attention should be given to the architectural effect, even if the result be to increase the cost somewhat.


Principle XI. For the sake of uniformity, and to conform to the unwritten laws of fitness, it is often necessary in bridge-designing to employ metal which is not really needed for either strength or rigidity.


Principle XII. Before starting a design, one should obtain complete data for same.


Principle XIII. A skew-bridge is a structure the building of which should always be avoided when it is practicable.


Principle XIV. The best modern practice in bridge-engineering does not countenance the building of structures having more than a single system of cancellation, except in lateral systems where the resulting ambiguity of stress distribution is of minor importance.


Principle XV. The employment of a redundant member in a truss or girder is never allowable under any circumstances, unless it be in the mid-panel of a span having an odd number of panels, in which case, for the sake of appearance, two stiff diagonals can be used.


Principle XVI. The use of a curved strut or tie in bridge-designing for the sake of appearance (or for any other reason) is an abomination that cannot for an instant be tolerated by a good designer.


Principle XVII. In all structural metal-work, excepting only the machinery for operating movable parts, no torsion on any member should be allowed if it can possibly be avoided; otherwise, the greatest care must be taken to provide ample strength and rigidity for every portion of the structure affected by such torsion.


Principle XVII. The gravity axes of all the main members of trusses and lateral systems coming together at any apex of a truss or girder should intersect in a point whenever such an arrangement is practicable; otherwise the greatest care must be employed to insure that all the induced stresses and bending moments caused by the eccentricity be properly provided for.


Principle XIX. Truss members and portions of truss members should always be arranged in pairs symmetrically about the plane of the truss, except in the case of single members, the axes of which lie in said plane of truss.


Principle XX. In proportioning main members of bridges, symmetry of section about two principal planes at right angles to each other is a desideratum to be attained whenever practicable.


Principle XXI. In both tension and compression members the centre line of applied stress must invariably coincide with the axial right line passing through the centres of gravity of all cross-sections of the member taken at right angles thereto.


Principle XXII. The principle of symmetry in designing must be carried even into the riveting; and groups of rivets must be made to balance about central lines and central planes to as great an extent as is practicable.


Principle XXIII. In proportioning members of bridges to meet stresses and combinations of stresses it is important to consider duly the quality, frequency, and probability of the action of said stresses or combinations of stresses.


Principle XXIV. In all main members having an excess of section above that called for by the greatest combination of stresses, the entire detailing should be proportioned to correspond with the utmost working capacity of the member, and not merely for the greatest total stress to which it may be subjected. In this connection, though, the reduced capacity of single angles connected by one leg only must not be forgotten.


Principle XXV. In every bridge and trestle adequate provision must be made for the contraction and expansion of the metal.


Principle XXVI. No matter how great its weight may be, every ordinary fixed span should be anchored effectively to its supports at each bearing on same.


Principle XXVII. The bridge-designer should never forget that it is essential throughout every design to provide adequate clearance for packing, and to leave ample room for assembling members in confined spaces.


Principle XXVIII. Although for various reasons engineers are agreed that field-riveting should be reduced to a minimum, such an opinion should not be allowed to militate against the employment of rigid lateral systems. All designs should be arranged so that the field-rivets can be driven readily


Principle XXIX. Rivets should not be used in direct tension.


Principle XXX. For members of any importance two rivets do not make an adequate connection.


Principle XXXI. Designs must invariable be made so that all metal-work after erection shall be accessible to the paint-brush, except, of course, those surfaces which are in close contact either with each other or with the masonry.


Principle XXXII. In multiple-track structures, if any bracing-frames be used between panel points to connect the longitudinal girders of adjoining tracks, they must be designed without diagonals, in order to prevent the transference of any appreciable portion of the live load from one pair of girders to any other pair of same.


Principle XXXIII. In bridges, trestles, and elevated railroads, the thrust from braked trains and the traction should be carried from the stringers or longitudinal girders to the posts or columns without producing any horizontal bending moment on the cross-girders.


Principle XXXIV. In trestles and elevated railroads the columns should be carried up to the tops of the cross-girders or longitudinal girders and be effectively riveted thereto.


Principle XXXV. Every column that acts as a beam also should have solid webs at right angles to each other, as no reliance can be placed on lacing to carry a transverse load down the column.


Principle XXXVI. In trestles and elevated railroads every column should be anchored so firmly to its pedestal that failure by overturning or rupture could not occur in the neighborhood of the foot if the bent were tested to destruction.


Principle XXXVII. All pedestals for trestles, viaducts, and elevated railroads should be raised to such an elevation as to prevent the accumulation of dirt and moisture about the column feet, and all boxed spaces in the latter should be filled with extra-rich Portland-cement concrete.


Principle XXXVIII. In designing short members of open-webbed, riveted work, it is better to increase the sectional area of the piece from ten to twenty-five per cent than to try to develop the theoretical strength by using supplementary angles at the ends to connect to the plates.


Principle XXXIX. Star-struts formed of two angles with occasional short pieces of angle or plate for staying same do not make satisfactory members. Better results are obtained by placing the angles in the form of a T.


Principle XL. In making estimates of weights of metal the computer should always be liberal in allowing for the weight of details.


Principle XLI. In general details must always be proportioned to resist every direct and indirect stress that may ever come upon them under any possible condition, without subjecting any portion of their material to a stress greater than the legitimate corresponding working stress.


Principle XLII.There is but one correct method of checking thoroughly the entire detailing of a finished design for a structure, viz.: “Follow each stress given on the stress diagram from its point of application on one main member until it is transferred completely to either other main members or to the substructure, and see that each plate, pin, rivet, or other detail by which it travels has sufficient strength in every particular to resist properly the stress that it thus carries; check also the sizes of such parts as stay-plates and lacing, which are not affected by the stresses given on the diagram, and see that said sizes are in conformity with the best modern practice.