U.S. Department of Transportation
Federal Highway Administration

TECHNICAL ADVISORY

T 5140.4
September 28, 1978

Par.

1. Purpose

2. Background

3. Recommendations

1. PURPOSE. To acquaint the Federal Highway Administration and States with problems recently associated with tied arch bridges and to emphasize the need for a thorough evaluation of alternate designs which provide more redundancy. The problems associated with these structures will also be made known.

2. BACKGROUND

   1. Tied arch bridges have experienced lamellar tearing in the hanger connections to the arch rib, due primarily to high restraint in the welded connections. Cracking has also been observed at weld details in the tie girder at floorbeam locations, and in the floorbeams at locationswhere diagonal struts have been provided from the bottom of the floorbeam to the stringers supported on top of the floorbeam.

   2. Several tied arch structures have been fabricated with electroslag welds in the tie girders and have required bolted splice repairs to the tie girder.

   3. Although these problems with electroslag welds are not indigenous to tied arch bridges, the serious consequences of weld cracking associated with the tie girder of a tied arch structure should not be overlooked. Repairs to rectify the above fractures have been very costly, time consuming and in many cases have inconvenienced the traveling public.

   4. While the tied arch structure may be economically competitive with other alternate designs, it is one of the most nonredundant structures, relying entirely on the capability of two tie girders to accommodate the total thrust imposed by the arch ribs.

   5. The effect of rib shortening in a tied arch is accentuated by the effect of tie lengthening and hanger stretch and the resultant moments in both the rib and tie girder can be largely eliminated by proper camber, provided the fabrication and erection account for this. If fabrication and erection procedures are such that reversing moments are not introduced into the structure, the actual stresses in the final structure may be quite different than designed.

3. RECOMMENDATIONS. Preliminary and detailed development which involves a tied arch structure should be given careful consideration in light of the previous comments. Existing structures having similar details should be inspected carefully, especially the tie girders.

W. J. Wilkes, Director
Office of Engineering