What was it about the construction of the NYC World Trade Centre buildings that made them collapse so easily in case of a very hot fire?

Answer by David Kahana:

The design of the towers was unique among steel frame buildings of the time. They were built like a tube within a tube, with the floors supported by trusses that tied on to supporting columns on the outer walls as well as on to the core columns making up the inner tube, where the elevator shafts ran.

The whole design maximized the open volume inside the building, so that there would be a lot of space for offices. This was not a steel cage design like many older steel frame buildings, there were no supporting girders running through the middle of the floors and under them. There were only relatively light floor trusses to keep the floors in place. There was no masonrywork.

This big open volume of course also helped with spreading the fire and making it much worse when the attack happened. There had been fires in the buildings before which were quite serious.

Both the inner tube and the outer tube were load bearing structures, with the weight distributed roughly fifty/fifty between them and carried down to ground.

They were commercially zoned buildings so they did not have to survive for very long in a fire, it wasn’t required. They depended on fireproofing around all of the steel work to insulate the steel from the heat of a potential fire. That fireproofing was sprayed on to the members of the floor trusses and there was also fireproofing built around the core columns as well so that they could stay cool and hold their structural strength, but only for the time that was considered necessary for evacuating the people. That time was a lot less than a day. In some places the insulation was known not to be adequate and it was in fact in the process of being upgraded when the planes hit.

The impacts of the airliners cut huge gashes through the outer walls, shifting the weight that the damaged walls were bearing through the roof truss to the core columns and to the other undamaged walls, which was what the design was made to do. A lot of the insulation was stripped from the floor trusses when the airliners went through. Core columns suffered significant damage on the impact floors, some were destroyed, and insulating structures were stripped away – an open tube in the core now ran up and down across several floors where the fire began to burn. First from the fuel, then from the office materials and the materials of the airliners. All of it could burn. And steel of the now overloaded and out of true core columns was in places open to the flame. The floor trusses were also open to flame.

Given all of this damage, it did not need to be an extremely hot fire to cause the collapse, which was quick, but I wouldn’t say was exactly so easy. But these were massive and pretty hot fires. It’s sometimes hard to see from the outside shots, but they went across several floors. They were a horrible inferno in fact. They were very bad fires.

The collapse when it happened can clearly be seen to have initiated near the impact floors high in the building, and it was initiated by the loss of integrity in the core. The core was stressed by damage, by additional weight, by fire, and by the distortion of the floor trusses due to fire as well. When it fails the building core unweights again through the roof truss onto the damaged outer walls which, having been well heated and which are already curving inwards at this point in time, then finally fail under the overload and progressively unzip all the way around the building. The now mostly unsupported top then falls downward onto the lower floors which can’t take the dynamic load and it’s all over.

But you should remember that almost all of the people below the impacts in both towers were evacuated, while almost nobody above the impacts survived.

The towers stood for long enough that they could be evacuated, where there was actually a route for the people to travel down. Above the impacts the people were trapped. The stairways were cut completely in one tower at the level of impact, in the other tower one stairway of three survived, and a few people actually made it down from the zone above the impact, or so I remember.

What was it about the construction of the NYC World Trade Centre buildings that made them collapse so easily in case of a very hot fire?

What is the greatest and toughest project ever completed in civil engineering?

Answer by Rakshita Nagayach:

I believe the Palm Island (Palm Jumeirah), Dubai was the most audacious project of its time. Building an island in the middle of an ocean was the bravest thing to do. The island has a breath-taking aerial view,along with the infrastructure built on it. But a lot of hurdles came in the way during the construction of the then largest man made island.

1) Foundation construction: The Palm developers wanted the foundations to be made with natural building materials i.e. only sand and rocks. The problem was that sand gets eroded as soon as it hits the water. Also to protect the island, a backwaters wall was to be constructed so as to prevent direct impact of waves on the island.

94 million cubic meters of sand and 5.5 million cubic meters of hard rock were needed to make a 2.5 meters wall above the sea level.

2) Climatic Factors: Massive sea waves, persistent winds and large currents are relevant in the Arabian Gulf. Though the Gulf is quite shallow near the coastline of Dubai (on an average, the depth in only 30 meters), But the Shamal (wind) coming in winters bring 2 meters high waves,becoming a threat to the project.

This threat was overcome by constructing breakwaters 3 m above the expected wave height and 11.5 km long.

3) Political Pressure: It was both on the engineers and the Palm Developers. The construction began in August 2001 and a month later, the World Trade Center attack wiped out almost all the tourism in the Middle East for about 3 months.
Also the engineers were given exactly 5 years to complete the project,which demands 15 years actually.

4) Massive Construction Equipment required: 9 Barges, 15 Tugboats, 13 Heavy land Bar machines and 10 floating cranes were used for the construction of the sea wall. The first step was to build the sea floor, by 3 Dredges, laying 7.4 m thick layer of sand when the sea is at the calmest. Rocks are immediately dropped to raise the breakwater.

5) Wave Action: Waves have an eroding nature of whatever comes in their way. The sloping layer of the sand laid takes out the force of the wave as they hit the water. Rubble formed the core of breakwater outer armour of rock pieces protect the entire wall.

Constant checks had to be made by the divers pinpointing every rock and crack, as every crack was dangerous. Every 27 m of the wall was checked by a diver periodically.

6) Shamal arrives: Storms with a speed upto 56 kmph bring torrential rains and violent thunderstorms with them, delaying the breakwaters by 3 weeks.

7) Availability of Sand: Although Dubai’s desert has abundant sand to be used for construction,the problem with desert sand is that its too fine to construct a city of a hundred thousand people. Marine sand available 6 nautical miles from the site has the properties of being resistant to wave impact, coarse and densely packed.
Dredges sucked up the sand from the Gulf sea bed and sprayed it on the site within 3 hours time. It took less than an hour to fill the 8000 tonnes tank with sand in the dredges.

Pumping, technically known as Rainbowing is the spraying of sand on the site.

8) Synchronicity in Wall building and Island building: If the breakwaters work progresses too quickly, it will cut-off the access for reclamation project, and if reclamation is done too fast, it is in direct contact with the sea waves.

9) Accuracy in shape of the Island: A single mistake in the shape would lead to failure of entire project. To keep a check on it, Dubai used it privately owned space satellite (only nation in the world to have a private satellite).GPS was used by daily perimetering the surface and create a grid reference by reading the signals from space and fixed position on land to check location of sand bars.

10) Water blockage: As the breakwaters trapped the island, the water was supposed to become stagnant,and ultimately home to algae and other unwanted biological hazards. So, 2 breaks in the sea wall were made, by making 4-lane road bridges over it on the observation that tides enter the wall twice a day and in 2 weeks, the entire water is replenished. A team of environmentalists keeps checking the water content on a regular basis.

11) Liquefaction of Sand due to seismic activities: Due to spraying, the sand is loose and not compact, which might lead to complete sinking of the island due to the occurrence of earthquakes.

Sand layers are compacted 12 m deep using 15 Vibrocompactors; probes drilling 200000 holes, shaking the ground around it and making it compact. As sand compacts and sinks, more sand is poured in.

12) Building the city in just 2 years: 40000 Asian workers in two 12-hours shift worked under 51 contractors building hundreds of villas, hotels and shopping malls.

13) Beach erosion and coast line depletion: The engineers have made sure that the beaches facing erosion will continuously be examined and sand would be sprayed on them.

Normally the waves keep the beach in a line but due to an obstacle, the wave pattern changes. In dome places,sand is deposited extending out of the sea,and in some places it is eroded away.
A dredger is appointed to suck up the sand where it is built across the coast and deposit it in areas where it has been eroded,thus maintaining the coastline.

14) Aquatic Life: To check the condition of fishes and corals, every 6 weeks divers check the waters. It was expected that the aquatic life would be affected badly, but to a surprise the results were completely opposite. Marine life is not only unaffected, the breakwaters have provided shelter for fish, attracting many other species.
Now the Palm developers plan to develop largest artificial reef in the world! In June 2004, 2 Jet planes were dropped in sea for people to dive on,and there’s a plan to drop a London’s Red double-decker bus!

Long live the spirit of Engineers!

What is the greatest and toughest project ever completed in civil engineering?

Webinars registration open ….

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Here is a link to the web site .

The first webinar shall be organized on :

” How to find out the cost of a RCC box footing ? “

This shall be dealt in very basics .

Background :

It is a common observation that when it comes to calculations , estimation and costing , many people find it rather difficult.

Thats why this topic has been selected so that adequate practice gets done through the webinars.

What is self-compacting concrete?

Answer by Joe Fernandes:

Self-compacting concrete is a flowing concrete mixture that is able to consolidate under its own weight. The highly fluid nature of SCC makes it suitable for placing in difficult conditions and in sections with congested reinforcement. Use of SCC can also help minimize hearing-related damages on the worksite that are induced by vibration of concrete. Another advantage of SCC is that the time required to place large sections is considerably reduced.

What is self-compacting concrete?

What is the difference between opc and ppc cement?

Answer by Sumanth Reddy:

Ordinary Portland Cement (OPC) and Portland Pozzolana cement (PPC) differ primarily in their  composition ultimately affecting its applicability.

PPC commonly has pozzolan material added to cement (either before clinkering or added to ground clinker). Pozzolan commonly used is fly as and is put upto 15%.

Pozzolans have an effect in reacting with the Calcium hydroxide formed after hydration of alite and belite phases in cement. Their composition is primarily Silica and alumina, though they are essential for formation of alite and belite in cement clinker, this external addition is an attempt to control the free calcium hydroxide presence in the paste.

In terms of utility, long term strength of such cements are strength gaining with the hydration period extending quite longer than OPC. On the downside, the early strength properties are going to be affected. The fresh properties are going to be slightly better because of action of fly ash as filler.

What is the difference between opc and ppc cement?

What are various causes of cracks in a basic reinforced concrete structure?

Answer by Rakshita Nagayach:

The various causes are:
• Structural cracks

1) Due to incorrect design
2) Faulty construction
3) Overloading

• Non Structural cracks

1) Moisture changes: Shrinkage effect,depending on the water content, cement concrete and aggregates.
2) Thermal movement: Concrete made in hot weather needs more water for same work-ability and hence results in more shrinkage.
3) Elastic deformation
4) Creep: Building items such as concrete and brick work when subjected to a sustained load not only undergo elastic strain but also develop gradual and slow time dependent deformation known as creep or plastic strain.
5) Chemical reaction: Certain chemical reaction in building materials result is appreciable change in volume of resulting products and internal stresses are set up which may result in outward thrust and formation of cracks.
6) Foundation movement and settlement of soil: Buildings on expansion clays are extremely crack prone.
7) Vegetation: Large trees growing in the vicinity of buildings cause damage in all type of soil conditions. If the soil is a shrinkable clay, the cracking is severe.

What are various causes of cracks in a basic reinforced concrete structure?

How can one identify structural cracks in a concrete construction?

Answer by Rakshita Nagayach:

Structural cracks due to shrinkage can be identified as:

1. Corrosion of the reinforcement: If the reinforcement is placed too near the surface, it has chances to corrode. The iron expands due to oxidation and conversion to Iron oxide, which in turn results in cracks. So if the reinforcement is visible from the concrete surface, there are chances of cracks present and upcoming.

1. Vertical cracks: These cracks mostly occur when the foundation has settled or disturbed unequally along its surface.
   
2. Linear cracks: These cracks gradually grow and though they appear to be linear, there are interruptions in the crack-line which implies the crack is towards spreading itself.
   
3. Network of cracks: These are due to the reaction of aggregate with some alkali hydroxide present in the concrete and can be easily seen by a network of cracks roughly growing as a circle in a group.
   

The reasons for these cracks can be read in Rakshita Nagayach’s answer to What are various causes of cracks in a basic reinforced concrete structure?

Thanks for the A2A Leo Pedro!

How can one identify structural cracks in a concrete construction?

Why do homes that have concrete slabs, that are cast on the ground, have an upper reinforcement?

Answer by Isaac Gaetz:

In the USA, the most common practice is to use a single layer of reinforcing, placed in the upper half of a slab on grade. The reinforcing is commonly welded wire fabric, not rebar. Typically the reinforcing will be placed at 2″ below the top of slab, at the first 1/3 point, or mid depth. The specific choice of the reinforcement depth will depend on the slab thickness (2″ and the first 1/3 are the same for a 6″ slab, for example).

For most soil conditions, uplift loading is not a significant concern, nor is flexure.  Instead the reinforcing’s primary role is to limit crack width and reduce shrinkage. If we only place reinforcing below the neutral axis, we’ll create large cracking at the top surface of the slab. This happens for two reasons:

1. First, the bottom concrete, enclosed on all sides by soil, moisture barriers, and the concrete above, naturally has less access to air and will generally cure more slowly than the concrete on top of it. Because the curing process involves the concrete losing moisture and shrinking, the top concrete with tend to shrink more and faster than the concrete below it.
2. Second, the reinforcing, if placed in the bottom of the slab, with further restrict the bottom concrete from shrinking. This will amplify the problem identified in the first point above.

Instead, we use reinforcing in the top of the slab to neutralize some of the tendency of the upper concrete to cure and shrink before the concrete below it.

As with all concrete, the goal isn’t generally to eliminate all cracking, it is to control it.

Larger reinforcing rebar, including multiple layers of reinforcing is typically only used in expansive soils, or in mat foundations.

Why do homes that have concrete slabs, that are cast on the ground, have an upper reinforcement?

What is the most efficient sequence of steps to design a Steel I-Girder Bridge (structurally)?

Answer by Isaac Gaetz:

Design is an iterative process, the trick is to strike a balance between depth of design and breadth of design. You have to make some decisions before you can move forward with the design, but if you go into too much depth early on, you’ll end up having to rework or throw out much of your work. It helps to have good intuition about design, so, knowing the approximate depth of the structure and beam spacing will be needed early, but having the final details will come much later in the process. Drawings will proceed similarly, you’ll need to develop design and schematic drawings earlier than construction level documents. Initially the idea is produce drawings that can convey the concept to various stake holders, who may not even be engineers or contractors.

What is the most efficient sequence of steps to design a Steel I-Girder Bridge (structurally)?

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