Featured

A Start to the Basics

This is the post excerpt.

Advertisements

Steel is a very complex object and the physics and knowledge that goes into constructing a building out of steel is amazingly complex. For example, steel literally sways in the wind and is designed to do so! There is a building in China called the Taipei 101 which is designed with a 730 ton ball connected to the steel structure to mitigate any seismic effects when the building sways, like it is designed to do. This will be discussed in depth later down the road but for now the focus is going to be aimed at the simple topics like common terms, load paths, and connections. 

Common terms are key when trying to understand what is being discussed. The most common terms used are going to be girder, column, bar joist, live load, dead load, point load, and uniform load. There are many more that will be covered in the “Terms” section, but these are slightly more important. A girder is a large steel beam used to support bar joists and act as a main part of framework. A bar-joist is a smaller beam of steel that acts as a connection between girders to support a floor, roof, etc. A column is a piece of metal (can be large or small) that holds up a girder and rests vertically on the ground. A Live load and dead load are very similar with one key difference, their ability to be taken out of the building. A dead load is the weight of the building itself: the material, the joists, the concrete, everything that is permanently part of the building is a dead load. A live load is anything that can be removed or added, i.e. people, desks, chairs, tables etc. A point load is a load that puts weight in one specific point on a girder or bar joist, and a uniform load is a load that spans a certain distance of a girder or bar joist. The quick, hand drawn picture below can offer a visual to those who are visual learners. 

Displaying FullSizeRender.jpg

A load path is basically the way a steel structure handles weight, disperses it to the sides/center of the building, and then how the load is transferred into the ground. For example, if there is just a simple structure that looks like a square without the bottom line, then the load path would be simple. The top girder or beam would disperse the weight of the load across to the support columns on the side which are then shot into the ground. As a reference The picture posted below shows how a structure with five columns, and two Girders would handle a five ton load and what the load path would look like. The path travels in a relatively simply manner but when the building is big enough, the load path becomes much more complex.

Displaying FullSizeRender.jpg

Connections are definitely the easier of concepts to understand, simply because there is set ways of doing it. The most common practice is to weld connections when fabricating a connection in the shop, and bolting connections while in the field (The actual job site). Bolting is the preferred method for connection while on site. There are two ways to ensure a bolt is done correctly, that is to either snug-tighten them, or have a slip critical bolt. They both act as a high strength connection and differences range between specifications given for the specific connection being made. Another way steel can be connected is through welding, however the costs and labor time required to perform welding keep it from being more popular than bolting. If welding, however, it should be performed on bare metal, and the weld material should have a higher strength than the metals being welded together. Shop welding is much more preferred over field welding though. Shear connections are very prevalent, if not the most prevalent form of connections in steel framing. Shear connections are a webbing that connect any member to another member that will then be subject to shear forces (Forces that try to shift two objects apart). An example of a shear connection is shown below in blue.

Image result for shear connection on beam

If you have any questions, feel free to always ask!

A992 steel, A325 X/N steel.

I bet just from reading the title you’re thinking “What do those stand for?” I really never went in depth explaining them because frankly i never had them explained to me but after reading and looking into it, and  I’m ready to share! So they are different specifications of the type of steel. Steel has different properties when made in different ways, with different chemicals, different amounts of slag, or different amounts of limestone. You can create A992 steel which is commonly used in beams and girders and columns. As it was described to me this steel is actually weaker than steel used to make tools and screws. This is because beams and girders need to be able to be cut, drilled, shaped and ultimately changed. This doesn’t mean its more likely to break in half. There’s precautions set in place and these beams can still carry enormous loads, even at only one foot in depth! For example, a W8x10 A992 steel can carry 21,9 kips per foot, safely! That’s 21,900 pounds per foot, with only 8 inches of depth and 10 pounds per foot! A325 X/N steel is the opposite. Its much tougher and is used when making bolts. Actually this is specific to bolts because of the X or N. The X and N signify how the connection will be. The X signifies that the connection with threads will exclude the shear plane. The N stands for not X, so the opposite connections is an N connection. These different metals clearly have different properties which means they also different Forces they can take. An A992 has an Fy of 50 kips per square inch while a A36 has 36 kips per square inch. The Fn is 65 Kips per square inch versus 58 kips per square inch.

At the end of the day the different metals get confusing and that completely okay! because when calculating the important thing to know is that different metals exist. If we ensure we calculate for the specific metal we have then we will be golden but if we calculate using wrong numbers (numbers given above as f’s) then we can have catastrophic accidents occur. Just remember A992, generally beams. A365, generally bolts.

MATH!!! YAY!!!: Beam/Girder Selection

In this blog post I will discuss how to determine what beam or girder should be selected when under a certain load. To start of its important to understand that there is two different kinds of basic loads a beam or girder can take. A point load or a uniform load. A point load is basically exactly what it sounds like. It’s when a load, no matter how big or small, and it is put on a specific point or points across the beam. A uniform load is a load that is spread evenly, or uniformly ;), across the entire beam. When a uniform load exists it also means there is a a span in which that uniform load spans. It’s basically the weight per foot multiplied by the span of the load that rests upon the beam. So if the span of the load is 10 feet and there is a Total load of 200 pounds per square foot, then the load is 2000 pounds per foot. Another key component is that on each end of the beam (in these basic situations) there is a resistance force, also known as the columns or anything holding the beam up. Now let’s discuss the actual math behind beam selections in the case of a W shaped beam.

This first example will consist of a uniform load.that is on a 16 foot beam. This beam has a load span of 10 feet and a total load of 300 pounds per square foot. To start multiply 10 feet by 300 pounds per square foot to find 3,000 pounds per foot, or 3 kips per foot ( A kip is 1000 pounds). Kips are easier to calculate and continue through calculations due to the fact that the number becomes 1000 times smaller. The first step is to find the maximum moment the beam will take under this load by take 3 kips per foot multiplied by the beam span squared divided by 8. This looks roughly like (wL^2)/8. This answer comes out to equal 96 kip feet. After finding this answer we can use it to find Z req’d which we need to have to determine a beam. This equation is the maximum moment, multiplied by 12 inches per foot to change the answer to inches out of feet. After this we divide by (50/1.67). The 50 and 1.67 are given numbers due to the steel being A992 steel. This answer ends up being 38.48 inches cubed. After this we look in the Steel Construction Manual book under page 3-23 and look in the Zx column to find a Z that is greater than the Z req’d one has, and can handle the load the beam will be under, while still being the lightest is possibly could be. This answer ends up at w14 x 26 (the 14 stands for the height of the beam in inches and the 26 stands for the pounds per foot of the beam. So now we have a beam selected for this load.

In this example i will explain a single point load on a 16 foot beam. In spirit of showing how these two answers will differ i will still use a load of 3 kips times the length of beam to find an equivalent to the uniform load due to the fact that the uniform load was 3 kips per foot. O this we put a 48 kip load on the center of the 16 foot beam. The steps to this problem are the same, but the equations to find the answers are significantly different. This equations looks like PL/4. The P is the point load, the L is the length and the 4 is a 4. So it looks like (48 kips * 16 feet)/4, which equals 192 kip feet.This next equation happens to be the same due to the fact that we have the right units and it will equal out to inches cubed which is a Z req’d of this beam. So we take (192*12)/(50/1.67) which equals 76.9536 inches cubed. In this circumstance there is a beam that would fit with a Zx of 78.4 inches cubed but I know this beam will not account for its own weight so i chose the W21 x 44, which has a much higher Zx of 95.4 inches cubed. This will definitely hold the beam load and the beam weight which makes it the perfect selection.  

Those are two example of how to solve loads on beams. In full disclosure this process has many more steps to ensure the beam actually is qualified to support everything it needs to and this is a very simplified version but it’s also a really cool idea of understanding the basics of how to select a beam. As always, if you have any questions, don’t be afraid to ask!

Truss design

Image result for truss

In honor of my most recent test covering the capabilities of trusses i will review trusses in this post. Trusses are the webbings that hold up roofs or floors or bridges, etc. Some example pictures of basic trusses are posted above. Trusses are designed to compress and pull in various ways to increase the amount of load that can be dispersed and controlled by the truss design. Trusses are not as simple as look, however. Each chord can be under tension stress or compressive stress and due to each type of load can cause a different type of torsion to the truss. This also means that the truss must be in equilibrium so that it means it can handle loads and not collapse on itself under its own weight, or the additional weight of anything placed on it. Their is different ways to calculate a truss but the consistent thing that comes from calculations is that each chord caries a load that enters each end of the web with however many pounds of force are on the truss. With that being said, knowing all of the weights and reactions in the truss is the first step in discovering what the chord can handle and bear. So finding the reaction can lead to creating a diagram separating each chord into an X, Y coordinate plane to allow them to be quantified and compared. For example, the top standard roof, we could look at the bottom left web gusset connection and draw out the diagram. It is clear that their is a horizontal chord and an angled chord. What is not seen is the reaction at the bottom and a possible load pushing down on the truss. So after knowing all forces and chords involved in each gusset we can separate all reactions into respective X, Y coordinates to allow them to be quantified easily without taking into account the angle, that we do not actually know. After creating an X, Y diagram it is easy to look at the pounds on the chord and use math to discover all the forces in each chord, which can then allow us to find the forces in all chords of the truss. Finding the forces should be a chain reaction following laws of physics that also balances out at the end, implicating that the truss does not move when the load is placed upon it. With this being said a lot of question must arise, and as always, feel free to ask any!

Willis Tower

In honor of the Cubs winning the World Series, I am going to take a quick second to explain the steel structure and concrete, along with the connections that exist within the building. To start, a little fun fact about the Willis tower is that if it were constructed today it would weigh %35 less than it does now. This is due to an increase in technology and steel fabrication meaning that a beam can hold the same weight and weigh less. Another fun fact is that there is enough concrete in the building to construct an 8 lane wide, 5 mile long highway. Thats pretty insane! So basically how the fabrication went up was centralized around 9 center columns leading to caissons that went to bedrock, ensuring stabilization. Around the outside was lateral and vertical support that creates a large checkered look up the building. This helps spread and take load to ensure excessive load paths to hold the lateral loading of wind, and weight loading of the steel, people and an other materials on the building. On each connections appears a shear plate that acts as the main connector taking a load and ensuring it is delivered down to the prevalent steel requiring the load.

Image result for sears tower construction

Image result for sears tower constructionImage result for sears tower construction

The Structure of the Twin Towers

9/11 was, and still is one of the worst tragedies in American history and will forever be in the minds of all of the world. With that being said there are also a lot of conspiracy theories out there revolving around 9/11 being an inside job by Bush. The structural integrity of the towers were also incredible! If the twin towers were made differently, 9/11 would have resulted in around 30,000 deaths. I understand that 9/11 is a touchy subject but the towers structural integrity and ability to withstand the forces they were put under the way they did was amazing.

To start off, the plane that impacted the side of the building put a load against the building that the structure was not designed, in any way, to take. It essentially tore a hole through 1 side of the tower and knocked out a little less than a fourth of the support columns. I know that doesn’t sound like a lot but the load path that buildings have are very important and in most buildings when two or three structural columns are knocked out, the building will collapse due to the load path lead straight down with gravity, rather than through a column. So the fact that the building was able to stay standing while missing that many columns is amazing. The building would have collapsed and tipped sideways, falling onto surrounding buildings and people, which would have resulted in many more deaths than there were. To explain in more depth why the towers were able to stay standing after losing so many structural columns was because of something called redundancy. Redundancy was an idea that came to after a building completely collapsed when one support column was compromised and the building completely collapsed. After this, the idea of redundancy was introduced to buildings which allowed for multiple load paths to exist in the case that the main load path is compromised. The Twin towers redundancy was excessively large, which is great because if it wasn’t the building would have essentially bent, and fell sideways.

The collapse of the towers has also been quoted as “explosions at the base to cause it to collapse the way it did” and “Jet fuel cant melt steel beams”. Here is why all that is wrong also. So, true, jet fuel cant melt steel beams but the thing is, is it didn’t melt steel beams, it actually just heats it up past its load bearing capacities. See, when steel heats up, its ability to handle loads decreases significantly. So essentially what happened was when the plane crashed through the building it severed the fire line ties which mean the sprinklers did not have water going to them, and it also meant that room was about to get very hot. The columns, as discussed were already at a minimum and when they began heating up on the other side and across the beams and joists the weight of the building above was too much for the structure at that temperature and the building collapsed in on itself. Essentially the reason the building collapsed was because the load bearing capacity of the columns and beams decreased dramatically with every degree they went up. This ended up causing the weight of the building to fall on itself, in turn creating a momentum straight down that the building just could not support.

The Twin towers ability to handle the impact of the planes and not tip and cause even more destruction is amazing engineering and the fact that the building then continued to collapse straight down is amazing in and of itself.

What Steel Can Be

So a common thought that I used to have, and I imagine people have in general, is what are the different things steel can be? Steel can be a lot of different things, but in the thought process of construction, it can be brought down to about 7 different objects. First off, structural steel is a mild steel and is easy to puncture or cut. Its mild steel because it needs to be able to be form fitted, and have holes to accommodate the bolts that will inevitably hold it in place. Objects that are hard steel are tools and drill bits that need to be as strong as possible and not break while being used.

The 7 different types of structural objects that can be made from mild steel and are used in construction consist of: Wide flange beams (Both W and S), plates, angle, T-shape (Again, either W or S), bars, Channels, and structural tubing.

Image result for Different shapes of steel

Wide flange beams are shaped like capital I’s and come in two different forms; both are pictured above as W, and S. A W wide flange beam is different than an S wide flange beam through one very small difference: whether it is curved, or sharp edged where the top and bottom flanges meets the middle line. So a W wide flange beam has sharp angles and is very square all around and parallel, while an S wide flange beam is more rounded and not parallel. Wide flange beams are used to hold up a roof, or a floor and act as support for great distances that need to be spanned.

Plates are as easy as they sound, they’re just plates of flat steel that are used to support steel throughout a flat surface with a designated length and width.Plates are used within brick walls or in any place that need extra support across longer distances.

Angle steel is plated steel that curves into an angle and can be used to support various objects. Angle steel is represented as L in the picture and looks exactly how it it sounds. Angle plates are used to either hold up subflooring or connect something to a wall and act to hold it at a 90 degree angle. Angle iron is also used in lintels, which means it spans across an opening of brick and supports the brick above the opening so it doesn’t collapse in on itself.

T-shape steel is a wide flange beam that has been cut in half to create two T shaped pieces of steel. The reason they can be either W or S like wide flange beams is because they are cut out of wide flange beams so they are shaped S or W based off of the Wide flange beam it was cut out of.  T shape is represented by WT or ST in the picture. T-shapes are most notably used in truss designs which hold up roofs. T shapes act to support the connections of a truss and strengthen a roof design.

Bars are long sticks of thinner steel that are smaller, more compact versions of plate steel. Bar steel can come as square/rectangular, or it can be round. Bars are honestly just bars of metal and are represented in the picture as h. Bars are used to support anything in need of a thing, yet long bar of steel to support it. Really, more or less, useless unless represented within a brick wall

Channels are represented in the picture as C and looks like a C. Channel steel is formed out of wide plate steel that is bent to fit the specific channel required int hat specific building. Channels act as a support for walls, trusses, or other objects that need such support.

Finally, Structural tubing Structural tubing, regardless of its name can be either round or square, but it was to be hollow. Structural tubing is represented by F and G in the picture and are primarily used as columns in buildings to hold up floors or roofs. They also act to hold up Wide flange beams, or floor joists and are very important in the grand scheme of supporting a structure.

 

Image result for Different shapes of steel

I have the picture twice to make it easier on the reader to find the correlating picture to the correlating piece of steel being discussed. As always, if you have any questions or want anything clarified, don’t hesitate to ask

History of Steel

The development of steel essentially started about 4000 years ago during the Iron age. Iron was stronger than bronze and began to take over and replace bronze as a metal used to produce weapons. Iron was beginning to be widely produced and stayed widely produced for the next 3700 years or so. Around the 19th century the problems that were present in iron became too prevalent to be ignored anymore. Iron was brittle and was being produced in inefficient ways, and during this time period, railroads were becoming too big for iron to stay the way it was. The railroad required a better produced iron, which caused mass funding to be put into testing and producing better quality products.

So to start, iron has different strength qualities depending on what its carbon content is. Iron, when too hot, begins to absorb carbon and take a carbon content of about 2.5%-4%. This Iron is called cast iron and because of its higher carbon content, is brittle and lacks the strength it needs. To combat this high carbon content, scientists discovered a method to slowly add oxygen, allowing the carbon content to be pulled out slowly. This, however, caused the iron to have an increased melting temperature causing some of the iron to collect/chunk in the furnace. The chunks would just be removed  and worked by a forger before being formed into sheets or rails. This process, while better, was still inefficient due to its massive labor and fuel requirements.

At this point, steel was still unproven as a structural metal, and its production was slower and more expensive than iron’s was. That was until a scientist named Bessemer created a solution that removed carbon from within the molten concoction, but the only problem was that it worked too well. There was an abundant amount of oxygen left within the steel and made it extremely brittle. This was a huge problem for Bessemer, but a slution came out when British metallurgist Robert Mushet created Speigeleisen. This would draw the oxygen out, and create a stronger steel. There was only one problem left at this point: a chemical called phosphorus made the steel brittle and a solution still had not been found to remove this from the mixture. That is, until 1876 when a scientist came up with the solution: Limestone. The addition of limestone would pull the unwanted chemical out of the molten steel.

After all of these developments and innovations, steel prices dropped significantly and as a result, the steel industry grew quite a bit. The industry didn’t completely boom until Andrew Carnegie and Charles Schwab invested and made quite of bit of money in the steel industry. Carnegie opened US Steel Corporation in 1901 and it was the first business in America valued at over 1 billion dollars.

Steel Really took stride in the early 1900 and was mostly recognized for its ability to be useful in the railroad industry. To this day it is used in almost everything built and is one of the best building materials in existence.