On 17 June 1958, eight months after the collapse of the Peace River Bridge (see Chapter 8), and while the same was being dismantled, yet another major bridge in British Columbia collapsed, however, this time a bridge under construction. In Vancouver, over the Burrard Inlet at the second narrows – between the Strait of Georgia at the Pacific Ocean and the mouth of the Fraser River – the construction of a new highway bridge had begun in November 1957 (Fig. 9.1).
The Second Narrows Bridge was built alongside an existing old bridge for com- bined railway and highway traffic built in 1925 (the first Second Narrows Bridge).
Fig. 9.1 The Second Narrows Bridge is located in Vancouver in British Columbia, Canada (see also Figs. 8.1 and 8.2).
86 m 142 m 335 m
Fig. 9.2 Elevation of the Second Narrows Bridge under construction.
Fig. 9.3 The temporary truss (i.e. the falsework) supporting the north anchor span during construction. (McGuire: Steel Structures. With kind permission of Pearson Education Publications).
This existing bridge had been hit by ships on numerous occasions, and it was there- fore decided that the second bridge – constructed for highway traffic only (having six lanes) – should be a high-level bridge. It was constructed as a continuous truss bridge (in all 1292 metre long), with the main 335 metre navigation span as a cantilever construction having two anchor spans (142 m each) on either side (Fig. 9.2).
At the time of the collapse the north anchor span was being erected, and in order to reduce the maximum cantilever length a centrally located falsework support was being used. In contrast to the anchor spans of the Quebec Bridge back in 1907 – which were constructed using an all-span length falsework (see Fig. 4.6) – it was in this case decided to limit the amount of extra supports to just one because of the water (Fig. 9.3).
Fig. 9.4 The collapse of the Second Narrows Bridge. (McGuire: Steel Structures. With kind permission of Pearson Education Publications).
On the afternoon of 17 June 1958, despite being supported, the north anchor span collapsed without any warning through the failure of the temporary truss (Fig. 9.4).
When the anchor span fell the adjacent span also collapsed, as it lost its support when the pier was pulled away. A total of 18 workmen and engineers were killed. A Royal Commission of Inquiry investigated the cause of the collapse and soon found the answer. It was not the buckling capacity of the temporary truss that was inadequate, as could have been expected; instead it was found that the lower transverse beam at the bottom of the falsework truss had failed. The purpose of this beam was to distribute the concentrated load – coming from the vertical members of the truss – in order to spread it over a larger area on top of the piles. The profile – or profiles, for there were probably more than one, lying parallel close to each other as a grillage – was a wide-flanged rolled standard steel girder (36 WF 160) (Fig. 9.5).
As the concentrated load is transferred directly through the web to the support there is no risk of web crippling (i.e. local buckling of the upper part of the web) as no shear force is present (i.e. the girder is not subjected to bending), instead the instability phenomenon to consider in design should be global buckling of the entire web – an instability phenomenon that is more or less similar to the Euler buckling case where both ends of a strut are fixed (Fig. 9.6).
If the upper flange, though, is not fixed in the transverse direction, the effective length factor is somewhere in between fixed end conditions (k= 0.5) and that of a free-swaying cantilever (k= 2.0), which have the result that the capacity to carry concentrated loads is lowered even further (Fig. 9.7).
862 26
17
26 305
Fig. 9.5 Cross-section of the transverse girder.
L k.L
Fig. 9.6 Overall buckling of the entire web when the load is transferred directly through the web (with a column effective length factor k equal to 0.5).
The exact conditions for the transverse girder(s) is difficult to ascertain in detail, but nevertheless, the capacity was far from being enough as the huge cantilever up above lost its temporary support and plunged into the water below (Fig. 9.8).
Swan, Wooster & Partners – the designer of the bridge – shamefully blamed the error in design on a young and inexperienced engineer, who, as a matter of fact, was among the casualties. This engineer was supposedly to have used the flange thickness (26 mm)
k 0.5 0.5 k 2 k 2
Fig. 9.7 The effective length factor k with respect to web buckling for different boundary conditions.
Fig. 9.8 When the web of the temporary transverse girder below buckled the entire weight of the cantilever above was transferred to the truss itself – which it had not been designed for – so it broke in bending and collapsed (see also Fig. 9.4).
instead of the web thickness (17 mm) in computing the resistance of the transverse girder to concentrated loads, but even if this is true there are two remarks to be made. First, that there, as a rule, never should be relied upon an unstiffened web to carry a concentrated load, whatever the real capacity is, and secondly, that any computation made by an inexperienced engineer at a company should, also as a rule, be checked (and double-checked) by his supervisor (a senior engineer at the office). The company should then stand united and not blame one of their employees if a mistake is made!
A couple of two small web stiffeners at each end of the transverse girder should have saved the bridge (stabilizing the web and the upper flange in these locations (Fig. 9.9). In figure 9.3 it is indicated that vertical web stiffeners actually were used, however, not directly under the load application points (and these indications could also very well be just bolting, holding the grillage tight together).
And finally, to summarize what really happened: too much of the concentration was focused on the construction up above of the cantilever truss, that a proper design of a somewhat negligible, but for the load-carrying capacity ever so crucial transverse girder down below was neglected.
Fig. 9.9 Vertical web stiffeners at the load application points would have saved the bridge, but were unfortunately missing.
Two years later the construction was, however, completed, and on 25 August 1960, the bridge was officially opened. In order to commemorate the lives of the workmen that were lost in the collapse in 1958 (and also to those who completed the bridge, one could imagine), the name of the bridge was changed to Ironworkers Memorial Second Narrows Crossing in 1994.