Through my research on https://interestingengineering.com/understanding-hyatt-regency-walkway-collapse this is my understanding on what happened during the Hyatt Regency Walkway collapse.
The structure of 3 sky walks failed and it is known as one of the worst structural collapses in history. It happened on Friday 17th July 1981. The 2nd and 4th sky walks gave way killing and injuring many people; on the night of the collapse there was a dance taking place in the hotel and so was busy with a lot of people. Each walkway was about 37 meters long and weighted about 29000 kilograms. The 2nd floor walkway was directly bellow the 4th floor walkway as per the design. On the night of the collapse it was documented that the 2nd floor walkway had about 40 people on it, and the 4th floor walkway had about 16-20 people on it.
During the construction of the walkway the contractors decided to alter the design which would mean that the connection between the 4th floor and the atrium roof would be double loaded. The support beams and tie rods that were used in the building had been designed to hold the weight of the skywalks and the people on them. However the new design with the addition of the extra weight of the atrium was the difference in the structure being stable and collapsing.
After the event Wayne G. Lischka, a structural engineer, started to investigate why the sky walks had collapsed in the first place. During his investigating he quickly realised that the changes that were made to the designs were the primary reason for the collapse. The two walkways were originally meant to be suspended form the atrium by one continuous rod. The manufacturers of the rods that the skywalks where suspended on showed concern at the changes at the time. They recommended that the tie rods should be threaded along the entire length of the rod, during construction the threads could be easily damaged, which would mean that you would be unable to use them anymore. Due to the risk of damaging the threads on the continuous tie rods the contractors decided to use two sets of tie rods. One set would now connect the 4th floor to the atrium roof and another set would connect the 2nd floor to the 4th floor. This change in the design would prove to be fatal, this change would mean that the 4th floor beams would have to support the 4th floor walkway as well as the 2nd floor which was directly underneath. This puts a lot of stress on the 4th floor beams and the tie rods and there is the combined dead weight of both of the sky walks, rather than a single tie rod supporting both of the skywalks. The box beams that split along the welds due to the added pressure applied by the new design, due to this break in the welds of the box beams the nuts that were being supported by, the nuts slipped through the gap created by the split.
Further investigation of the collapse discovered that there was low levels of communication, very little or no calculations and overall negligence, all of this also contributed to the collapse as well.
The diagram above shows the original design and what the design was changed to, this is the significant change that eventually lead to the collapse according to the investigators.
Tension structures
There are 4 different classes of buildings, class 1, 2A, 2B, 3. Each of these buildings have different requirements for robustness.
Categories in Scotland as as follows, Class 1 is for agricultural buildings. Class 2A buildings are buildings that come under commercial and shared residential buildings that are no higher than 4 stories, Class 2B buildings covers buildings that are 4 stories or higher but no higher than 15 stories. Class 3 buildings are buildings that are non-domestic that have no place in the other categories. This is as stated in the non-domestic handbook 2016.
When designing a building you should design it with the appropriate materials and the designers should carry out analysis on wither or not the the structure will be robust or not as if not then the design should be re thought even if this means the building takes longer to build or more costs are incurred. At the Hyatt Regency, the designers clearly never thought about this and would have rather have saved money, hassle and time by making the change to the walkway that prover to be fatal
In the non-domestic handbook section 1.2.1 disproportionate collapse its states that
“To ensure that buildings are designed and constructed to sustain a limited extent of damage or failure without a disproportionate level of collapse from an unspecified cause, the following procedure should be followed:
determine building risk group 

assess additional measures 

design and construct additional measures.â€
Gov.scot. (2018). Building Standards technical handbook 2017: non-domestic buildings – gov.scot. [online] Available at: https://www.gov.scot/publications/building-standards-2017-non-domestic/1-structure/12-disproportionate-collapse/ [Accessed 28 Nov. 2018].
A good example of a successful tensile structure is Denver airport. the reason why this is a good example is because at the time it was the largest fabric roof of its time and since then it has stood the test of time and all the extreme weather that it has to deal with throughout the year.
“Fentress says, recalling the process of creating the world’s then-largest and most famous fabric roof. “This freed us to create a superlight cutting-edge structure, a canopy saturated with light. In doing so, it saved the airport thousands of tons of steel, USD$115 million in budget, and reduced the construction schedule by nine months.†â€
In-text: (Architonic, 2018) Bibliography: Architonic. (2018). Denver International Airport by Fentress Architects | Airports. [online] Available at: https://www.architonic.com/en/project/fentress-architects-denver-international-airport/5100647 [Accessed 26 Nov. 2018].
The reason why tensile structures are so strong is due to the supporting steel masts and the ridge cables attached to the 17 pairs of masts. Cables anchored to the ground outside hold the membrane down to stop the wind from lifting the structure up. The reason why the roof is so strong is because when a load is a applied to the membrane it will slightly change shape, or the curvature. The stress being put on the structure in one direction will in turn resist the load being placed in the opposite direction therefore providing a strong stable structure. All of the above gives this unconventional structure robustness.