New York WTC 1 & WTC 2 structural design and collapse overview.

Abstract

This paper describes the structural systems, the site, the design innovations, the steel used, the fire resisting materials applied, and finally explains what did caused the collapse of the WTC 1 and WTC 2. The structural design and collapse of WTC7 is not tackled in this paper. The major four structural subsystems in the towers 1 and 2 are the exterior walls, the core, the floor system, and the hat truss. At the time of design and construction, a number of innovations were first used in the design of the towers. These innovations are the framed tube system, the deep spandrel plates, the viscoelastic dampers to supplement the framed tube in limiting wind induced oscillations and the unified shape of external columns used before finally describing the fire resistive materials used during and after the building construction. Lastly, we'll describe the collapse due to the impact, the fire originated by the jet fuel and the sagging of the upper floors above the plane impact zone resulting in the full collapse.

Introduction

Based on the technical investigative papers conducted by the NIST, we summarized major topics on the design and collapse of the 110 Story WTC1, WTC2, and the 47 story WTC7 buildings. The structural design of each building are described to provide a foundation for understanding the effects of the aircraft impact, growth and spread of fires, and ultimately the collapse of each building.

The site

The two towers WTC1 (north tower) and WTC2 (south tower) were completed in 1970 (as the tallest building in the world at that time) and each were 110 stories high and the footprint was a square of 64mx64m. WTC7 was 47 story building on the land owned by the port authority of New York and New Jersey and was completed in 1987. Below the 11 western acres of the site and partially underneath WTC1 and WTC2, was a 6 story underground structure. The structure was surrounded by a wall that extended from the ground level down 21.3 m to bedrock and served to hold back waters of the Hudson River. This wall enables rapid excavation for the foundation and blocked groundwater from flooding toward the site under construction.

Overview of the structural design

In addition to the gravity loads (dead + live loads), the building need to resist lateral loads and excessive swaying from periodic hurricane force winds. An approved approach of extensive and detailed studies were conducted in wind tunnels to estimate the wind loads for the design of these buildings. An additional load was required by PANYNJ to be considered was the impact of a commercial airlines at a flying speed of 600 mph.

There were four major structural subsystems in the tower: the exterior wall, the core, the floor system, and the hat truss. Framed tube concept was used for the exterior structural system. Columns supporting the building were located at both the external faces and within the core. The core contained the elevator, the stairwells, and utility shafts. The floor system provided stiffness and stability to the framed tube system in additional to supporting floor loads.

The first major structural subsystem was the exterior framing that consisted of 236 narrow columns, 59 on each face from the 10th floor to the 107th floor. Each column, composed of 4 plates (0.36 m wide) welded on their sides, is distant 0.66m from the column next to it with a framed glass window in each gap. The columns were fabricated in welded panels, three stories tall and three columns wide to enable rapid construction. Adjacent columns were connected at each floor by steel spandrel plates. 

The upper part of the building had less wind load and building mass to support, hence, the thickness of the steel plates became as thin as 6mm near the top from 76 mm at the lower floors. Similarly there were 10 grades of the steel used with yield strength ranging 248 MPA to 600 MPA as dictated by the calculated stresses due to the gravity and wind loads.

The second structural subsystem was the core, measuring 41 m by 26.5 m and extended to the full height of the building. The long axis of the core in QTC 1 was oriented in the east-west direction, while the long axis of the core in WTC2 was oriented in the north south direction. The steel grades used ranged from 248 MPA to 290 MPA. The four massive corner columns bore nearly 20% of the total gravity loads on the core columns. A grid of steel beams connected the core columns and supported the core floors.

The third major structural subsystem was the floors between the external walls and the core. In addition to supporting gravity loads, they provided lateral stability to the exterior walls and distributed wind loads among exterior walls. Tenant floors had truss systems consisting of steel bar trusses with knuckles of the main truss web extended 76mm above the top chord to bond with the lightweight concrete cast in place on a fluted steel deck. The floor trusses and fluted metal deck were prefabricated in panels that were typically 6.1 m wide and hoisted into position. The bottom chords of the main trusses were connected to the spandrel plates of the exterior wall by viscoelastic dampers reducing the sway and the vibration.

The forth major subsystem is the hat truss located from the 107th floor to the roof each tower and was a set of steel braces. Its purpose was to support the antenna and provide additional connections among the core columns and between the core and perimeter columns providing additional means for load distribution.

Innovations

At the time of design and construction, the towers includes a number of features considered innovative:

Framed tubes

The framed tube concepts structure consists of closely spaced exterior columns tied together at each floor by deep spandrel beams creating a rigid wall like structure around the building exterior. Its behavior shows the characteristics of both pure tube (carrying portion of the gravity loads) and pure frame systems (resisting shear forces of the wind loads). Under the effect of wind alone, columns on the windward side are in tension and on the leeward side are in compression. The overturning moment of the lateral wind loads are primarily resisted by tube action. The columns on the walls parallel to the wind direction are in tension on the windward side and compression on the leeward side. The shear force from the wind loads is primarily resisted by frame action along the 2 faces parallel to the direction of the wind.

The floor diaphragms carry lateral forces to the side walls of the building (allowing tubes action to take place) and provide lateral support for the stability of the columns.

Deep spandrel plates

Instead of using spandrel beams as common, band of deep plates as spandrel members were used to tie the perimeter columns together.

Uniform external geometry

Atypical to high rise building to the have the same appearance of tall, uniform columns from the base to the top, this was achieved by varying both the strength of the steels and the thickness of the plates that made up the perimeter columns.

Wind tunnel and human perception testing and wind loads

Specific to the project, a unique wind tunnel testing program was made to estimate how the building perform under wind loads. Tests on the two tower models showed that the wind response of each tower was significantly affected by the presence of other tower and the tests results led to the inclusion of stiffer perimeter columns and the addition of viscoelastic dampers.

Viscoelastic dampers

Tests of wind-induced conditions that would be tolerated by building occupants led to a new ground of human perception that low building accelerations caused discomfort. To limit the oscillations, 10000 dampers were inserted between the spandrel plates and truss lower chord.

Long span composite floor assemblies

Composite floor system constructed with open web, lightweight steel trusses and lightweight concrete and achieving composite action between the steel truss and concrete slab with knuckles (that fails well beyond the design capacity of the trusses) was an innovation at the time of design and construction.

Vertical shafts wall panels

The compartmentation system used in the vertical shafts was unique in that it eliminated the need for any framing. The walls consisted of gypsum planks placed into metal channels at the floor and ceilings slabs with metal tongue and groove channels attached to the long sides that serviced as wall studs.

Structural steels

The steel of the perimeter columns & spandrel plates yield strength ranged between 248 and 690 MPA whereas the yield strength of the core and floor truss system ranged between 248 and 345 MPA. All measures values, specifications of the materials were found compliant and within the factor to safety. Even some the areas such as the floor trusses, steel with yield strength of 345 MPA were installed whereas the specified was 248 MPA.

Passive fire protection

The WTC were classified as Class 1B which requires the columns carrying loads to have 3hrs fire endurance and the floor system to have 2 hr rating when tested in accordance with the ASTM E 119 [12]. This can be achieved by the either the application SFRM or rigid fire rated gypsum panels. The SFRM was innovative in 1960. In 1969, The PANYNJ directed a 13mm thick coating to insulate the floor trusses and achieve class 1A rating even though WTC were classified as class 1B.

In 1995, and after studies performed by PANYNJ to establish requirement for retrofit sprayed insulation, the study concluded that 38mm thick mineral fiber will provide 2 hr fire rating consistent with the class 1B requirements. PANYNJ implemented a new policy with this thickness to be applied to all new constructions and renovations. 

In the years between 1995 and 2001, thermal protection was upgraded on 18 floors of QTC1 including those where the major fires occurred on 11/9/2001 and 13 floors of WTC2 that did not include the fire floors.

At the core, where gypsum panels or fire barriers (with 2hr fire rating) weren’t fixed, SFRM was applied with a thickness ranging from 35mm to 60mm. and SFRM were used to fill gap in walls and floors through which flames and smoke might pass. Exterior walls were connected to the floors with no gap.

Following a 1975 fire, sprinkler systems were installed in both towers and completed before 11 Sept 2011.

The collapse

The two aircraft hit the towers at high speed and did considerable damage to principal structural components (core columns, floors, and perimeter columns) that were directly impacted by the aircraft or associated debris. However, the towers withstood the impacts and would have remained standing were it not for the dislodged insulation (fireproofing) and the subsequent multi-floor fires. The robustness of the perimeter frame-tube system and the large size of the buildings helped the towers withstand the impact. The structural system redistributed loads from places of aircraft impact, avoiding larger scale damage upon impact.

The hat truss, a feature atop each tower which was intended to support a television antenna, prevented earlier collapse of the building core. In each tower, a different combination of impact damage and heat-weakened structural components contributed to the abrupt structural collapse.

In WTC 1, the fires weakened the core columns and caused the floors on the south side of the building to sag. The floors pulled the heated south perimeter columns inward, reducing their capacity to support the building above. Their neighboring columns quickly became overloaded as columns on the south wall buckled. The top section of the building tilted to the south and began its descent. The time from aircraft impact to collapse initiation was largely determined by how long it took for the fires to weaken the building core and to reach the south side of the building and weaken the perimeter columns and floors.

In WTC 2, the core was damaged severely at the southeast comer and was restrained by the east and south walls via the hat truss and the floors. The steady burning fires on the east side of the building caused the floors there to sag. The floors pulled the heated east perimeter columns inward, reducing their capacity to support the building above. Their neighboring columns quickly became overloaded as columns on the east wall buckled. The top section of the building tilted to the east and to the south and began its descent. The time from aircraft impact to collapse initiation was largely determined by the time for the fires to weaken the perimeter columns and floor assemblies on the east and the south sides of the building.

WTC 2 collapsed more quickly than WTC 1 because there was more aircraft damage to the building core, including one of the heavily loaded comer columns, and there were early and persistent fires on the east side of the building, where the aircraft had extensively dislodged insulation from the structural steel.

The WTC towers likely would not have collapsed under the combined effects of aircraft impact damage and the extensive, multi-floor fires that were encountered on September 11, 2001, if the thermal insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.

 In the absence of structural and insulation damage, a conventional fire substantially similar to or less intense than the fires encountered on September 11, 2001, likely would not have led to the collapse of a WTC tower.

Prepared by Dany EL DADA

References:

1-     Final report of the collapse of world trade center towers (NIST)

2-     What Did and Did Not Cause Collapse of World Trade Center Twin Towers in New York? Zdeněk P. Bažant, Hon.M.ASCE1; Jia-Liang Le2; Frank R. Greening3; and David B. Benson4