Friday, July 17, 2015

Alphabet Blocks and Civil Engineering 101

   Children are born engineers. Everything they see, they want to change. They want to remake their world. They want to move dirt and pile sand. They want to build damns and make lakes. They want to launch ships of sticks. They want to stack blocks and cans and boxes. The want to build towers and bridges. They want to control the universe. They want to make something of themselves.

Dr. Henry Petroski
Professor of Civil Engineering
Duke University
American Scientist, Volume 91
May-June 2003
   I am interested in how K–12 educators plan to implement the engineering component of the Science-Technology-Engineering-Mathematics (STEM) effort. The same tradition in public education that supports the teaching of mathematics and science at the K–12 levels does not exist for the teaching of engineering at those levels.
   Much of the STEM curriculum I've looked at focuses on design. But, as I heard one engineering professor state, "Not all design is engineering and not all engineering is design."
   There are electrical, aeronautical, marine, chemical, civil, and software engineers. But from that short list the branch that I would choose to start to build a curriculum around is civil engineering. That is the type of engineering that kids do without formal, adult instruction or supervision. Kids design and build structures in their world of play.
   If kids are 'born' engineers as Dr. Petroski believes then kids are born civil engineers. The task then becomes one of identifying what civil engineers know and do and then translating their knowledge and skill into age appropriate, activity based curriculum.
   As I researched what civil engineers know and do, I learned that there are five basic construction forces and six basic structural members. Many of these are conceptually appropriate at the K-8 level and all are appropriate at the 9-12 level.
    For example, to the civil engineer, a stack of blocks is a structural member called a column. The dominant force acting inside the column is compression, one of the five construction forces. A stable tower will not collapse from internal forces (the weight the blocks) but it is susceptible to external forces—another youngster can come along and knock down the tower (see video at end of post).
   Consider a single cube sitting on a flat surface. In the diagram, the cube is transparent so we can focus on its bottom face. The full weight of the cube acts on the bottom face (red). Since weight is a force, and the cube is made of a homogeneous material, the weight
is distributed over the area of the bottom face.
  Science defines pressure as force per unit area. This is expressed in the following formula.

   If a block is stacked on top of the first block, then the pressure on the bottom face (red) of the bottom block doubles. 

   And so on through as many blocks as there are in the stack. This is a linear relationship. Note that the pressure on the bottom face of any block equals its weight and the weight of the blocks stacked above it. This weight presses down on every block in the stack except the top block. Hence the term compression.
  The civil engineer calls this pressure stress.

   The force acting on the bottom face of the bottom cube of the tower is equal to the weight of all of the cubes in the stack. That, of course, equals the weight of the number of cubes in the stack.
   The greatest internal stress (pressure) acts on the material near the bottom of the bottom block. The material that the block is made of has to be able to withstand this stress. And the foundation (ground) the column is standing on also has to withstand the total stress of the column. Hence the need to build on a firm foundation.
   Not all forces acting on the column are internal forces. Wind, earthquake, and collision can exert external forces on the column.
   Next to the pleasure of building a tall column of blocks is the pleasure many feel in knocking it over. In the following video, a horizontal force is applied a column of cylindrical blocks. A friend captured the collapse of the column, in slow motion, on his iPhone. Slowing down the collapse reveals that several interesting events happen in the short time it takes for the column to collapse. Can you spot them?

Saturday, July 4, 2015

Building Block Towers

   As promised in the previous post on Alphabet Blocks, this post will focus on a tower-building activity. This activity was just one of the many enjoyable times grandmother and I had when our three grand kids from Georgia came to visit. 
   Next summer, there’s a chance I might teach a course about construction forces, forces like compression, tension, etc., to K–8 teachers. For the course I wanted to build a model of Trajan’s column in Rome but I needed a set of cylindrical blocks. These are not traditional building blocks and I was unable to find a set of cylindrical blocks for sale.
   I made a quick visit to one of our local home improvement stores and found a wood dowel four feet long with a diameter of two inches. A friend with a table saw cut the dowel—I think it’s used as a closet clothes hanger rod—into two-inch lengths. I painted each block with two coats of Deft Clear Wood Finish. The Deft seals the wood and makes the blocks easy to clean with a damp cloth.  The dowel is made of a good hard wood and the finished blocks make a durable set with a nice, smooth feel to them..
   To make a scale model of Trajan’s column I needed 9 of the two-inch tall blocks. When cut, the rod made 23 blocks and that turned out to be just the right number for the kids to use to build a tower.
   The towers that the kids built in the following pictures are examples of a column. The column is one of the six structural members used in all of the structures that civil engineers build. The column is also a compression member but more about that in the next post.
  Asking the question, "Who can build the tallest tower?" was all the motivation the kids needed to get them involved in the activity. We agreed that the height of every tower was simply the number of stacked blocks without the tower toppling over.
   Emma is three years old and wanted to be the first to build a tower.
   We were amazed that she was able to stack 18 of the cylindrical blocks. She was so proud of herself but alas, that block she has in her hand in the third picture was the block that toppled the tower.
   Next up was Kate, She is six and she built a tower of 22 blocks but that last block—seen on the floor—toppled the tower.
   Asher, the oldest of the three grand kids is eight. He used every one of the 23 blocks in his tower plus the piece I had made to model the top of Trajan's column.
  If you look closely at the pictures of the columns for each youngster you will notice that there is a large number of blocks that are offset in Emma's column. For her to get a column as tall as the one she built she had to make allowances higher up the column for the offsets lower down the column. She has no vocabulary to communicate to others  how she built her tower but she has some form of understanding of tower building that predates language.
   Notice that both Kate's and Asher's columns each have very small offsets. In fact. both Kate and Asher were careful to match the base of the block being added with the top of the block at the top of the column. They too do not have the vocabulary to describe the method each used to build their tower. They have yet to develop concepts like center of gravity, center of mass, etc. but they do know, from experience, what "to balance" means and they apply that meaning when asked to build a tower of blocks.
   In the next post I will describe elements of the math, science, and engineering inherent in block stacking.