Understanding Materiality // Andy McAllister
5/15/2015
When the Alma team began schematic design, we knew that the primary structure would be steel because of its efficient and cost effective benefits. We did not however anticipate the implications that we encountered because of our decision to use steel. In design school, it is easy to design with and use steel in a hypothetical project. We have never really been faced with what it actually means to use a certain material. Having an understanding of materials and their properties is paramount in understanding a project. One must consider implications such as production of a certain material, size of the space that parts will be fabricated and manufactured in, tools needed to manufacture a designed part, and general material properties. While the Alma team did consider many of these implications, we did not anticipate how serious some would be.
The struggles began as soon as the steel arrived by truck. We unloaded the steel and planned our course of action. It was a difficult task because the facility, the Seaton Hall shop, was not big enough to accommodate the member sizes. We did foresee this happening and made prior arrangements with another facility close to the shop. However, we quickly realized that their equipment was also inadequate for cutting the steel tube. This meant that our only option was to manually cut a 45 degree miter in 4”x8”x20’ HSS (Hollow Steel Section, aka steel tubing) with a plasma torch, outside in the winter! Taking this route added twice as much time in cleanup and preparation. All of these unanticipated challenges aside, we managed to complete all of the 45 degree miter cuts in a single day. We learned that when specifying steel, a great deal of planning and coordination must be in place to ensure a seamless manufacturing process. A quick note of all tools and accommodations must be carefully considered as well. Negligence to do so adds time in cleanup and can be dangerous if not done correctly.
Our next task was the most daunting task on the project. The task was to weld 200 steel plate tabs on 5- 2”x4”x20’ HSS beams. That’s 40-3”x4”x1/4” steel tabs, every 6” on center, per beam. Working with a structural engineer gave us a false sense of security that what was on paper would translate directly into a manufactured end product, without any trouble. Unfortunately, this was wrong. There was a 3” deflection over the course of the beam, resulting in an unusable beam. It is typically acceptable to work with tolerances of up to a fraction of an inch, but this far surpassed this. We speculated that the heat transfer was the factor, so we adjusted our method on the next beam. For beam #2, we alternated ends of the beam with each weld, leaving more time for the weld to cool. We received the same results as beam #1; a 3” deflection over the course of the beam. For the remainder of the beams, we adjusted even more and the results were no better. It took us 5 days to weld all of the tabs, and they all were equally warped. At this point, mild panic began to set in.
We had no other choice except to figure out how to straighten up the beams. When it comes to tasks like these, there are only a number of things that can be done before you have to start over. In this case, our options were to cold press them back into shape or use heat in strategic locations. Cold pressing is a technique that uses tons of force to press steel to a desired deflection. The idea of using heat is that it brings the steel back to a yielding state, making the steel somewhat malleable. We made jigs that would simulate a cold press method with inconclusive results. The only other option was to use heat to simulate the heat transfer endured on the opposite side of the beam. In theory, if the beam bowed using x amount of heat in y location on one side, then mirroring that method and applying equal variables should cancel out the opposing bend. So we flipped the beams over and began to simulate the heat transfer of the welds every 6” on center. The deflection grew slightly worse at first, but as the steel cooled, the beams straightened out.
We learned that the amount of heat transfer from the welds caused the beams to suffer from heat distortion. When steel is heated up it expands. When it cools it contracts. If the steel is heated up “red hot”, then the rate of contraction becomes greater than the rate of expansion. This causes the steel to pull towards the weld causing that surface to bow. The bow is a result of heat distortion. The technique we used is called heat straightening, and it’s essentially using those same principles to balance the initial deflection. We used an acetylene torch to simulate the heat transfer of the weld on the opposite side. I previously mentioned that the deflection worsened before the beams straightened out. That was a result of the steel heating up and expanding. As it cooled, the opposite side pulled, leaving the beam equalized. In practice, engineers design with tolerances and pre-defections that equal out when loaded. Whether we could have planned for this is hind sight. If we learned anything from this project, we learned a great lesson general chemistry and the properties of structural steel.
The fact that the steel was structural steel came along with many more implications, and we battled them for the duration of the steel fabrication. All of the holes that we needed to make in the steel tube were uncommon sizes, so acquiring proper drilling equipment was difficult. Structural steel is inherently stronger, so the bits also had to be stronger. The Alma team persevered and eventually completed the steel work necessary for galvanization. Having a better understanding of properties of structural steel would have allowed us to better prepare ourselves. Understanding materiality is paramount to fundamentally understanding a given design. Historically, architects assumed the role as architect/maker. Architects wore many hats during a project, including designing and constructing their designs. Today, architects rarely design and build their work. It has become a niche market known as Architect-led Design+Build. It is important to merge the architect and the maker because to be able to fully understand your design is to understand how it will come together. Understanding materials and methods can inform craft, details, and can provide innovative solutions when exploited. It can also help the architect communicate more clearly through documentation. It is our duty to be experts on our designs. It should also be our duty to know everything that pertains to the making of our designs.
Written by Andy McAllister