The Hartford Civic Centre Roof Collapse

Approximately three weeks ago, the post: Dangers of Using Software’s in Engineering became live on this blog. It has since generated a lot of controversies, with some professionals receiving it with open hands while others simply disagreed completely with the content of that post. For the records, “structures centre” is not in any way against the use of software in engineering. In fact, Software tools have made an immense contribution to the structural engineering profession, have removed many mundane tasks and have also allowed more complex behaviours to be examined. However, the fact remains that, software packages are only as good as the engineer who drives them. They are no replacement for actual structural engineering knowledge.

In this post, the collapse of the roof of the Hartford Civic Centre will be discussed. The role played by the state of the art software package employed in carrying out the analysis & design and finally, the lessons learnt.

The Hartford Civic Centre Building

The roof of the Hartford Civic Center Stadium, Connecticut, USA, was an innovative concept – a 91.4 m × 110 m space frame suspended 25.3 m over a 10,000 seat arena. Just four pylons, each set out from the edge of the building, was used as supports, providing a 13.7 m long cantilevered perimeter. This had 2300 members and a grid of 6.4 m wide, 9.14 m × 9.14 m modules (Figure 1).

Space frame of the civic centre building
Figure 1: Space frame module for Hartford Civic Center Stadium

There were a number of key features in these modules. First, the roof panels were not directly attached to the top chords, but instead were connected to posts protruding from these chords at discreet nodes. In using posts of different heights, a drainage gradient was created in the walls, and these posts would also reduce the bending moments being moved from the roof panels to the space frame.

Secondly, the diagonal members were used to provide lateral support to the top chords, with the intention of decreasing their unbraced length by half, from 9.14 m to 4.57 m.

Thirdly, this lateral support was crucial, as the top chords consisted of four steel angles shaped in a cruciform – a cross-section fundamentally weak in buckling.

In order to complete the design, the structural design firm, Fraoli, Blum & Yesselman, did indeed use a state-of-the-art software analysis package. In fact, the City of Hartford was persuaded to purchase the package, citing construction savings of half a million dollars through its use. When the design was finished, the Bethlehem Steel Corporation was given a construction contract and the Gulick-Henderson inspection and testing firm was retained to ensure successful completion. Construction began in 1972, with the space frame, together with all services, such as electrical ducts and ventilation ducts, being mounted at ground level, then mounted in place – a novel approach at the time.

Design Concerns

However, when the space frame was assembled (and still at ground level), Gulick-Henderson informed the designers that it was deflecting more than the 310 mm expected. The issue does not appear to have been addressed, and it was jacked 25.3 m up to the top of the pylons. At this stage, the maximum sag was measured and found to be not only larger but twice as large as anticipated. In the light of the press, the designers answered that these variations had to be anticipated in view of the simplification of the assumptions of the theoretical calculations.

It was then that the contractor installing the fascia panels discovered that the space frame had been significantly distorted, with the holes for the fascia support brackets not lined up with the space frame. For once, the designers did not express any concern, and the project manager even reminded the contractor that they were responsible for all project delays. The contractor then cut/extended the brackets and welded them in place, essentially working around the distortions.

In addition, multiple complaints were received from members of the public apparently the roof deflections were so pronounced that they were clearly visible and unsettling. The town of Hartford, worried, confronted the designers, who again defended the adequacy of their work.

Five years into the night of 18 January 1978, when the Civic Center had been exposed to its heaviest snow load since construction – heavy, but still only half of the design load. At 4:19 that morning, with the arena empty, the 1270tonnes space frame collapsed in its entirety (see post cover mage). What was most disturbing, though, was that just six hours ago, more than 5,000 people were sitting below, watching a basketball game.

Cause of Failure

The investigation that followed was undertaken by Lev Zetlin Associates, Inc. (LZA), who found that the structure had basically started to collapse as soon as it had been completed. A large number of design flaws have been identified, which are addressed comprehensively by a number of other writers. However, in this post, we are going to focus only on the major cause, the buckling capacity of the top chords, specifically the level of restraint provided by the bracing diagonal.

The primary issue was the assumption. While the designer and software package used a length of 4.57 m for the top chords, the investigation discovered that this was far from the case in practice. As the diagonal bracing was in the same inclined plane as the top chords, the deflection was confined to only one plane – the outer top chords were essentially free to deform horizontally. Furthermore, the lack of a post at some of these locations eliminated any possible constraint that the roof panels may have offered (Fig 1).

While the software package assumed an unbraced length of 4.57 m, in fact, the top chords were found to be actually unbraced, with a length of 9.14 m. The problem was further compounded by changes in the diagonal-to-top-chord connection details, where the diagonal connection points did not fit into the top chords see (figure 2).

Connection details, as built and as designed
Figure 2: As-built and designed connection details

These issues resulted in a significant overload: the outer top chords on the north-south and east-west sides were overloaded by 213% and 852% respectively. As the top chords buckled, the progressive collapse of the space frame resulted.

Lessons from Failure

There are so many lessons to be learnt from the collapse of the Hartford Civic Centre. However, only a few of these will be mentioned:

First, you would agree with me, that the engineers for the Hartford Arena solely depended on computer analysis to assess the safety of their design. The roof design was highly susceptible to buckling which was a mode of failure not included in that particular software package. This further supports the statement at the beginning of this post that “a software package is only useful when the engineer who drives them fully understands the implications of using them”. Do not over-rely on software packages

Secondly, the role of a professional should not be downplayed. Even though the architect recommended that a qualified structural engineer be hired to oversee the construction, the construction manager declined, citing increased cost and that he would oversee the project himself. After the collapse, he disclaimed all the responsibility on the grounds that a design error caused the collapse. He believed he was only responsible for ensuring that the structure was constructed correctly and not the performance of the structure.

As a result of the construction manager refusal to hire a qualified structural engineer for the purpose of inspection, no one realized the structural implications of the bowing structure. This collapse emphasizes the importance of having not just a qualified structural engineer in place but one who has a good understanding of the structure that is being built enough to recognise warnings of bad design and point them out before they become catastrophic.

Thirdly, when carrying out projects of this magnitude, a project peer review is usually sought, to be conducted by an independent designer. However, this was not done in the case of the Hartford Civic Centre Building. If a second opinion had been sought the design deficiencies responsible for the arena’s collapse probably would have been discovered.

Finally, we can conclude that the greatest cause of the Hartford Civic Centre collapse was as a result of “overconfidence”. The excessive deflections apparent during the construction were brought to the notice of the designers multiple times. The designers, very confident in their design and the software package used did not take time to recheck their work. An argument can be made for the Hartford designers giving that they specifically requested for the software package in order to complete the design. Why should they then doubt the result obtained from the package? To do so was tantamount to doubting the very essence of purchasing the software in the first place. However, we can all agree that an ethical engineer would not have blindly kept faith in his analysis result instead would’ve paid closer attention to unexpected deformations and investigate their causes.

References

1) Johnson R. G. (2015) Hartford Civic Center [Online] Available at https://failures.wikispaces. com/Hartford+Civic+Center+(Johnson) (Accessed: July 2015.

2) Brady S. (2015) ‘Hartford stadium collapse: why software should never be more than a tool to be used wisely, 93 (9), pp. 20–22

Thank you for Reading! Let us know in the comment section if there are other lessons to be learnt from this failure.

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