Prefabrication in Bridges and Aircraft

The following two articles give examples of the advantages of using pre-fabrication in their respective fields, bridges and aircraft. I chose to look at these two industries for two very specific reasons. First, because of the incredible stresses/loads they must resist, the parts used in each must be extremely strong and meet rigid standards. Second, should failure occur, the loss of life is nearly guaranteed. Therefore, the parts must be reliable. I can apply these same criteria to my design for tornado-resistant structural connections. The following bullet points are close to or shown exactly as published in the original source.

Connection Details for PBES: Chapter 1 – General Topics. US Department of Transportation, Federal Highway Administration. 22 Aug. 2013. Web. 16 Nov. 2013.

Benefits of Prefabrication in Bridge Construction:

  • Constructed in a controlled environment using high quality materials and standardized production processes.
  • Improved quality leads to an extension of the structure service life.
  • By reducing the amount of construction that takes place at the site, the amount of time that construction crews and motorists are exposed to the dangers of work-zones is also reduced.
  • Can keep a project on schedule even with fewer available working days and other environmental limitations at the site; complete more construction during a short construction season.
  • Identical repeating elements reduce costs.

Canaday, Henry. Plastic Revolution: Automation, Innovation Expand Use of Composites. Air Transport World, May 2013, 43-45. Web. 16 Nov. 2013. <atwonline.com>.

  • Less cost per man hour of labor, but
    more capital investment. As volume increases, these
    factors should balance out at lower unit costs.
  • More accurately made components; assembly of parts becomes easier and faster.
  • Composites do not corrode, so they should save operators
    maintenance costs. Potential for the use of new materials.
Pre-cast piers transported to the site, rather than cast high over a body of water.

Pre-cast piers transported to the site, rather than cast high over a body of water.

Robotic arm laying fibers for composite materials.

Robotic arm laying fibers for composite materials.

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Fixation Through Form (and Material)

Meijs, Maarten, Knaack, Ulrich. Components and Connections: Principles of Construction. Basel, CHE: Birkhaeuser, 2012. Web.

After determining the function that a connection/joint must perform, the designer/engineer can begin to develop its geometry and select its material. I had this book in a bookmarked folder for several weeks now and have finally been able to read through the chapter on connections. I’m glad that I didn’t get to see it until now, or else I would have really jumped ahead of myself. This construction book is packed with information regarding the options available to designers when developing connections, such as position, fixity, forming and reforming technologies, material, etc. Pros and cons of each option are also discussed, leading to some really helpful insight into just how creative I can be in this process. For instance, I can use direct form-locking connections in conjunction with a material connection, using a geometry of built up components angled just so so that no moments arise through load transfer. This is very exciting! I knew that I wanted to stay away from indirect form-locking connections, where an extra member is used to connect components, such as a dowel or nails, because they feel unnatural in a digitally fabricated design and because they are insufficient to carry the concentrated loads they must transmit. However, before reading this, I didn’t even know it had such a name. Even more importantly, however, rather than feeling like I would be taking an easy way out or being untrue to my goals, I now wish to use some adhesive agent with at least one of my sculpted connection details. Rather than gluing together a butt joint, by carving out a specific geometry between two components, I’d be increasing the surface area for the adhesive to bond to. Digitally fabricated, integrated form-locking design is still the necessary step in making stronger connections.

Other considerations brought up in this book are the assembly and dis-assembly of the connection and feasibility of construction. Can the parts be easily transported to the site? Will special equipment be needed to assemble the building that would undermine pre-fabrication’s cost effectiveness? Are the parts ever meant to be taken apart? How will demolition and recycling be affected? Perhaps without intending to do so, the book gave me some fuel to use in the argument for pre-fabrication (highly controlled environment in labs specially equipped), yet reminded me to be realistic. My construction technique should be as realistic as it is effective.

Shear Connections with rods, angled to eliminate bending moments along the rod.

Shear Connections with rods, angled to eliminate bending moments along the rod.

Timber Construction

Timber Construction

Page 84 Page 80

Wind Forces in a Tornado-like Vortex

Sabareesh Geetha, Rajasekharan, Matsui Masahiro, and Tamura Yukio. “Characteristics Of Internal Pressures And Net Local Roof Wind Forces On A Building Exposed To A Tornado-Like Vortex.” Journal Of Wind Engineering & Industrial Aerodynamics 112.(n.d.): 52-57. ScienceDirect. Web. 16 Oct. 2013. DOI: 10.1016/j.jweia.2012.11.005.

From this article in the Journal of Wind Engineering & Industrial Aerodynamics, I was able to learn about the types of forces a building is subjected to in a tornado. The investigation modeled buildings with different percentages of openings on walls and roofs. Then the study tested the models in a tornado simulator under conditions of an EF5 tornado, which means wind speeds are 200+ mph. Using multipe pressure sensors inside and on the model’s exterior, data was collected and the results compared. The study found that the pressure on the roof varied based on the buildings proximity to the core of the tornado. It also identified uplift as the dominant load condition near the tornado’s core, and radial loading as the dominant condition further from the tornado’s core. Because a tornado moves, any product that I design will have to address both radial loading, which induces bending stress, and uplift, which induces axial tension.

Because the study used a scaled down model, the tornado had to correspond to the same scale. The scale used was 1:1000. However, I do not intend to design/model a full house. I would like to take a chunk of a house where the wall meets the roof and  test how well they hold together. With more investigation, I can find the wind pressure’s force and use those numbers to apply load to my designs, rather than test only for scaled down loads. It would also be of some benefit to test single elements under tension and bending; rather than a chunk of a house, I could try to pull two pieces apart. I will still have to look into the equipment available to me for testing procedures.

Integral Attachment Theory – Designing Plastic Parts

“Design Solutions Guide.” BASF Corporation, Engineering Plastics. 2007. Web. 23 September 2013. http://www2.basf.us//PLASTICSWEB/displayanyfile?id=0901a5e1801499d2

“Technical Expertise: Snap-Fit Design Manual.” BASF Corporation, Engineering Plastics. 2007. Web. 23 September 2013. http://www2.basf.us//PLASTICSWEB/displayanyfile?id=0901a5e1801499d5

I first encountered this guide over the past summer working on a UROP with Larry Sass. We were asked to develop a connection detail that could work throughout the house we were fabricating. One of my colleagues had used some of the images as inspiration for our project, though we didn’t get into the material of the guides. I’ve spent the last two weeks reading through the sections and scanning the tables. There is a lot of information in these guides!

The most useful information in these guides are the structural equations that need to be considered when designing your part’s geometry. I was happy to see that much of the structural considerations were topics that I am already familiar with and comfortable calculating, such as stress, strain, moments, and torque. Also really useful are the tables included that list the equations needed to calculate moments and certain constants by sectional properties. Furthermore, the guides include classification and physical properties of plastic materials. This will be very handy in choosing the appropriate grade of plastic in developing a connection design. Properties such as loading capacity, thermal expansion, moisture absorption, and impact resistance will directly affect my decisions.

The second guide focuses on snap-fit design. Snap-fit, or press-fit, uses force to connect one part to another through some slot (see image). I’ve touched on this a bit in an earlier post. Initially, I assumed this is the form my connection detail would adopt, but from the first guide I now have new ideas for ways that two parts can connect. They can screw into each other. I also wonder whether material expansion can be exploited in design: can one piece be a mold for the following piece, yet the mold doesn’t allow the second part to be extracted?

While there is a lot of useful information in these guides, I’m left with more questions. The examples used in the guides are small parts; aspirin bottles/caps, car door handles, tool handles, etc. The forces they assume are also very small. I wonder if these same principles can be applied to pieces 2 to 3 times larger and under hundreds of times more force! I will have to find examples of more similar use parts, maybe airplane manufacturers have used plastic. That could be interesting…

snap-fit

Preparing for a Hurricane – Is it worth it?

Source:

Jaffe, Greg, Martha Brannigan, and Douglas A. Blackmon. “Should Building Codes Be Tightened in Zones Prone to Hurricanes? (cover story).” Wall Street Journal – Eastern Edition 16 Sept. 1999: A1. Business Source Complete. Web. 19 Sept. 2013.

Summary:

When the issue of building codes is discussed, it seems that the price tag for stricter rules is scarier than 110 mile per hour winds. As it stands, who pays the most when a hurricane strikes? Is it the home owner or the insurance company?

When a home is destroyed, any insured homeowner can rebuild without spending their own money (depending on the terms of their policy). However, when it comes to new construction, the homeowner would have to pay extra money for shutters, impact-resistant glass, deeper concrete foundations, etc. It seems that homeowners don’t want to have to spend this money, nor do they want to lose insurance benefits for not having these reinforcements in place. The insurance company would, of course, like every policy holder to have hurricane measures in place, reducing the payout they are required to make. But does this make home reinforcement less effective? While there is an upfront cost to the homeowner, doesn’t the homeowner save when it comes to the hassle of rebuilding, life safety, and personal loss?

While the debate seems to be focused on who has to pay what, from an outside point of view, the point of money is mute. Any measures that can be taken to protect your home, neighbors homes, personal belongings, and life are completely worth it. The issue that I will have to contend with in my research will most likely be the possibility of implementation. Likely, my research plans will focus on new constructions and prefab. Hopefully the prefab aspect of my research will be able to keep costs down and not undermine affordable housing in hurricane zones.

Learning From Destruction

Source:

Williams, Jack. “Hurricane Proof House: How One Man Rebuilt After Hurricane Charlie.” Weatherwise 62.4 (2009): 24-29. Inspec. Web. 19 Sept. 2013.

Summary:

In this article, one homeowner gets a look at the susceptibility of his own home in the face of a hurricane. In 2004, Hurricane Charlie ripped through Jim Minardi’s home while he was taking shelter in his neighbor’s living room across the street. After the storm, the two houses served as a stinging example of the contrast between a simple construction and one meant to exceed building codes, even though the codes already had hurricane considerations built in. The article then describes Minardi’s chance at having his home rebuilt with several hurricane-proof systems as part of a television program. Here are the improvements that were made:

-Cast-in-place concrete walls, with reinforcement embedded into the concrete slab foundation.

-Roof rafters “firmly” attached to the walls, though there is no mention of how. In a ‘normal’ construction, few nails are used to attach the roof to the tops of walls. Under extreme uplift conditions, the roof can be easily ripped off of the house, instead of the the entire house behaving as a single system.

-Thicker wood used in the roof and more heavy duty covering, attached with stainless steel screws rather than glue. The roof is hip roof with bracing between its trusses. The old gabled roof left the ends to high-impact winds and the trusses were not behaving as a single system.

-Windows: Impact resistant glass in frames embedded in the concrete walls, held in place with 3.5″ long screws every 10″ along the frame. Previously, the windows had only three 1.5″ screws on each side of the frame. Flying debris can be one of the most dangerous elements in a high-wind storm, opening the home to water intrusion.

-3′ stem wall around foundation in case of storm surge.And backup generators to keep air conditioning running, preventing mold from growing.

The walls, foundation,windows, and roof have all been updated and reinforced. However, what really stands out to me is the effort to join all of these parts together, creating a single home system, rather than multiple independent systems within the home. This reinforces my instinct to focus on the connection details in home systems. Another point that seems important to me in this article is that the home that survived Hurricane Charlie was build to EXCEED code even though the code had already been updated to consider hurricane loads.

 

Structural Tube Assembly – US Patent

Source:

Weiland, Edward E. “Structural Tube Assembly.” Patent 3776523. 4 December 1973. Web. 12 September 2013. <http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=3776523.PN.&OS=PN/3776523&RS=PN/3776523>

Summary:

This patent is particularly relevant to my research because it is a perfect example of a pre-fabricated construction technique that utilizes geometry to join parts rather than additional bolts and screws. One metal tube is sculpted to fit into a slot in a second metal tube. The portion of the first tube inserted into the second is split into a prong-like shape. After inserted, the two prongs are forced apart to conform to the shape of the interior of the second tube. To increase the strength of this connection, another tubular member with a constricted end is inserted between the spread prongs, preventing the prongs from returning to their original shape.

The method used here depends on the material’s ability to be molded, such as metals and maybe plastics. This leads to the question as to why home framing isn’t built more in steel? Skyscrapers and institutional buildings use massive amounts of steel in their structures. Is it because they can afford it or because they have to be safer? If it’s a cost issue, wouldn’t a salvageable/recyclable material be more cost effective in the long run? If it’s a safety issue, shouldn’t a person feel safest in their home?

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