CHAPTER VI. 

 BRIDGE ABUTMENTS AND PIERS. 



Introduction. An abutment is a structure that supports one end of a bridge span and at the 

 same time supports the embankment that carries the track or roadway. An abutment also 

 usually protects the embankment from the scour of the stream. 



A pier is a structure that supports the ends of two bridge spans. Piers must be designed 

 so as not to interfere with the flow of the stream, and care must be used to prevent undermining 

 the pier by the scour of the stream. 



TYPES OF ABUTMENTS. Masonry abutments may be classified under four heads, 

 Fig. i, (a) straight or "stub" abutments; (6) wing abutments; (c) U abutments; (d) T abutments. 



(a) The standard straight abutment of the N. Y. C. & H. R. R. R., shown in Fig. I, is an 

 excellent example of an abutment of this type. The earth fill is allowed to flow around the ends 

 of the abutment as shown. Straight abutments should not be used where the water will wash 

 the fill away. 



(6) A standard wing abutment of the N. Y. C. & H. R. R. R. is shown in Fig. i. The length 

 of the wings is determined by the width of the roadway, the allowable slope of the sides of the 

 embankment and the angle of the wings. The angle that the wings make with the face of the 

 abutment ordinarily varies from 30 degrees to 45 degrees for standard conditions. For skew 

 bridges and for unusual conditions the angle of the wing is variable. 



(c) A standard U abutment of the N. Y. C. & H. R. R. R. is shown in Fig. I. This is a 

 wing abutment with the wings making an angle of 90 degrees with the face of the abutment. 

 The wings are tied together by means of old railroad rails as shown. The wing walls run back 

 into the fill, which flows down in front of the wings. If the slope is liable to be washed away by 

 the scour of the stream the wings should be extended farther into the bank. 



(d) A standard T abutment of the South Bend and Michigan Southern Railway for a skew 

 span is shown in Fig. I. The T abutment is essentially a straight abutment with a stem running 

 back into the fill; the stem carries the roadway, supports the abutment, and prevents water from 

 finding its way along the back of the abutment. A T abutment may be considered as a U abut- 

 ment with the two wings in one. 



STABILITY OF BRIDGE ABUTMENTS WITHOUT WINGS. A bridge abutment 

 must be stable (i) against overturning, (2) against sliding, and (3) against crushing the material 

 on which the abutment rests, or the masonry in the abutment. The problem of the design of a 

 bridge abutment is essentially the same as the design of a retaining wall, for which see Chapter V. 

 The method of design will be shown by giving the calculations for a straight concrete abutment 

 for West Alameda Avenue Subway, Denver, Colo. 



Design of Concrete Abutment for West Alameda Avenue Subway, Denver, Colorado. The 

 height of the abutment is 21 ft. 6 in. from the bottom of the footing to the top of the bridge seat, 

 and 25 ft. o| in. to the top of the back wall. The following assumptions were made: Weight of 

 concrete, 150 Ib. per cu. ft.; weight of filling, w = 100 Ib. per cu. ft.; angle of repose of the filling, 

 ii to i (<f> = 33 42'); surcharge 800 Ib. per sq. ft., equivalent to 8 ft. of filling; maximum load 

 on foundation 6,000 Ib. per sq. ft. 



Solution. After several trials the dimensions given in Fig. 2 were taken. The stability of 

 the abutment was investigated for two conditions: (a) with a full live and dead load on the bridge 

 and on the filling, and (6) with no live load on the bridge and no surcharge coming on the filling 

 above the wall, it being assumed that a locomotive is approaching the bridge from the right, and 



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