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  #1  
Old Posted Nov 18, 2009, 8:27 PM
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Interior Willis (Sears) Tower Design: A Question?

When one looks at construction pictures of John Hancock Center or the World Trade Center Towers in New York, the actual CORE of the building can be seen. But in all the photos of the Sears Tower under construction that I have seen, I have never seen the actual "CORE" of the tower. Is this because it doesn't have the same type of "CORE" that say, the World Trade Center Towers had. You know the steel reinforced concrete shaft that extends from the base to the top of the roof? I know it has to have atleast a small shaft for the two Express Elevators that take visitors straight from the 2nd Level Basement to the 103rd Floor. So my question is does the tower have a traditional "CORE" like one would think about in a skyscraper, or does each tube have a small core? If anyone knows about this I would love to hear from you because I understand the "bundled-tube" design, but I still realize the tower has to have a core because of the local elevators, plumbing, vents, pressure, and other stuff.

If it does have the same traditional core that we have all grown to love, is this visible in this picture as the elevator bank on the Franklin Street Lobby?


Is the outer wall of the core basically (not exactly) the "Granite Wall" that has the flags hanging off of it? Or back in the elevator banks?


Perhaps this is a dumb question, perhaps it's not. But I have always wondered if Sears had a similar core to other supertalls and if it served the same function that those "newer" cores were serving in their respective skyscrapers. I have had a hard time finding any info on the internet.
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Last edited by Lexy; Nov 18, 2009 at 9:19 PM.
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  #2  
Old Posted Nov 18, 2009, 10:24 PM
ChicagoChicago ChicagoChicago is offline
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  #3  
Old Posted Nov 18, 2009, 11:37 PM
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Intresting post and thank you for linking that! It appears that the base of the core on the bottom floors is not rectangular like you would typically think. It's more of a cross shaped core and as the tower rises and starts shedding floors, the core itself shrinks and becomes smaller and smaller. Until you reach the 90th floor and it essentially becomes what many would believe to be a "stereotypical" high-rise core from the 90th to the roof. Fascinating! I would love to be able to do a "behind the scenes" tour of Willis Tower and see the inner workings of the building and how it functions. I have always been fascinated by that!
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Old Posted Nov 19, 2009, 6:33 AM
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The World Trade Center towers did not have a steel reinforced concrete shaft.
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Old Posted Nov 19, 2009, 6:08 PM
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Quote:
Originally Posted by nequidnimis View Post
The World Trade Center towers did not have a steel reinforced concrete shaft.
I thought all the reports stated that the towers had a "steel reinforced cast concrete core structure"? Wouldn't that mean that they were constructed with "steal reinforced concrete"?

Either way, the question I have posed is about the Willis (Sears) Tower Core Structure and it's importance in supporting the structure itself. With the WTC Towers used only as an example and nothing more.



ON a side note:

I have been searching till my fingers hurt for pictures of the Sears Tower core while under construction. I would love to be able to the elevator shafts because that stuff has always fascinated me! LOL! I have always been curious how that worked in a tower like this one. I know that sounds weird, but I love the engineering and design behind the tower almost as much as I love the tower itself!
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Old Posted Nov 21, 2009, 5:30 AM
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Lexy,

The concept of tube and bundled-tube both allow that a structural interior is not required - at least in the way that you are thinking. In both of these systems the function of interior core is moved to the outside edges, where sets of closely spaced perimeter columns and rigidly connected beams provide resistance to the major structural loads including horizontal shear. This is the force that pushes the building back when the wind blows, but there also needs to be a means of carrying that load to the ground so it has to have a vertical component as well. Even so, tube structures also often use interior columns but they are assigned as gravity-load columns (meaning pure vertical loads derived from floors) and do not otherwise contribute to the lateral load resisting system.

Bundled tubes were conceived after it was realized that the major drawback to "single" tube design was the large accumulation of shear forces at corners - so-called shear lag - which reduced some of the basic efficiency. To counter this, smaller tubes were "tied" together, breaking up the total shear-lag into lesser pieces and improve the overall efficiency.

In other framing systems, a structural core might be employed - the type that you are thinking - that is reinforced concrete. However, as buildings get taller, this system becomes less efficient and tube concepts are favoured. In these cases, the continuous rigid plane of the concrete or block wall induces horizontal rigidity.

Another alternative are braced frames (e.g. ESB) and even braced tubes (e.g. John Hancock). In these systems the diagonal elements of X- and/or K-bracing provide the necessary load path to the ground.

So while tubes are efficient structurally, the trade off is the loss of open area at the perimeter - meaning possibly compromised window area. Recall that the WTC had 39" column centres but only about 20" left for glass. At the time, they weren't sure if it would be accepted but obviously it was. Similarly, the large diagonals on the exterior of Hancock were initially a concern because they thought offices with the reduced window area would be hard to sell. Strangely enough - once the building became accepted and even an icon - those offices now sell at a premium.

Your final question - as to why there is an apparent lack of a reinforced core in the construction photos - is that there is none (in tubes). Again, the rigidity necessary for stability is provided by the closely spaced exterior columns and heavy beams. There are "cores" where the elevators and mechanical services are located, but mostly comprised of conventional construction. The core in the WTC was simply steel column (to carry interior "gravity" loads) and covered with gypsum board.

This is perhaps the great irony of the WTC design: the rigidity of the tube system employed allowed that a structural core was not required - one that may have also prevented spread of the fire (i.e. the core of the building was easily compromised as a result of the attack and allowed greater distribution of flammable fuels). By the same token, it was the rigidity of the tube system, and it's incredibly over-redundant frame, which allowed the building to even stand after loss of great pieces of the exterior frame.
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Old Posted Nov 21, 2009, 4:01 PM
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Quote:
Originally Posted by Kelvin View Post
Lexy,

The concept of tube and bundled-tube both allow that a structural interior is not required - at least in the way that you are thinking. In both of these systems the function of interior core is moved to the outside edges, where sets of closely spaced perimeter columns and rigidly connected beams provide resistance to the major structural loads including horizontal shear. This is the force that pushes the building back when the wind blows, but there also needs to be a means of carrying that load to the ground so it has to have a vertical component as well. Even so, tube structures also often use interior columns but they are assigned as gravity-load columns (meaning pure vertical loads derived from floors) and do not otherwise contribute to the lateral load resisting system.

Bundled tubes were conceived after it was realized that the major drawback to "single" tube design was the large accumulation of shear forces at corners - so-called shear lag - which reduced some of the basic efficiency. To counter this, smaller tubes were "tied" together, breaking up the total shear-lag into lesser pieces and improve the overall efficiency.

In other framing systems, a structural core might be employed - the type that you are thinking - that is reinforced concrete. However, as buildings get taller, this system becomes less efficient and tube concepts are favoured. In these cases, the continuous rigid plane of the concrete or block wall induces horizontal rigidity.

Another alternative are braced frames (e.g. ESB) and even braced tubes (e.g. John Hancock). In these systems the diagonal elements of X- and/or K-bracing provide the necessary load path to the ground.

So while tubes are efficient structurally, the trade off is the loss of open area at the perimeter - meaning possibly compromised window area. Recall that the WTC had 39" column centres but only about 20" left for glass. At the time, they weren't sure if it would be accepted but obviously it was. Similarly, the large diagonals on the exterior of Hancock were initially a concern because they thought offices with the reduced window area would be hard to sell. Strangely enough - once the building became accepted and even an icon - those offices now sell at a premium.

Your final question - as to why there is an apparent lack of a reinforced core in the construction photos - is that there is none (in tubes). Again, the rigidity necessary for stability is provided by the closely spaced exterior columns and heavy beams. There are "cores" where the elevators and mechanical services are located, but mostly comprised of conventional construction. The core in the WTC was simply steel column (to carry interior "gravity" loads) and covered with gypsum board.

This is perhaps the great irony of the WTC design: the rigidity of the tube system employed allowed that a structural core was not required - one that may have also prevented spread of the fire (i.e. the core of the building was easily compromised as a result of the attack and allowed greater distribution of flammable fuels). By the same token, it was the rigidity of the tube system, and it's incredibly over-redundant frame, which allowed the building to even stand after loss of great pieces of the exterior frame.
Kelvin, your post is most appreciated! I really think this answers my question best. Basically, the elevator shafts within the Sears Tower are located within their own "cores". If they are local, Skylobby feeding elevators then obviously the "shaft" they reside in terminates at that Skylobby floor and above that become rentable/usable space in some cases. Basically there is a "core" to the Sears Tower, or a "Spinal Cord" that runs the length of the building but it's not structurally the same as the WTC Towers in that it acts to house the two Express Elevators, Stair wells, and Plumping/Electrical/Ventilation shafts and that's really it. It is not there for the purpose of structural reinforcement so to speak. That is what the outside curtain walls of the different "tubes" are for. They (the vertical skeleton girders of each tube) take the lateral loads and the horizontal movement is limited by the "X-bracing" (mechanical floors) and horizontal girders on all the floors of the Sears Tower. I would love to see a "walk around" video of the Sears Tower under construction. You know, one made on site while they are working on the tower. Just so you could see how it "looks" without all the Sheetrock. Heck, even some photos made while inside the building during construction would be real enlightening!!
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  #8  
Old Posted Apr 8, 2011, 7:32 AM
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System of double-decker express elevators provides effective capacity of vertical transport, passengers in two skylobbies transferred to a single local elevators for individual levels of experience.
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