L2 Hyatt Regency Hotel

Added: 2026-05-31

[00:00:00]hello everyone and welcome back this week's lesson is going to be primarily covering gravity loads and gravity load paths a topic you've hopefully covered in the past in previous structures courses but it's an important one so we'll be going over it again but before we jump into that conversation I wanted to transition from our conversation in the previous lessons on roles and responsibilities recall that last week we laid out the specific roles that Structural Engineers and Architects have on a project however it's always important to remember that we all have a shared responsibility to ensure that the building is safe for the public so with that said I've got a question for you let's Flash Forward a few years into the future you are a practicing architect registered and let's say that you are designing this lovely new Atrium for a hotel and let's say you have this concept of having these tiered walkways suspended from cables hanging down from the roof structure as shown on the screen your structural engineer provided you with details calling for a single hanger Rod that runs continuously from the roof through the upper tier walkway and all the way down to the lower tier walkway this Rod has bolts below each of the walkways that will allow the gravity load to transfer back up into the rod and eventually to the structure in the roof above so so far it looks so good you're happy with the result and your drawings make it through plan check no problem however during construction the general contractor approaches you with a sour look on their face they are not too happy at all with the idea of purchasing and handling a single hanger rod that's going to be multiple stories in length the likelihood of such a lengthy piece of relatively thin metal getting bent during the construction process is quite High so they proposed an alternate solution they submit to you and your team their proposed solution as an RFI so this would be the original detail with the bar continuous through the structure of the walkway and this is their proposed solution in their RFI so you can see here that they've broken up this single hanger rod into two hanger rods one that goes from roof to the upper walkway and then a second one that extends down to the lower tier walkway now your structural engineer looks over the proposed detail and gives it the thumbs up they explain to you that if you trace the gravity load path all the way up the single hanger Rod versus all the way up the double hanger Rod you end up with the same amount of force in this upper rod in both scenarios so in the continuous you have the weight of one walkway going up the rod and then above the second walkway you have the weight of two going up that Hangar Rod up there the same thing would occur here you would have the weight of one and up above you still have the weight of two walkways so as long as they designed this single hanger rod for the weight of both walkways which they did splitting it into two would make no difference to the hanger rods both rods will need to carry the weight of two walkways they tell you there is no significant change here and in fact splicing long pieces of Steel is something we do quite often when building structures so your general contractor is telling you that this will save both time and money and your structural engineer is saying that it will not affect their design my question to you is this do you approve of this change keep in mind that if you say no to the change you will need to explain your reasoning to an owner that will likely be very upset at the idea that you are rejecting a cost savings alternative approved by your structural engineer so in other words you better have a pretty good reason for saying no on the other hand if you said yes I have even worse news for you in the summer of 1981 the Hyatt Regency Hotel in Kansas City Missouri hosted a gathering in the large Atrium space part way through the night catastrophe struck as the walkways on the second and fourth floors collapsed killing over a hundred people instantly and injuring over 200 more at its time this event was the worst structural failure in United States history following a thorough investigation of the collapse experts determined that the critical flaw that led to the collapse was a change made to a structural detail proposed by the contractor and mistakenly approved by the structural engineer of course that detail that was changed was the one we just discussed now I mentioned earlier that splicing steel elements is completely normal in structural design and that is true every steel building you've ever been inside of has had some kind of splice detail but those buildings aren't going to collapse anytime soon so what made this splice different well let's reconsider what the structural engineer told us earlier they said that the proposed change would not affect the amount of force in the rod that statement is true up above the second walkway both rods are carrying the weight of two walkways so the failure did not occur in the rods what the structural engineer failed to consider is the change in the amount of force in the bolt holding the second walkway recall what I told you about how the gravity load makes its way into the rod in the first place the way that the load is transferred from the structure of the walkway into the hangar Rod is via these bolts underneath the walkway so underneath here there is a bolt in the case of the single hanger rod each bolt is responsible for transferring the weight of only one walkway into the hangar rod however in the case of this detail note how this bolt here will be responsible for not only carrying the weight of the hanger rod and the walkway above but also the weight from the walkway below in other words changing this detail effectively doubled the amount of force on this volt you can see that very clearly in this video demonstration and I'll provide a link to the full video in the additional reading section for this week's lesson so note how when he pulls up on the lower walkway here the upper walkway is not affected at all similarly when he pulls up on that upper walkway the lower walkway is not affected at all thus they are independent of one another however in the second scenario so this is the change that was proposed note how when he pulls on the lower walkway again the upper doesn't move however when he pulls on that upper walkway the lower walkway moves along with it so again the original design this bolt here is carrying the weight of only one walkway however here the load on the bolt is doubled another way to visualize these two scenarios as he explains in the video is to imagine that you and a friend are both hanging from a rope in the first design the original design both you and the friend are gripping the Rope independently in the second scenario in the design change your friend is holding on to your ankles the total weight that the Rope experiences is the same in both cases however obviously in the second scenario it will be significantly more challenging for you to maintain your grip on the Rope now I'm walking you through this example for two main reasons one we are going to be talking about gravity loads and gravity load paths in the upcoming lessons and this example shows how crucial it is for us to understand exactly how our floor loads travel from their origin all the way down to the foundation any weak links no matter how small or insignificant they may seem can have a devastating impact on the whole system in this example the engineer could have provided a rod that was five times as strong as the one that was used they could have bulked up this floor deck to a massive size they could have bulked up this girder underneath to a massive size but ultimately the bolt here would still fail and the structure would still collapse so every piece of the gravity load path is important the other reason I share this with you is to reinforce what we talked about in lesson one that is you have responsibility to ensure that what is getting built under your watch is safe for the public even if approving this particular RFI wasn't your direct role you still have a responsibility to check it as the architect it's ultimately your name your reputation on the line not to mention your conscience as well so yes what we do is fun and interesting we get to shape the built environment we get to push the boundaries of what is possible but always keep in mind that we have this responsibility to keep people safe as well all right that's all for this video I'll see you next time