Chapter 4- Kemper and Hartford area roof collapse

Question 1

What is a definition for structural redundancy and

why must it be considered a basic requirement in the design of any large structure

Answer 1

1. structural redundancy allows the load to be carried in more than one structural member. (through a side path).
2. must be in large structures whose failure may cause extensive damage and loss of life.

Question 2

23. In the collapse of the Kemper Arena roof on 4th June 1979 what was the poor design detailing of 36 (out of the 42) hanger assemblies that were used to transfer the roof’s loading to the three space frames ?

What destructive action did this poor detailing generate?

And how did the other 6 hanger assemblies have an acceptable detailing to transfer safely both gravity and wind loadings

Answer 2

1. The hanger assemblies (36 out of 42) had a design flaw where the details did not include adequate drainage provisions. This allowed water to accumulate around the hanger connections. The accumulated water led to significant corrosion of the hanger rods and other components. The lack of drainage led to prolonged exposure to moisture, accelerating the corrosion process. there was also a lack of redundancy
2. increased stress concentration and material degradation due to the corrosion.
3. the 2 hinge hangers allowed the roof to act like a pendulum, while the single hinge hangers limited the horizontal roof motion to the bending deflection, while also having better drainage system and corrosive protection

Question 3

Because of the poor design detailing to 36 of the hanger assemblies should four A490 high-strength bolts per hanger have been used to connect the base plates to the trusses’ top chords

Answer 3

Yes, high-strength bolts should have been used to connect the base plate to the trusses to prevent the single joint from being limited to horizontal forces. Originally, the 36 hangers were hinged at the top and connected rigidly to the top chord of the trusses.

Question 4

What is ponding

and why is it to be avoided in roof structure design

Answer 4

1. ponding is the acclimation of water on a flat roof surface.

2.this causes issues as added an additional load to the roof surface leading to increased stress where deflection might occour.

Question 5

With the bolting to hanger assembly No. 1 shown in Fig. 4.5 being the first to fail what would have happened to the actions (forces and moments) carried by this hanger had there not been a lack of redundancy in the roof structure

Answer 5

the single hanger failed due to bolt fatigue.
the adjoinign hangers were unable to carry the additional pull ( force) from the failed hanger.
this caused a chain reaction as the result in lack of redundancy.

Question 6

To what has the root cause of Hartford Arena roof collapse on 17 January 1978 been attributed?

Answer 6

inadequate design and construction of the roofs space frame structure.
there was a design flaw in the method connecting the roofd space frame to the concrete support frame.
the failure occurred in the box beam jointand the plate did not have enough strength.

Question 7

In the construction of the steel space frame for the Hartford Arena roof, the frame’s top horizontal bars were configured in the shape of a cross, built up from four steel angles (Fig. 4.13). By giving a reason to justify your answer, explain if this is a good cross-sectional shape for members taking compressive force

Answer 7

1. the shape was not good for compressive strengths.
2. this is due to the cross bending and twisting under relative small stresses.
3. the force transmitted between diagonal and horizontal bars caused bending stressed in the bars.
   as the top bars intersected at one point while the diagonals at another.
4. the cross could buckle easily

Question 8

A unique aspect of the construction procedure was to assemble the roof’s frame on the ground and to lift it up by means of hydraulic jacks, fixed to the top of the four corner pylons (see Fig. 4.11). In January 1973 the roof, now in its final position, but not yet burdened by the weight of the roof deck, was measured to have a deflection at the centre twice that predicted by the computer analysis. How did the engineers justify their lack of concern, and would you expect this to be the situation today when analysing the same problem by computer analysis

Answer 8

1. the engineers believed that the weight of the roof deck was added and it would cause the structure to settle, reducing the overall deflection .
2. in calculations they did not account for structures initial deflection without the roof deck was an indication of potential issues in the overall design of construction
3. with todays technology and computers engineers have a more through approach as they can predict deflections.

Question 9

What was the difference between the configuration of the actual structure to the space frame of the Hartford Arena roof and that assumed by the designers’ in the mathematical model?

What form of failure did the designers therefore overlook, when using the results of the computer analysis?

Also, how could the structural problem have been avoided

Answer 9

1. the structural model assumed that all chord bars are braced laterally by the inclined secondary diagonals which was the case for the interior frame. but along the frame the diagonals and top bars were in the same inclined plane, hence the buckling. therfore should have been 4 times stiffer.
2. the designers overlooked the progressive failure as one member failure. The transferred load was too much for the adjacent member, so it also failed.
3. this could have been avoided with more stronger members to carry the load, therefore redundancy

Question 10

Following the collapse of the Hartford Arena roof, a computer model with correct buckling lengths and stiffnesses of all bars was “loaded” in steps. In terms of the load value on the roof of the day of failure being 100%, what conservative load value was calculated for the first bar to fail and what was the final load value for progressive collapse?

Do you consider this gives the roof structure an acceptable margin of safety and give a justification to your answer

Answer 10

1. the calculated load was 13% below the total load actually on the roof that day.
   the conservative live load was 20% above the actual measured snow and ice load .
   the final load was somewhat above this value to cause the buckling of the first bar.
2. this does not give the roof an acceptable safety margin as would have required 5000 bars to prevent the roof from collapsing