Collapse of the World Trade Centre Towers Written by G Charles Clifton, HERA Structural Engineer 17th September 2001,revised 19th September,minor revision on impact force made 8th October,minor revisions made 11th December. Introduction Construction of the World Trade Centre Towers began on August 5 1966 and they were officially opened on April 4 1973.Fig 1.shows the two towers prior to the attack.As will have been forever seared on the memory of all readers, they were destroyed in a terrorist attack on 11 September 2001.The method of destruction was simple and devastating,namely suicide attack by aircraft. The resulting images of the towers burning and collapsing were ones no-one ever expected to see The first airplane hit the North Tower at 8.46am local time and that tower collapsed at 10.28 am or 1 hours after the impact. The second tower was hit at 9.03 am but collapsed more quickly,at 9.59 am. Fig 1 World Trade Centre Towers Before Attack (Associated Press photo) This article has been written by Charles Clifton,HERA Structural Engineer and gives my thoughts on the possible sequence of damage and collapse.I am writing this from 17 years of experience in the research,design and construction of steel framed buildings.A significant part of the research has been determining the behaviour of steel framed buildings under the extreme events of severe earthquake or severe fire.This has given me some insight into what may have happened to these towers under the much more severe event of a direct hit from a near fully loaded large modern airplane.It is important to note that the explanation given is only my considered opinion, based on the information available six to eight days after the event. C:\WINDOWSTEMP\Collapse of the World Trade Center Towers Rev3.doc
1 C:\WINDOWS\TEMP\Collapse of the World Trade Center Towers Rev3.doc Collapse of the World Trade Centre Towers Written by G Charles Clifton, HERA Structural Engineer 17th September 2001, revised 19th September, minor revision on impact force made 8th October, minor revisions made 11th December. Introduction Construction of the World Trade Centre Towers began on August 5 1966 and they were officially opened on April 4 1973. Fig 1. shows the two towers prior to the attack. As will have been forever seared on the memory of all readers, they were destroyed in a terrorist attack on 11 September 2001. The method of destruction was simple and devastating, namely suicide attack by aircraft. The resulting images of the towers burning and collapsing were ones no-one ever expected to see. The first airplane hit the North Tower at 8.46am local time and that tower collapsed at 10.28 am or 1 ¾ hours after the impact. The second tower was hit at 9.03 am but collapsed more quickly, at 9.59 am. Fig 1 World Trade Centre Towers Before Attack (Associated Press photo) This article has been written by Charles Clifton, HERA Structural Engineer and gives my thoughts on the possible sequence of damage and collapse. I am writing this from 17 years of experience in the research, design and construction of steel framed buildings. A significant part of the research has been determining the behaviour of steel framed buildings under the extreme events of severe earthquake or severe fire. This has given me some insight into what may have happened to these towers under the much more severe event of a direct hit from a near fully loaded large modern airplane. It is important to note that the explanation given is only my considered opinion, based on the information available six to eight days after the event
2 Subsequent material received from then until 11th December has led to minor changes in terms of Correcting the times of impact and collapse and a brief note on the orientation of the map presented in Fig 2 Elaboration on the structural load distribution in the North Tower after the impact(given that the initial explanation is overly simplistic) Correcting some typographical errors Before presenting those details,some details of the building are given, followed by brief details of the impact.The effect of the impacts can only be assessed in light of these details,in particular the devastatingly high local impact force on the buildings from the planes.This is followed by my assessments of the effects of this impact on each of the two towers,which showed some significant differences. There has already been considerable speculation on the severity of the fire and its role in the collapses.On the basis of what I have seen and heard reported to date,it is my opinion that the effect of the fire was of much less importance than the effect of the initial impact,especially on the first tower to be hit the North Tower).The reasons behind this opinion follow details of the effects of the impacts on each tower and the article ends with a personal footnote on the tragedy and a reference. Details of the Buildings Fig 1 shows the two towers in service.These towers were the principal buildings in a complex of city development.Fig.2 shows the location of these towers on the World Trade Center site,along with the direction of impacts. North North Towei West East we South Fig.2 Site Plan of World Trade Centre Development Showing Location of the Two Towers (The direction of arrows adjacent to each tower shows the approximate direction of the planes on impact) This map should be orientated 40 degrees clockwise from the stated directions-the North face is actually closer to North-East. C:\WINDOWSTEMP\Collapse of the World Trade Center Towers Rev3.doc
2 C:\WINDOWS\TEMP\Collapse of the World Trade Center Towers Rev3.doc Subsequent material received from then until 11th December has led to minor changes in terms of · Correcting the times of impact and collapse and a brief note on the orientation of the map presented in Fig 2 · Elaboration on the structural load distribution in the North Tower after the impact ( given that the initial explanation is overly simplistic) · Correcting some typographical errors Before presenting those details, some details of the building are given, followed by brief details of the impact. The effect of the impacts can only be assessed in light of these details, in particular the devastatingly high local impact force on the buildings from the planes. This is followed by my assessments of the effects of this impact on each of the two towers, which showed some significant differences. There has already been considerable speculation on the severity of the fire and its role in the collapses. On the basis of what I have seen and heard reported to date, it is my opinion that the effect of the fire was of much less importance than the effect of the initial impact, especially on the first tower to be hit ( the North Tower). The reasons behind this opinion follow details of the effects of the impacts on each tower and the article ends with a personal footnote on the tragedy and a reference. Details of the Buildings Fig 1 shows the two towers in service. These towers were the principal buildings in a complex of city development. Fig.2 shows the location of these towers on the World Trade Center site, along with the direction of impacts. Fig. 2 Site Plan of World Trade Centre Development Showing Location of the Two Towers (The direction of arrows adjacent to each tower shows the approximate direction of the planes on impact) This map should be orientated 40 degrees clockwise from the stated directions – the North face is actually closer to North-East. North Tower South Tower North West East South
Each tower was 411 metres high,63.5 x 63.5 metres square on plan,core 24 x 42 m.There were 110 storeys in each tower. The towers were one of the best examples of "tube tower construction",a structural form ideally suited to providing the strength and stiffness required for very tall buildings.On each facade a rigid moment-resisting frame was formed comprising 59 box-section columns,spaced at 1.02 metre centers. connected by deep spandrel beams.The frames did not run into the corners, however,there a shear connection between the two adjacent frames was provided so that the frames,together with the floors,formed a torsionally rigid framed tube fixed to the foundations.This framed tube carried all wind loads. The floors spanned without intermediate columns to the core,which was supported on 44 box-section columns designed and detailed to carry vertical loading only.Fig 3 shows this concept in an isometric view,while one of the 450 x 450 mm exterior frame box columns is shown in Fig.4. 1720 Horizontal section through an extemnal column with window frame connection 42 43- Fig.3 Fig.4 Isometric View of Building Cross Section Through (from [1]) Exterior Box Column (from [1]) The numbers in the figure are taken from [1] The numbers in the figure are taken from [1] and denote: and denote: 13-Perimeter frame 36-Steel column 17-Core box columns 450 mm square 38,39-Fire resistant plaster 20-Floor slab 40-Aluminium facade 42-Window glass 43-Window frame The access and services were carried through the central core,as shown in Fig 5. The floor system comprised 900 deep bar joists spaced at 2.04 m centres and braced by secondary joists.These secondary joists then supported a profiled deck on which was poured a 100 mm thick light-weight concrete slab.The top of the bar joists stood above the soffit of the decking and was cast into the concrete slab to make the bar joists composite in a similar manner to the Speedfloor system. C:\WINDOWSTEMP\Collapse of the World Trade Center Towers Rev3.doc
3 C:\WINDOWS\TEMP\Collapse of the World Trade Center Towers Rev3.doc Each tower was 411 metres high, 63.5 x 63.5 metres square on plan, core 24 x 42 m. There were 110 storeys in each tower. The towers were one of the best examples of “tube tower construction”, a structural form ideally suited to providing the strength and stiffness required for very tall buildings. On each façade a rigid moment-resisting frame was formed comprising 59 box-section columns, spaced at 1.02 metre centers, connected by deep spandrel beams. The frames did not run into the corners, however, there a shear connection between the two adjacent frames was provided so that the frames, together with the floors, formed a torsionally rigid framed tube fixed to the foundations. This framed tube carried all wind loads. The floors spanned without intermediate columns to the core, which was supported on 44 box-section columns designed and detailed to carry vertical loading only. Fig 3 shows this concept in an isometric view, while one of the 450 x 450 mm exterior frame box columns is shown in Fig. 4. The access and services were carried through the central core, as shown in Fig 5. The floor system comprised 900 deep bar joists spaced at 2.04 m centres and braced by secondary joists. These secondary joists then supported a profiled deck on which was poured a 100 mm thick light-weight concrete slab. The top of the bar joists stood above the soffit of the decking and was cast into the concrete slab to make the bar joists composite in a similar manner to the Speedfloor system. Fig. 3 Isometric View of Building (from [1]) The numbers in the figure are taken from [1] and denote: 13 – Perimeter frame 17 – Core box columns 450 mm square 20 – Floor slab Fig. 4 Cross Section Through Exterior Box Column (from [1]) The numbers in the figure are taken from [1] and denote: 36 – Steel column 38, 39 – Fire resistant plaster 40 – Aluminium façade 42 – Window glass 43 – Window frame
4 The bar joists spanned between the perimeter frames and the core,as shown in Fig 6.Fig 7 shows an isometric of part of the floor and exterior wall, illustrating some of the details described above. 131415 10 11 · Structural system for typical floor aL Fig.5 Fig.6 Plan View of Typical Floor(from [1]) Structural System for Typical Floor The numbers in the figure are taken from [1]and denote: 11,12-Lts from [1]) 10-Open plan offices The numbers in the figure are taken from [1]and denote: 13-Perimeter frame 14-Bar joists 900 mm deep 15-Secondary joists 16-Horizontal floor bracing 17-Core box columns Assembly of the external wall units(alternately staggered in one-storey heights)and floor units Fig.7 Isometric View of Floor and Exterior Wall System The gravity and lateral load-resisting systems were designed to deliver the strength and stiffness required from a 110 storey building with minimum dead load.This was achieved very well,with a steelwork weight of only 44.5 kg/m2 floor area.The very light and open structure,superbly engineered to meet the design serviceability and ultimate limit state conditions on a building of this height and size,probably made the buildings more vulnerable to collapse from C:\WINDOWSTEMP\Collapse of the World Trade Center Towers Rev3.doc
4 C:\WINDOWS\TEMP\Collapse of the World Trade Center Towers Rev3.doc The bar joists spanned between the perimeter frames and the core, as shown in Fig 6. Fig 7 shows an isometric of part of the floor and exterior wall , illustrating some of the details described above. Fig. 5 Plan View of Typical Floor ( from [1]) The numbers in the figure are taken from [1] and denote: 11, 12 – Lifts 10 – Open plan offices Fig. 6 Structural System for Typical Floor ( from [1]) The numbers in the figure are taken from [1] and denote: 13 – Perimeter frame 14 – Bar joists 900 mm deep 15 – Secondary joists 16 – Horizontal floor bracing 17 – Core box columns Fig. 7 Isometric View of Floor and Exterior Wall System The gravity and lateral load-resisting systems were designed to deliver the strength and stiffness required from a 110 storey building with minimum dead load. This was achieved very well, with a steelwork weight of only 44.5 kg/m2 floor area. The very light and open structure, superbly engineered to meet the design serviceability and ultimate limit state conditions on a building of this height and size, probably made the buildings more vulnerable to collapse from
5 the aircraft impact than would have been the case for a more inefficient and heavier structural system. Passive fire protection was provided to the columns by vermiculite plaster and to the underside of the floor systems by a fire rated suspended ceiling. Each tower had an effective floor area of 319,000 m and used 87,000 tonnes of steelwork. The Impacts Each tower was hit by a Boeing 767.The impact on the North tower was near the centre of the North face at around the 95th storey,with the plane hitting the side of the building square on and flying level.The impact on the South tower was some 15 storeys lower,with the plane hitting the South face near the South East corner and impacting at an angle to the face of the tower.Fig 2 shows the impact directions and Fig 8 shows the plane about to hit the South Tower. Fig.8 Plane About to Hit South Tower (Photo from CNN) The potential force of the impact from each plane can be approximately calculated and the figures are very large.The weight of each plane would have been approximately 150 tonnes,according to the media reports and Boeing data on this type of plane.The plane would have been traveling at around 800 kms/hour at impact.This gives a momentum of 150x800/3.6=33,333 tonnes.m/sec.If the plane was arrested by the building in effectively 0.6 seconds,which is a reasonable estimate based on a linear deacceleration over the 63.5 m width of the building,then the force exerted on the building is the momentum/effective time to arrest,ie Force=33,333/0.6=55,555kN.To put that in perspective,the ultimate limit state design wind pressure over the entire height of the building is 220 kg/m2 This gives a ULS wind force on one face of the building of 58,400 kN.Thus the potential force of impact from the plane is 95%of the design ultimate limit state wind load on the building!Especially in the case of the North Tower,not much of the plane was ejected from the building,so it is reasonable to assume the most of that potential force was absorbed by the building.Also the above calculation also does not take into account any additional force C:\WINDOWSTEMP\Collapse of the World Trade Center Towers Rev3.doc
5 C:\WINDOWS\TEMP\Collapse of the World Trade Center Towers Rev3.doc the aircraft impact than would have been the case for a more inefficient and heavier structural system. Passive fire protection was provided to the columns by vermiculite plaster and to the underside of the floor systems by a fire rated suspended ceiling. Each tower had an effective floor area of 319,000 m2 and used 87,000 tonnes of steelwork. The Impacts Each tower was hit by a Boeing 767. The impact on the North tower was near the centre of the North face at around the 95th storey, with the plane hitting the side of the building square on and flying level. The impact on the South tower was some 15 storeys lower, with the plane hitting the South face near the South East corner and impacting at an angle to the face of the tower. Fig 2 shows the impact directions and Fig 8 shows the plane about to hit the South Tower. The potential force of the impact from each plane can be approximately calculated and the figures are very large. The weight of each plane would have been approximately 150 tonnes, according to the media reports and Boeing data on this type of plane. The plane would have been traveling at around 800 kms/hour at impact. This gives a momentum of 150x800/3.6=33,333 tonnes.m/sec. If the plane was arrested by the building in effectively 0.6 seconds, which is a reasonable estimate based on a linear deacceleration over the 63.5 m width of the building, then the force exerted on the building is the momentum/effective time to arrest, ie Force=33,333/0.6=55,555kN. To put that in perspective, the ultimate limit state design wind pressure over the entire height of the building is 220 kg/m2 . This gives a ULS wind force on one face of the building of 58, 400 kN. Thus the potential force of impact from the plane is 95% of the design ultimate limit state wind load on the building! Especially in the case of the North Tower, not much of the plane was ejected from the building, so it is reasonable to assume the most of that potential force was absorbed by the building. Also the above calculation also does not take into account any additional force Fig. 8 Plane About to Hit South Tower (Photo from CNN)