In summer 2016, SAME published a special issue of The Military Engineer commemorating the service and contributions of military engineers in the Vietnam War. To accompany the publication, we are featuring on Bricks & Clicks a special series entitled TME Looks Back: Vietnam featuring past articles, photos, and other material that first appeared in the magazine during the 1960s and early 1970s.
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Pipe Truss Bridge at Da Nang
By Lt. E. C. Lenz, Civil Engineer Corps, United States Navy
Situated on the coast in the middle of the I Corps, Republic of Vietnam, the Da Nang area is geographically divided by the Da Nang River (Song Han) into east and west land areas. On the west side lies the City and Port of Da Nang, the Vietnamese Army I Corps Headquarters, the Da Nang Air Base, and many other military installations from which most of the air and ground operations begin in that region. The military installations on the east side are almost entirely shipping, receiving, and support facilities with the exception of the III Marine Amphibious Force Headquarters and the Marble Mountain Air Facility. Most of the Vietnamese population on the east side receives support from the port and agricultural areas on the west side.
When the deepwater piers at Observatory Point on the east side were completed, a bottleneck of a major American supply route to Da Nang was broken. Further streamlining the flow of logistical supplies from the United States to their ultimate destination in Vietnam, the Naval Support Activity began a containership program. This meant that quantities of supplies could be brought into Da Nang, but the expeditious movement to the west was deterred by the river and was complicated by the ever-increasing flow of Vietnamese supplies from west to east. The existing bridge built by the French could not support heavy one-way loads such as containerized vehicles and other military traffic, and was too narrow for two-way traffic of wide equipment. A bridge which could support all of this was a necessity.
Plans were made and authorization was received for such a structure from the Officer in Charge of Construction, Republic of Vietnam (OICC, RVN) and the order for its construction was given on June 30, 1966.1 The structure was to be a Class 60 bridge with a total span of 1,680 feet made up of parallel pipe truss spans. It would provide for two lanes of traffic, but the bents were designed to carry four lanes for future expansion.
The three major bridge components are jackets, cap trusses, and spans. To avoid overtaxing the limited skilled labor market in Vietnam and thus delay the work, these components were fabricated and assembled in Poro Point in the Philippines.2 Poro Point had previously been used for similar work. Jigs were prepared, pipes were sized and cut to dimensions, and components were assembled and sent by barge to Da Nang.
On-site work, prior to and during delivery of the components, included dredging 124,800 cubic yards of fill for the bridge approaches, driving 1,200 feet of steel sheet piles for bulkheads and tie-back walls, and the construction of the bridge abutments. Dredging began on September 5, 1966, and was completed on January 6, 1967.
The bents are an adaptation of offshore structures used by the petroleum industry. Each bent consists of a jacket template (through which the bearing piles are driven) and the cap truss. The jackets are alike except for the length which varies according to the depth of water in which they are placed. The main members of the jacket are two rows of four vertical 26-inch-diameter pipes which will extend 5 to 10 feet into the river bed and 2 feet above mean high high water (Figure 1). It was assumed that each jacket bent would act as a rigid frame with the cap truss transferring wind and longitudinal loads to the jackets, and the jackets in turn transferring the loads to the piles.
The jackets were set by two barge-mounted cranes, one at each end. In this operation, the jacket was lowered into the water but suspended above the river bottom. A 24-inch-diameter pipe pile was then placed through the vertical jacket sleeve. After its exact location was surveyed, the pile was driven to a shallow depth. The pile location at the opposite end was then determined and the pile driven. At least four piles were driven to secure each jacket. All piles were then driven to bearing depth, welded to the jackets, and cut off at the specified height. Thus the jacket templates become an integral part of the supporting system.
Cap trusses with the same vertical configuration as the jackets were then set on and welded to the piles. The two rows of four vertical 24-inch pipes had beveled steel plates to facilitate field connection to the bearing piles. (The support beam of the cap truss was designed assuming the jacket structure was placed one foot off the center line of the bridge as such placement of the jacket would develop the critical load on the beam. Actually, field placement of the jacket varied only fractions of an inch off the center line.) The cap trusses are all alike with double beams welded together spanning the two middle piles and a single beam spanning the remaining sections (Figure 2).
Four 24-inch pipe piles were then driven to the proper bearing depth at either approach for abutment placement. In forming the abutments, H-piles were driven partly into the dredged fill to support the concrete formwork of the retaining wall which extended above the bridge approach fill. The formwork was stripped and after the concrete was cured, fillmaterial was placed to the full height of the wall. Continued settling of the fill forced the H-piles to move downward and, as a result, the bridge abutments were put in stress. Numerous stress cracks showed on the front face of the concrete abutment in which the bearing piles were encased. It was found that each abutment had been dislocated toward the river; the east and west abutments 5 and 7 inches, respectively.
To eliminate the stress, the fill was excavated to below the toe of the abutment and the concrete encasing the piles was removed so that the H-piles could be driven to bearing depth. In addition, batter piles were driven and tied into the abutment with the H-piles.
The first four of fourteen spans arrived on April 17, 1967, some two months behind schedule due to material shortage in Poro Point. With a Navy 100-ton floating crane and crew, the first four spans were set in an exceedingly smooth operation (Figure 3), with the total elapsed time of placement being only 4 hours. The span length of 120 feet, acting as a simple beam, was chosen to minimize the number of bents and thus hold down the cost of the bridge. A parabolic low truss bridge span was selected for the following reasons. The vertical clearance was not restricted; the weight of the completed span is less than that of any other type truss; the amount of material required for the structure would be less by comparison with other types; and fabrication and assembly procedure would be relatively simple, and could be conducted away from the construction site.
The top and bottom chords of the truss are continuous 18-inch-diameter pipe sprung into a parabolic curve (Figures 2 and 4). The span is divided into ten panels, each 12 feet long with the exception of the two end panels which are 10 feet, thus leaving a chord overhang of 2 feet. The chords were not closed to a point, to avoid difficulty with the floor beam framing. Vertical web members and the first diagonal at either end are 12-inch pipe and the rest of the diagonals are 10-inch pipe. Each span is 30 feet wide and 120 feet long, and the truss height is 14 feet. The apex of the top chord is 8 feet above the reference line and the apex of the bottom chord is 6 feet below the reference line. Total span weight without decking is 78 tons.
The spans rest on a fixed bearing connection at one end and an expansion bearing connection at the other allowing for 2 inches (1.5 inches for temperature change and .5 inch for expansion due to live load) of maximum expansion per span. The bearing connection is welded to a support beam which is parallel to the length of the bridge, held by the support beam of the cap truss. This system, therefore, made allowance for adjustments in any horizontal plane in case the bents were misplaced.
Prior to fabrication and assembly of the last two spans an error in placing the abutments was discovered which made it necessary to increase the length of the two end spans by 2 feet less the riverward movement previously noted.
The deck frame consists of floor beams and stringers. The beams are tied to each vertical web member of the parabolic truss with pipe trusses at the five middle floor beams to provide lateral support. The stringers have a 6-by-8-inch nailer to which the decking is spiked. The decking consists of two layers of 4-by-12-inch timber, the second layer being offset at 30 degrees to the first. A wearing course of 2-by-12-inch treadway completes the deck.
With the completion of the decking, security lighting, guard rails, hand rails, and paving of the approaches, the bridge was opened on July 25, 1967.
From a military point of view the bridge design is excellent. It requires a minimum of work at the site thus freeing troops for other duty, and its sectional components may easily be repaired or replaced if damaged. Other advantages include site adaptability, low cost, and speed of erection. Although the construction period for the bridge was somewhat lengthy, this may be attributed to material and manpower shortages caused by other higher priority projects. A conservative estimate for completion, assuming that required manpower and all components are available, is six weeks for a bridge of this length.
In an active military situation the necessity for bridges at particular locations is changed by events. A bridge of this type may be moved by simply dismantling the major components. The dismantling would involve cutting welds at three major points— the bearing connection, the jacket at the pile, and the cap truss at the pile. The components could then be taken elsewhere by barge as the military situation dictated. Thus an important innovation for a heavy-duty bridge has been added to military construction techniques.
The possibilities of parabolic pipe truss bridging for civilian needs appear to be widespread. The limiting factors would be site accessibility, load characteristics, and the size of river traffic. The mass production of components would reduce the cost considerably and provide almost “instant” bridges wherever and whenever desired. Where skilled bridge labor is scarce and the cost of bringing in personnel is prohibitive, this type of bridge is ideal because only a small crew of builders is needed.
Although somewhat unusual in appearance this parabolic truss bridge is an outstanding success and a most significant engineering development to emerge from the warfare in Vietnam.
1 By RMK-BRJ (Raymond International, Morrison-Knudsen, Brown and Root, and J. A. Jones).
2 By Atlantic, Gulf and Pacific Company as a subcontractor.
[reprinted from TME / September-October 1968]