Grit1645

Nov 20 2008, 10:40 AM

What are you asking for, mynameis? The core columns were not filled with concrete.

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The building's design was standard in the 1960s, when construction began on what was then the world's tallest building. At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells. Steel beams radiate outwards and connect with steel uprights, forming the building's outer wall. All the steel was covered in concrete to guarantee firefighters a minimum period of one or two hours in which they could operate - although aviation fuel would have driven the fire to higher-than-normal temperatures. The floors were also concrete. The building had to be tough enough to withstand not just the impact of a plane - and the previous bomb attack in 1993 - but also of the enormous structural pressures created by strong winds.

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Fire Resistance of Concrete-Filled Steel Columns

Construction Technology Update No. 6, May 1997

by V.K.R. Kodur

Filling a hollow steel column with concrete increases the column's load-bearing capacity as well as its fire resistance. Concrete reinforced with steel fibres or bars both offer advantages over plain-concrete filling. This article discusses the fireprotection advantages and other benefits achieved with the various types of concrete filling, with emphasis on steel-fibre technology.

Steel hollow structural section (HSS) columns are very efficient in resisting compression loads and are widely used in the construction of framed structures in industrial buildings. Steel is vulnerable to fire, however, and in the past, building codes normally required fire protection for these columns. This effectively eliminated the potential for architects to create designs using exposed steel.

Research conducted over a ten-year period at NRC's Institute for Research in Construction (IRC) using a large test furnace has shown that filling steel columns with concrete will increase both their loadbearing capacity and their fire resistance. The need for external fire protection for the steel is eliminated allowing architects and engineers to expose the steel in their designs — without jeopardizing fire safety. Added benefits include an increase in usable floor space and a reduction in fire-protection costs.

The new Museum of Flight in Seattle, Washington, home to one of the most extensive aircraft collections in the world, owes its sense of transparency and openness, in part, to its use of concrete-filled steel columns. One of the design requirements for the museum was that the exhibits be visible from the outside and the sky from the inside. This was achieved by enclosing the building in glass and supporting it by an unobtrusive structure (see Figure 1). The architects, aware that the greater fire resistance of concrete-filled steel columns would make them an obvious choice to fulfill this structural requirement, used IRC test data and computer programs to demonstrate to code authorities in the Seattle area that the required minimum one-hour fire-resistance rating at full design loads could be obtained.

Figure 1. The Museum of Flight in Seattle, Washington uses a bar-reinforced concrete-filled steel-column structure to help achieve its sense of openness and transparency.
Demonstrated Advantages

In recent studies supported by the North American steel industry, IRC researchers tested and developed computer models for both square and circular HSS columns. They investigated the influence of significant factors such as type of filling (plain concrete, bar-reinforced and steel-fibre-reinforced concrete), concrete strength, type and intensity of loading, column dimensions and slenderness ratio.

These studies demonstrated that HSS columns filled with steel-fibre-reinforced concrete have greater fire resistance than those filled with plain concrete and that their use permits more cost-effective construction. Steel-fibre-reinforced filling offers the following advantages over plainconcrete filling:

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greater tensile strength added to the composite system,
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less cracking under service conditions, and
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greater resistance to deterioration from material fatigue, impact, shrinkage, and thermal stress.

Behaviour of Concrete-Filled HSS Columns in Fires

The performance of concrete-filled steel columns is unique. At room temperature, the load is carried by both the concrete and the steel. When the column is exposed to fire, however, the steel carries most of the load during the early stages because the steel section expands more rapidly than the concrete core. At higher temperatures, the steel section gradually yields as its strength decreases, and the column rapidly contracts at some point between 20 and 30 minutes after exposure to fire. At this stage, the concrete filling starts carrying more and more of the load. The strength of the concrete decreases with time and ultimately, when the column can no longer support the load, either buckles or fails in compression. The time at which the column fails determines its fire-resistance rating.
Fire Performance of Different Types of Concrete Filling

Plain concrete
The fire resistance of columns filled with plain concrete is limited to between one and two hours. Failure occurs because of a reduction in the compressive strength of the concrete with increased temperature together with rapid crack propagation in the concrete, resulting in premature failure of the concrete core. Fire resistance of longer than one hour can be achieved by reducing the load levels. One cautionary note is that the fire resistance of these columns is very sensitive to eccentric loads, i.e., where loads act away from the longitudinal axis.

Steel-fibre-reinforced concrete
The fire resistance of steel columns can be improved significantly by filling them with steel-fibre-reinforced concrete instead of plain concrete (see Figure 2). Fire-resistance ratings of up to three hours can be obtained without any reduction in the load. The presence of steel fibres, about 2 percent by mass, reduces cracking in the concrete and contributes to the compressive strength at elevated temperatures, thus preventing premature failure of the concrete core.

These benefits can be attributed to the superior mechanical and thermal properties of steel-fibre-reinforced concrete at elevated temperatures, and to the containment effect provided by the steel fibres to the concrete core.

The increased cost of using steel- fibrereinforced concrete rather than plain concrete as a filling for HSS columns can often be justified by the numerous advantages offered, including:

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better deformation behaviour, resulting in gradual rather than sudden failure (see Figure 2);
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increased load-carrying capacity — of between 10 and 20 percent;
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increased fire resistance — of between
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2 and 3 hours — even under eccentric loads;
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decreased buckling;
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suitability for a wide range of column dimensions.

Bar-Reinforced Concrete
Columns filled with bar-reinforced concrete offer many of the same advantages of columns filled with steel-fibre-reinforced concrete. They are, however, more expensive because of the labour involved in placing the reinforcing bars. They are also more difficult to work with in confined spaces with regard to achieving sufficient concrete coverage of the reinforcing bars.

Figure 2. The figure shows the comparative fire-resistance capacity of a typical HSS column with three types of concrete filling. The variation in axial deformation with time when the column is subjected to fire demonstrates the superior deformation behaviour of steel-fibre-reinforced concrete (ductile, or gradual, failure) compared to that of plain concrete filling (brittle, or sudden, failure).
Predicting Fire Resistance

Data generated from the IRC fire tests have been used not only to determine the influence of different parameters on the fire resistance of concrete-filled columns but also to validate computer programs that can predict the fire resistance of these columns.

In turn, data from these computer studies have been used to develop simple design equations that can be used to calculate the fire resistance of HSS columns filled with concrete.

The 1995 edition of the National Building Code (NBC), Appendix D, Section D-2.6.6., recognizes the fire resistance of steel HSS columns filled with plain concrete and includes design equations for calculating their fire resistance. Similar equations for columns filled with steel-fibre-reinforced concrete and with bar-reinforced concrete will likely be included in future editions of the NBC.

These equations can simplify the structural design process and thus encourage the use of these types of concrete filling. By varying parameters such as load, column-section dimension or concrete strength, it is possible to achieve an optimum design that is not only economical but also based on rational design principles.

In the meantime, computer programs developed by IRC for predicting the fire resistance of concrete-filled columns can assist designers and regulators in evaluating the fire resistance of building elements in cases where no specific data exist. In addition to the Museum of Flight in Seattle, two recently built schools in Ontario (see Figure 3), in which HSS columns filled with bar-reinforced concrete were used, represent successful applications of IRC’s computer programs.

Figure 3. A recently built school in Hamilton, Ontario in which HSS columns filled with bar-reinforced concrete were used
Conclusions

The use of concrete filling in hollow steel columns is an effective way to increase the fire resistance of these columns and to reduce fire-protection costs, while giving architects the freedom to create pleasing designs using exposed steel. For many tall buildings, there will be no need to increase the diameter of the columns for lower floors because of the greater load-bearing capacity afforded by the concrete filling. Other benefits include increased usable floor space and reduced construction costs because this type of column can be prefabricated and erected in all types of weather.

JFK

Nov 20 2008, 12:00 PM

Well, looking at the prints and not knowing the exact dimentions of the steel used in that column, it had to have come from the 80th floor or below. That is where the major transisition was from box column to I beam construction.

There was no concrete inside or over those columns.

mynameis

Nov 20 2008, 12:02 PM

JFK

Nov 20 2008, 12:00 PM

Well, looking at the prints and not knowing the exact dimentions of the steel used in that column, it had to have come from the 80th floor or below. That is where the major transisition was from box column to I beam construction.

There was no concrete inside or over those columns.

What's the possibility of HSS columns filled with concrete? I'll do some searching. I want to know when this technique was used first. Then go on from there.

Grit1645

Nov 20 2008, 01:08 PM

JFK is right, mynameis. The columns were not encased, nor were they filled. The hollow section just remained hollow after they were in place. In general, the column fireproofing seems to have consisted of several layers of drywall.

Grit1645

Nov 20 2008, 01:08 PM

JFK is right, mynameis. The columns were not encased, nor were they filled. The hollow section just remained hollow after they were in place. In general, the column fireproofing seems to have consisted of several layers of drywall.

mynameis

Nov 20 2008, 02:34 PM

Evidence of this being a non HSS building is from? NIST? UL? 911 Commission?

Grit1645

Nov 20 2008, 02:54 PM

mynameis

Nov 20 2008, 02:34 PM

Evidence of this being a non HSS building is from? NIST? UL? 911 Commission?

Pictures of the debris. Pictures of the construction. If the column section in your first photograph had been filled with concrete, trust me, you would know it.

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At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells. S

mynameis

Nov 20 2008, 03:00 PM

Grit1645

Nov 20 2008, 02:54 PM

mynameis

Nov 20 2008, 02:34 PM

Evidence of this being a non HSS building is from? NIST? UL? 911 Commission?

Pictures of the debris. Pictures of the construction. If the column section in your first photograph had been filled with concrete, trust me, you would know it.

Dust and concrete were everywhere during the collapse. Lots of water doused flames and lots of moving were done to the material without any forensic investigation that would determine your conjecture. We do not and cannot know this as an argument. We can however assess if HSS was used before or near the construction of the Twin Towers and if this was constructed with reinforced concrete? I would think it would be a simple question to answer factually? That search and rescue website seems to think so.

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At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells. S

Grit1645

Nov 20 2008, 03:25 PM

mynameis

Nov 20 2008, 03:00 PM

Grit1645

Nov 20 2008, 02:54 PM

mynameis

Nov 20 2008, 02:34 PM

Evidence of this being a non HSS building is from? NIST? UL? 911 Commission?

Pictures of the debris. Pictures of the construction. If the column section in your first photograph had been filled with concrete, trust me, you would know it.

Dust and concrete were everywhere during the collapse. Lots of water doused flames and lots of moving were done to the material without any forensic investigation that would determine your conjecture. We do not and cannot know this as an argument. We can however assess if HSS was used before or near the construction of the Twin Towers and if this was constructed with reinforced concrete? I would think it would be a simple question to answer factually? That search and rescue website seems to think so.

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At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells. S

I don't know how you propose to make that determination outside of the photographs. Having seen plenty of steel with concrete in/on it, I can tell you that it would be quite apparent in the pictures if that were the case, regardless of how much damage/water/moving around they underwent. The concrete adheres to the steel, it would not all be jarred out that easily or that completely. I worked on a site where they made connections to encased steel, and they had to jackhammer the concrete off to open up bare steel to weld the new connections to.

Simply determining whether it was commonly used around that timeframe or not doesn't help much, because each building's structural composition is determined by the designers from several available options.

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Temperatures at 800C

But as fires raged in the towers, driven by aviation fuel, the steel cores in each building would have eventually reached 800C - hot enough to start buckling and collapsing.

The protective concrete cladding on the cores would have been no permanent defence in these extraordinary circumstances - keeping the intense heat at bay for only a limited timespan.

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The perimeter structures of most high-rises erected since the 1960s resemble tubes. Inside, a massive hollow core made of steel and/or concrete contains many of the services: elevators, stairwells, and bathrooms. Because the core and perimeter columns carry so much of the load, the designers could eliminate interior columns, with the result that there is more open floor space for the tenants. . . .Engineers reduced, or eliminated, the use of concrete [although it is more fire-resistant than steel] in supporting the structure [of these high-rises].

The floors in most of the high-rise buildings erected since the sixties are much lighter in weight than the floors in the older buildings. . . .

In typical high-rise office floor, three or four inches of concrete covers a corrugated-steel deck, whose weight is supported. . .in the case of the Twin Towers, by long “trusses”—lightweight strips of steel that are braced by cross- hatched webs of square of cylindrical bars, creating a hollow space below each floor surface. This space allows builders to install heating and cooling ducts within the floors, rather than in a drop ceiling below them [the floors]—an innovation that means the developer can increase the number of floors in the entire building. #18

But these innovations, which builders welcomed, had potentially deadly consequences that firefighters foresaw:

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Raw Materials

Reinforced concrete is one important component of skyscrapers. It consists of concrete (a mixture of water, cement powder, and aggregate consisting of gravel or sand) poured around a gridwork of steel rods (called rebar) that will strengthen the dried concrete against bending motion caused by the wind. Concrete is inherently strong under compressive forces; however, the enormous projected weight of the Petronas Towers led designers to specify a new type of concrete that was more than twice as strong as usual. This high-strength material was achieved by adding very fine particles to the usual concrete ingredients; the increased surface area of these tiny particles produced a stronger bond.

The other primary raw material for skyscraper construction is steel, which is an alloy of iron and carbon. Nearby buildings often limit the amount of space available for construction activity and supply storage, so steel beams of specified sizes and shapes are delivered to the site just as they are needed for placement. Before delivery, the beams are coated with a mixture of plaster and vermiculite (mica that has been heat-expanded to form sponge-like particles) to protect them from corrosion and heat. After each beam is welded into place, the fresh joints are sprayed with the same coating material. An additional layer of insulation, such as fiberglass batting covered with aluminum foil, may then be wrapped around the beams.

To maximize the best qualities of concrete and steel, they are often used together in skyscraper construction. For example, a support column may be formed by pouring concrete around a steel beam.

A variety of materials are used to cover the skyscraper's frame. Known as "cladding," the sheets that form the exterior walls may consist of glass, metals, such as aluminum or stainless steel, or masonry materials, such as granite, marble, or limestone.
Design

Design engineers translate the architect's vision of the building into a detailed plan that will be structurally sound and possible to construct.

Designing a low-rise building involves creating a structure that will support its own weight (called the dead load) and the weight of the people and furniture that it will contain (the live load). For a skyscraper, the sideways force of wind affects the structure more than the weight of the building and its contents. The designer must ensure that the building will not be toppled by a strong wind, and also that it will not sway enough to cause the occupants physical or emotional discomfort.

Each skyscraper design is unique. Major structural elements that may be used alone or in combination include a steel skeleton hidden behind non-load-bearing curtain walls, a reinforced concrete skeleton that is in-filled with cladding panels to form the exterior walls, a central concrete core (open column) large enough to contain elevator shafts and other mechanical components, and an array of support columns around the perimeter of the building that are connected by horizontal beams to one another and to the core.

Because each design is innovative, models of proposed super tall buildings are tested in wind tunnels to determine the effect of high wind on them, and also the effect on surrounding buildings of wind patterns caused by the new building. If tests show the building will sway excessively in strong winds,
An example of a skyscraper ground floor design and 6uilding frame.
An example of a skyscraper ground floor design and 6uilding frame.
designers may add mechanical devices that counteract or restrict motion.

In addition to the superstructure, designers must also plan appropriate mechanical systems such as elevators that move people quickly and comfortably, air circulation systems, and plumbing.
The Construction Process

Each skyscraper is a unique structure designed to conform to physical constraints imposed by factors like geology and climate, meet the needs of the tenants, and satisfy the aesthetic objectives of the owner and the architect. The construction process for each building is also unique. The following steps give a general idea of the most common construction techniques.
The substructure

* 1 Construction usually begins with digging a pit that will hold the foundation. The depth of the pit depends on how far down the bedrock lies and how many basement levels the building will have. To prevent movement of the surrounding soil and to seal out water from around the foundation site, a diaphragm wall may be constructed before the pit is dug. This is done by digging a deep, narrow trench around the perimeter of the planned pit; as the trench is dug, it is filled with slurry (watery clay) to keep its walls from collapsing. When a section of trench reaches the desired depth, a cage of reinforcing steel is lowered into it. Concrete is then pumped into the trench, displacing the lighter slurry. The slurry is recovered and used again in other sections of the trench.
* 2 In some cases, bedrock lies close to the surface. The soil on top of the bedrock is removed, and enough of the bedrock surface is removed to form a smooth, level platform on which to construct the building's foundation. Footings (holes into which the building's support columns can be anchored) are blasted or drilled in the bedrock. Steel or reinforced concrete columns are placed in the footings.
* 3 If the bedrock lies very deep, piles (vertical beams) are sunk through the soil until they are embedded in the bedrock. One technique involves driving steel piles into place by repeatedly dropping a heavy weight on their tops. Another technique involves drilling shafts through the soil and into the bedrock, inserting steel reinforcing rods, and then filling the shafts with concrete.

A. Diaphragm wall. B. Footing. C. One type of foundation for a skyscraper uses steel piles to secure the foundation to the ground. D. The slip form method of pouring concrete.
A. Diaphragm wall. B. Footing. C. One type of foundation for a skyscraper uses steel piles to secure the foundation to the ground. D. The slip form method of pouring concrete.

* 4 A foundation platform of reinforced concrete is poured on top of the support columns.

The superstructure and core

Once construction of a skyscraper is underway, work on several phases of the structure proceeds simultaneously. For example, by the time the support columns are several stories high, workers begin building floors for the lower stories. As the columns reach higher, the flooring crews move to higher stories, as well, and finishing crews begin working on the lowest levels. Overlapping these phases not only makes the most efficient use of time, but it also ensures that the structure remains stable during construction.

* 5 If steel columns and cross-bracing are used in the building, each beam is lifted into place by a crane. Initially, the crane sits on the ground; later it may be positioned on the highest existing level of the steel skeleton itself. Skilled workers either bolt or weld the end of the beam into place (rivets have not been used since the 1950s). The beam is then wrapped with an insulating jacket to keep it from overheating and being weakened in the event of a fire. As an alternative heat-protection measure in some buildings, the steel beams consist of hollow tubes; when the superstructure is completed, the tubes are filled with water, which is circulated continuously throughout the lifetime of the building.
* 6 Concrete is often used for constructing a building's core, and it may also be used to construct support columns. A technique called "slip forming" is commonly used. Wooden forms of the desired shape are attached to a steel frame, which is connected to a climbing jack that grips a vertical rod. Workers prepare a section of reinforcing steel that is taller than the wooden forms. Then they begin pouring concrete into the forms. As the concrete is poured, the climbing jack slowly and continuously raises the formwork. The composition of the concrete mixture and the rate of climbing are coordinated so that the concrete at the lower range of the form has set before the form rises above it. As the process continues, workers extend the reinforcing steel grid that extends above the formwork and add extensions to the vertical rod that the climbing jack grips. In this way, the entire concrete column is built as a continuous vertical element without joints.
* 7 In a steel-skeleton building, floors are constructed on the layers of horizontal bracing. In other building designs, floors are supported by horizontal steel beams attached to the building's core and/or support columns. Steel decking (panels of thin, corrugated steel) is laid on the beams and welded in place. A layer of concrete, about 2-4 in (5-10 cm) thick, is poured on the decking to complete the floor.

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In the November/December 2001 issue of Designer/Builder, Mallot gives a deeply disturbing interview to Kingsley Hammet who writes: "Prior to the advent of the World Trade Center towers, high-rise buildings shared two vital characteristics. They were supported by a grid of steel columns, generally spaced about thirty feet apart, and each interior column was encased in a tough cladding of concrete to create a fireproof skin designed to withstand a four-hour inferno. (The four-hour fire rating is the code rule for the columns and major beams in any large building.) As designed by architect Minoru Yamasaki, New York's Twin Towers incorporated neither of these traditional features. And as far as Malott is concerned, it was the failure of their substitutes - not the initial crash, not the exploding jet fuel, and not the subsequent fire alone -that lead to their collapse and the enormous loss of life . . . "As Malott watched the tragedy unfold, he surmised that the sequence of events went something like this. when the planes slammed into the exterior of the buildings, the fuselages and engines broke through a number of the outside columns while the wings disintegrated as though being forced through a cheese grater. The bodies of the planes crashed across the unobstructed floors, smashed into the central cores of the buildings, and blew the sheetrock off the supporting columns and from around the stairwells, completely destroying the elevator shaft wails. Thus, in the first seconds, the four-hour-rated fireproofing was stripped from the steel core structures and with it went all hope that the buildings could survive a fire. "After an hour of this inferno, the now-naked steel columns of the central core at the impact floors were heated to about 1,600 degrees, which is the point at which steel loses almost all of its structural strength. The relatively skimpy floor system, with hung sheetrock, small-diameter steel bar joists, and the thin layer of concrete, offered little barrier to the raging flames despite having been rated as fire-resistant for four hours. Three floors may have collapsed within the impact area, further tearing fireproofing away from the core columns.

Once the first couple of core columns began to buckle, Malott speculates, they threw all of their load not onto a neighboring ring of strong columns protected with fireproofing (which in this design did not exist), but onto the adjacent columns in the exposed core, which were similarly denuded of fireproofing by the initial impact and also were failing under the intense heat. 'The outside of the building did not fail. It did not get hot enough,' Malott says. 'It was the core that failed.'

"It's time now to go back and rethink the entire concept of the high-rise structural system, Malott says. Buildings such as the World Trade Center towers cannot be built to minimum code specifications And architects must now truly consider the impact of a fully loaded aircraft or other impact/explosion/fire combination striking another tower. Future high-rise buildings must be designed with a redundant system of interior support columns so no failure of any critical part - be it the core, the skin, or the floor -leads to the catastrophic collapse of the entire building . . .

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[DESIGNER BUILDER 2405 MacLovia Lane, Santa Fe, NM 87505]

[The story below, so far as we know, was the first time the corporate media has let its audience know that the collapse of the World Trade Center towers might have been due to improper construction rather than to the impact the planes. . . But while the Shirtwaist Triangle fire early in the last century (which killed 150 people) produced major building reforms, the whole tendency since September 11 has been to ignore the culpability of those responsible for the towers' construction.]

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Also unique to the engineering design were its core and elevator system. The twin towers were the first supertall buildings designed without any masonry. Worried that the intense air pressure created by the buildings’ high speed elevators might buckle conventional shafts, engineers designed a solution using a drywall system fixed to the reinforced steel core. For the elevators, to serve 110 stories with a traditional configuration would have required half the area of the lower stories be used for shaftways. Otis Elevators developed an express and local system, whereby passengers would change at "sky lobbies" on the 44th and 78th floors, halving the number of shaftways.

(Taken from www.skyscraper.org)

JackD

Nov 20 2008, 07:10 PM

mynameis -- I think many other lower tall buildings are often built with a mixture of steel core columns near the foundation, and then concrete with reinforcing steel rods through them.

The towers had a different mandate -- they had to not only be tall, but to be strong, and resist 100+mph sustained winds, swaying through many feet of displacement. it was apparently NO FUN to be high up in the buildings as they got to swaying in a strong wind, back and forth 10 feet or more....

A concrete core, or "filled core columns" would not allow for this flexibility --- whereas the steel cage can flex gently, and return to center, concrete cannot flex -- it cracks, spalls, and crumbles. Thus, any additional "strength" by using a concrete core is offset not only by the extra massive weight (

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Steel-core design

The building's design was standard in the 1960s, when construction began on what was then the world's tallest building. At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells.

All the steel was covered in concrete to guarantee firefighters a minimum period of one or two hours in which they could operate - although aviation fuel would have driven the fire to higher-than-normal temperatures. The floors were also concrete.

mynameis

Nov 20 2008, 08:19 PM

Forgive me for not entirely believing this. There are conflicting reports. Another view, but not an engineer.

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Steel-core design

The building's design was standard in the 1960s, when construction began on what was then the world's tallest building. At the heart of the structure was a vertical steel and concrete core, housing lift shafts and stairwells.

All the steel was covered in concrete to guarantee firefighters a minimum period of one or two hours in which they could operate - although aviation fuel would have driven the fire to higher-than-normal temperatures. The floors were also concrete.

Grit1645

Nov 20 2008, 08:50 PM

Just an observation, mynameis, but people who do not work with structural engineering or construction often refer to things by the wrong terminology.

For example, the vertical support elements of a building are COLUMNS, but laypeople are always saying "beams" even as they do in the above sketch. Beams are generally horizontal members that carry slabs, etc. Girders are sometimes distinguished from other beams, and they carry beams. Kind of like the distinction between ships and boats (ships can carry boats on board).

Another common mistake is to refer to concrete as cement. Cement is an ingredient of concrete, which usually also consists of things like stone, gravel, water, sand. Reinforced concrete structures are made with concrete and steel rebar (not "concrete and steel beams").

mynameis

Nov 20 2008, 09:50 PM

I don't jump to conclusions, but there are conflicting reports as the core is misrepresented. I don't know, you don't know, you assume and provide no proof from the construction methods of the world trade center. Then suggest that many tons of friction and shock during the collapse would still have concrete inside after pulverization if this concrete cladding / casing doesn't exit. I doubt the concrete was poured inside the steel as reinforcement. Outer concrete casing has made a significant case for misrepresentation in my view. I think the construction photo will out. Anyone know sites that show the debris from the upper floors?

Grit1645

Nov 21 2008, 12:33 AM

Actually, I DO know, but whatever.

Grit1645

 

"Actually, I DO know, but whatever."