14 September 2001
The crisis we have all experienced in the last few days is overwhelming on a personal level for everyone. My family and I listen to and watch the stories that have emerged, the immensity of tragedies at so many levels, and are broken by the horror and the sadness of it all.Our lives and the lives of our children and families will never be the same.Despite the urgings of our leaders that we must return to our normal patterns of life, that is impossible. Yet we carry on. This morning I was standing on the construction site, remembering what seemed like huge piles of demolition debris being collected just a few weeks ago. Despite all the pictures, I can't even imagine what it must be like in New York right now.
We are nearing the end of the first phase of the science center project --demolition, construction of temporary facilities for those displaced, and a massive relocation of underground utility services. What comes next will be the excavation for the foundations for the structure of the buildings to rest upon. I have listened to a number of reports on the news about the structure of the World Trade Centers and the buildings which surround that site, some of which have collapsed or are in danger of collapsing. Jon Morrison, a principal with CVM, the structural engineering firm which has designed Kohlberg, the Trotter renovation, and now the new science center offered to sit down with me and talk about buildings; why they stand up and why they sometimes fall down. I thought people would appreciate his insight. There are also two very interesting books I read six or so years ago, "Why Buildings Stand Up" and "Why Buildings Fall Down", written by Mario Salvadori,. They are fascinating to read. I thought I would start this story with a citation from the latter.
"According to the Old Testament, the early inhabitants of the earth, the ancient Babylonians, were of one language and of one speech. But our earliest forefathers were not content. So ambitious were they that they were determined to build a city with a tower reaching heaven that God, offended by their pride, broke their single speech into so many different languages. The Babylonians, unable to understand one another, were stymied in their plan and their tower collapsed. The God offenders were scattered over the face of the earth: 'Therefore is the name of it called Babel' (from the Hebrew balal: to mix up). Thus was the first structural collapse."
I was amazed to read again a story early on in the text, about what was at the time the tallest building in New York, the Empire State Building. On July 28, 1945, the very day the United States ratified the United Nations Charter, a B-25 bomber left Massachusetts for a flight to Newark. Lost in the clouds and fog of that day, the plane struck the Empire State Building 913 feet above the ground, on the 79th floor, ripping a hole in the north face. One of the two motors was propelled across the width of the building, through the opposite wall, and down through the roof of a building across Thirty-third Street, starting a fire that destroyed parts of that structure.There were thirteen fatalities.The center of the impact aligned almost exactly with a column on the face of the tower; the right motor passed on one side and the left on the other. The column itself was barely damaged. Had the plane struck the column a bit higher or lower, it might have struck and bent the column and a collapse may have ensued. The author continues: "No plane has hit a skyscraper since 1946, ... although the question of the future probability of such a catastrophe is real" (amazingly enough, there was a second crash of a military plane with another NYC skyscraper just a year later). What happened in 1945 (or better put, didn't happen) was the result of redundancy inherent in the frame structure. This a good place to turn to my conversation with Jon.
Buildings perform functions, primarily protecting people from the weather and providing an infrastructure in which people can do the work we do. The function of a building is enabled by the construction of surfaces which separate the inside from the outside, walls and roofs for the most part. The envelope of a building is called its skin. The structural elements -- columns, beams and floors-- are called the skeleton, on which the skin is wrapped.
It is in the development of structure that architecture has undergone a revolution. We have moved from an era of trial and error to determine structural integrity to one where mathematical formulas rule. In most buildings, functional and structural purposes are achieved though different components of different materials. In older buildings (such as Parrish or Trotter), the exterior walls are part of the structure, supporting the horizontal members (beams) that hold each floor. In newer structures such as Kohlberg Hall, the stone walls are really only skin, with the structure, both columns (the vertical elements) and beams made of steel. "Curtain walls", most commonly associated with the skins of high-rise buildings, consist of thin metal struts encasing large glass panels. We have such curtain walls on campus as well, although obviously on a smaller scale--the walls of Kohlberg Commons or the Lang Music Concert Hall are curtain walls, as will be the new walls of the science commons. The curtain wall is then structurally supported by an underlying frame of concrete or steel,
Structure supports loads. An engineer's first task is to determine which loads might act on structure and how strong they might be. These include dead loads, the elements of a building itself which must be supported --the columns, floors, etc. In addition to its dead load (the load which will always be there), a building must support various live loads such as people, furniture and equipment. Finally, there are dynamic loads (as opposed to static loads) --wind gusts, earthquakes or explosions, for example. Dynamic loads can often be exceedingly dangerous because they can have a much greater effect than the same loads applied slowly. Taller buildings also oscillate --in the case of the World Trade Centers, for instance, the period of oscillation was ten seconds, meaning it takes that long for the structure to sway back to its original position. Wind loads act horizontally and with tall structures in particular, require a structure which bear the load differently than loads which are vertical and gravity-based. Even for buildings such as Kohlberg and the new science center (only three stories in height), wind loading needs to be accounted for. Earthquake loads are also primarily horizontal, like wind loads. In this region, they must also be accounted for in significant ways.
The purpose of structure is to carry the loads of the building to the ground. It does this through two elementary actions --pushing and pulling. Elements are either pulled by a load (and stretched --in tension) or pushed by a load (and shortened-- in compression). There are no perfectly rigid structural materials, so they are always either in tension or compression. Structural materials must not only be strong, but also elastic, meaning they must return to their original shape once the load disappears Failure occurs when structure is deformed, meaning the loss of its elastic quality, something which can happen over time or through a sudden event.
Steel, an alloy of iron and carbon, and reinforced concrete are the most common structural materials today for non-residential structures. As I noted above, Kohlberg's structure is steel, as is the LPAC. Lang Music, Martin and Dupont are examples of concrete structures. The new science center will be a combination of the two. In certain areas, like the new chemistry addition, the structure will be steel, insulated, covered and out of view. In other areas, such as the floor of the new commons, we have opted for reinforced concrete, which can be made thinner thus providing additional ceiling height for the physics department below. In other areas, the structure is left exposed as part of the architecture. The Commons will be the most extreme example of this, with its exposed concrete piers rising up to the thick wooden timbers supporting the roof. Another interesting structural element will be an independent concrete column, isolated from the rest of the building's structure, supporting the astronomy dome on the roof. This accomplishes a vibration-free zone. The structure will be founded on reinforced concrete spread footings, resting on natural bedrock --these are really just large concrete pads resting on solid material, about two feet thick and as deep as ten feet below the building. We have begun excavating for these footings already and will continue into the fall. Concrete should start to be poured before winter hits.
Steel is a common and excellent structural material in many ways, but has some limitations. Its melting point at 1200 degrees F. means that it must be treated with a fire-protective insulative coating, generally sprayed on. In the case of the World Trade Centers, the heat caused by the burning of massive amounts of jet fuel was more than sufficient to destroy the insulation and melt the structural steel columns. The buildings were able to withstand the huge destructive impact of the planes because of the redundancy built into the structure allowed it to remain standing. It was the eventual collapse of the remaining structure from the intense heat that brought the building down.
It has been reported that the floors collapsed in on themselves, dropping straight to the ground, one floor following another a split second later. This is known as a progressive collapse, or pancaking, with the sheer weight of the floors above causing each floor below to buckle. Jon thinks everything about the attack was based on more than just a working knowledge of structural engineering. The impact point was where there was mass enough above to create a progressive collapse, and also high enough to make fighting the fire impossible. The collapse of the buildings, Jon notes, was inevitable, as was the relative time frame of that collapse. The relief effort was thus doomed and the loss of life involved in that effort another tragedy.
I asked Jon if buildings could be designed in a way to protect them from terrorist activities. He said that in recent years, since Oklahoma City and the embassy bombings in Africa, such requirements for certain categories of buildings do exist. For example, he has designed the new pavilion for the Liberty Bell according to new regulations and requirements specifically intended to protect the building from collapse from explosives.
I want to thank Jon for taking the time to talk to me. He has been a tremendous resource for the College for years in many ways. In particular, he has volunteered many times to work with our engineering students on various academic projects.
Send message to the chair of the Science Project User's Group , Rachel Merz (firstname.lastname@example.org)
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last updated 9/14/01webmaster