A Structural Analysis of:
The Brooklyn Bridge Ryan Grace - Shuiping Xiong Structures III Prof. Matthew Dates Fall 2016
Inquiry of
The Brooklyn Bridge
Ryan Grace - Shuiping Xiong
Structures III - Prof. Matthew Dates
As we progress through our architectural careers, we both agreed that we have become more interested with the things which cannot be seen. Examples of this can be found in the detailing of connections, the sizes of steel which are used to hold up buildings, and one of the most interesting aspects of structure to us is the foundational supports of buildings and bridges. When one is walking down the street and stops to admire a structure, the exterior design above grade is the admirable focus, however, one should not forget the unsung hero is the structural foundation holding the structure up and in place. Many questions begin to arise when thinking about the foundation of a building, or bridge. How deep down does it go below grade? What was dug up during the excavation process? What unseen circumstances did they encounter? The list goes on, however, we will highlight the additional questions in the following paragraphs. One of the most interesting structures to us was the Brooklyn Bridge which is located in New York City.
Source: http://3.bp.blogspot.com/-D4PntK8a3ag /T2TBXeXDa6I/AAAAAAAAJaU/pVtsf8Yn9ww/s160 0/brooklyn-bridge-2.jpg
The Brooklyn Bridge, created originally by John Roebling and opened in 1883, peaks our interest as architectural students. Simply knowing how old this bridge is and the limitations they had in construction methods back then makes this bridge fascinating to knowing it is still in operation and being used every day by New York City pedestrians. For example, knowing they had to lower workers in caissons to excavate the sand by hand (see below) helps puts things in perspective on how relatively quickly technology/methods are evolving. In the world of 3D printing and modeling, etc. this bridge has intrinsic value knowing it was not produced with any special technology. The following questions are the reason why we would enjoy working on this bridge for the semester project. It has always been a wonder in our eyes, and we believe this is the perfect opportunity to thoroughly investigate it. There are several questions which we would like to explore while looking at this bridges design: Source: http://paintingandframe.com/uploadpic/others/big/1-brooklyn_bridge_caisson.jpg
The first group of questions we have can all be related to methods and materials. What materials were used for the foundation and what is it sitting on? Was any specific type of backfill used or is it the sand which was there before? What are the life expectancies and limitations of such materials? In addition, how are the materials underwater holding up compared to their similar parts above ground? 133 years ago they probably did not anticipate the cars/buses which are constantly going over it today. Is it still holding up well? Also, how do the materials underwater weather compared to the materials above water? Next, this broader grouping of questions can be categorized with water as the subject. After taking a studio where we learned about sea level rise and its impacts on city inhabitation, we would like to know how much/little of an effect the surrounding water has to help hold up the bridge in place. What would be needed if the water is not there? Knowing the weight of water and speculating at how much volume is surrounding the foundation of the bridge, we wonder how this was factored in the structural calculations. Do storm surges greatly affect this? Will sea level rise help or hurt the structure of the bridge? Freezing/thawing of the water over a century has bound to have had some impact on the materials underwater.
Grace, Xiong. Initial sketches
These questions will help begin to frame our research and investigation into the future making of a model. We are not necessarily limiting ourself to these questions/criteria, yet we are open to any ideas which may open up once our research continues with this project.
Grace, Xiong. Initial sketches
Masonry
The Brooklyn Bridge:
Loading Diagram
Ryan Grace - Shuiping Xiong
The two iconic Gothic towers are composed of Limestone and Granite.
Structures III - Prof. Matthew Type: Dead Load Table C3-2: Granite & Limestone are both 165 lb/ft³ Load Path: Force travels into the caissons which sit on top of the bedrock
Load Charts
Steel Cables wl1
wl2
l1
There are four main cables supporting the Brooklyn Bridge. These are composed of a total of 19 wires twisted together which results in a wire 15” in diameter.
wl1
l2
l1 RC=1.10 wl
RB=1.10wl RA=0.400 wl
RD=0.400 wl
Shear 0.400 wl
0.500 wl
0.600 wl 0.500 wl
Moment
0.500 wl
-0.100 wl2
0.500 l2
Steel Wire Rope & Diagonal Stays
These wires are connecting the main cables and the deck of the Bridge
-0.100 wl2 +0.025 wl2
0.400 l1
0.400 wl
Type: Dead Load Table C3-2: Steel: 492 lb/ft³ Load Path: Force travels into the top of the Gothic towers
+0.080 wl2
0.500 l2
Gravity Loads. Compression. Tension.
Type: Dead Load Table C3-2: Steel: 492 lb/ft³ Load Path: Force travels into the four main Steel Cables
Deck
The deck of the bridge is composed of steel and concrete
Type: Dead Load Table C3-2: Steel: 492 lb/ft³. Concrete: 150 lb/ft³ Load Path: Force travels into the steel wire rope and diagonal stays
Pedestrian Activity
The Brooklyn Bridge, as of 2014, sees 120,000 vehicles, 4,000 pedestrians and 3,100 bicyclists cross the Brooklyn Bridge every day. Type: Live Load Load Path: Forces travel into the deck
The Brooklyn Bridge:
Bending Tool Analysis Ryan Grace - Shuiping Xiong
Initial Findings: The caissons appeared to be under an immense amount of compressive force.
Structures III - Prof. Matthew
Background: Our area of focus for the bending tool analysis studies the foundational support of the Brooklyn Bridge. The foundational support was made possible by the use of caissons. Caissons were large wooden boxes which were sunk in place by placing granite blocks on the top of them. The bottom remained open as workers dug out the sand below them in order to lower the caisson to bedrock. Bending Tool (see photo to the right): Our tool is simple, yet helped to simply explain how forces are acting on these caissons. As the rings of tape move closer together, compression is being shown, vice versa, as they move apart, they are in tension. In addition, we marked lines on the tape which act similar to the tape. Farther apart = tension, closer together = compression.
Compression
Number of lines exposed before force: (8)
Compression tension
Compression
Before: Caisson under vertical forces Source: Grace, Xiong
Compression
Caisson under horizontal forces Source: Grace, Xiong
The question that arose was:
Number of lines
With these compressive forces constantly exposed after pushing inward on the caisson, how were force: (9) they designed that they did not collapse in? During and after construction? After: Caisson under vertical forces Source: Grace, Xiong
Research Continued After furthering our research into how they prevented the caisson from collapsing inward, we realized they had two different strategies. During construction, while the inside was empty and was only filled with workers, they used compressed air to counter-act these compressive forces. The workers would enter in an air lock chamber (highlighted below) and then they would pressurize the inside. Once they hit bedrock with the foundation, they filled the foundation with concrete, which has great compressive strength. Orange dashes highlight air chamber locks. Arrows represent the pressures entering and counteracting the water forces.
Source: Grace, Xiong
Blue foam represents concrete
Source: Grace, Xiong
Study of Different Forces: We have two main forces acting on our object. First is the force of the Granite stone, and then the weight of water acting on it from all sides. We began by placing a piece of glass on top of our tool. Glass was used so we could see through it and observe how the tool behaved. This glass represented the granite stone, and the water. In addition to the weight on top, the weight of the water would have been constantly pushing on all sides of the side of the caisson.
Before: Section of Caisson Under Construction
(Source:https://www.jamesmaherphotography.com/new-york-historical-articles/ brooklyn-bridge/)
After: Caisson under vertical forces Source: Grace, Xiong
The Brooklyn Bridge:
Structural Drawing (Scale1:600) Ryan Grace - Shuiping Xiong
Structures III - Prof. Matthew
The Brooklyn Bridge:
Structural Drawing (Scale1:50) Ryan Grace - Shuiping Xiong
Structures III - Prof. Matthew
The Brooklyn Bridge:
Caisson Experimentation Ryan Grace - Shuiping Xiong
Caisson
Structures III - Prof. Matthew
Air Caisson
Air
Air Air
Loading Balance/Distribution:
Cant access work platform
Therefore, we experimented with several different sizes and shapes of materials to examine which method works best
Caisson
Source: Grace, Xiong
Air
Too heavy of weight / too much material at once and they would have sank the caisson immediately and it would not have been accessibly by workers.
Background: Air Air We have entered this final analysis of the Brooklyn Bridge under one main/general underWater standing of how the caissons work. As you push a cup upside down into water, the air remains inside the cup and water does not enter (see Figure 1). We believe they were able to do this Water until they hit grade and continued this during the digging process until they hit stable ground. (Figure 1a) Cup prior to submerging in water
Initial Exploration: Loading of the Caisson
Caisson
(Figure 2) Unaccessible Work Scenario Source: Grace, Xiong
Air
Water Water
(Figure 1b) Cup after submerging in water. Note: Boat is remaining at the bottom of the cup.
Man Hole Excavation
Air Pump
Water
Water
Source: Grace, Xiong
Caisson
Caisson
Air
(Figure 3) Attempt at loading with steel. Result: Sank immediately to the bottom. Take away: Add weight slowly, also maybe not such a heavy material Source: Grace, Xiong
Air
Explanation: Water is denser than air. Therefore, air will want to remain above the water. so as air is trapped inside of the cup and pushed down, it has no place to go except to stay within Water the boundaries of the cup.
Main questions we asked ourself:
Water
Caisson
Air
How were they able to successfully load the top of the caisson so it sank?
Caisson
Air
How were they able to maintain air pressure inside the caisson at all times?
(Figure 4) Attempt at loading with rocks on the same platform. Result: Shaky, due to the weight being higher up off of the water Take away: Try to keep the weight low Source: Grace, Xiong
Final Exploration: Maintaining Air Pressure of the Caisson Basic Shape Exploration:
Was the rectangle shape caisson simply picked based off of the design for the piers? Since they did build the caissons off shore and floated them on the water prior to the sinking, we let two similar cups float without any assistance...
(Figure 7a) Round cup regularly would ‘roll over’ upon a forward push. Source: Grace, Xiong (Figure 5) Attempt at loading with smaller rockers on a lower and wider platform. Result: Remained stable throughout loading. Slowly sank as we loaded the platform Take away: The caisson must have displayed these similar characteristics or else it would not have been stable throughout construction Source: Grace, Xiong
Cant access work platform
Uniform Incremental loading was the solution!
As weight on top of the caisson was increased, it needed to be able to gradually push down the caisson, while maintaining that the top was still above the water level. Therefore workers could always access the work platform. (See Figure 6)
(Figure 7b) Rectangular cup would maintaining stability. Perhaps this lead into the design of the piers, not the other way around! Source: Grace, Xiong
Further Exploration:
We knew from previous studies that they had different chambers for different construction purposes: 1) Excavating the dirt 2) Allowing workers to enter and exit 3) Keeping the caisson pressurized (Figure 6) Incremental Caisson Loading Diagram Source: Grace, Xiong
Note: Our caisson model consists of plastic Tupperware containers with three straws which will represent these three chambers. Hot glue was applied around the perimeter of the chamber/container intersection to give us an airtight seal.
Man Hole Excavation
Air Pump
Airlock system exploration: We will first test the airlock system which they used to get the workers in and out. (Note: All three chambers were blocked) (Figure 9) Photos showing success of pinching straw to represent an airlock. Source: Grace, Xiong
(Figure 8a) Caisson with open chambers. Water immediately entered the caisson upon submerging. Source: Grace, Xiong
9a. Standard blocking of top of the chamber
9b. Pinching below the standard blocking
9c. Can now open the top and access inside the chamber
9d. Our ‘person’ we used
9e. Placing the person inside the chamber
9f. Person is now inside. Then we blocked the top of the chamber
9g. We then released the bottom pinching closure
9h. Person is now inside the caisson. Air pressure has been maintained
(Figure 8b) Caisson with closed chambers. Water will not enter the caisson. Source: Grace, Xiong
Moving forward, we knew we would need to have the tops of the three chambers closed at all times in order to maintain the air pressure inside the caisson. So then we ask...
How were workers able to enter/exit the caisson? How were they able to get soil/grade out of the caisson?
Grade excavation exploration:
Based on previous studies, it appeared that while the manhole and airlock chambers were closed, the dirt excavation chamber was open above. (See Figure 10a)
Eureka moment (hopefully...)
With the chamber being plugged at the bottom, could this act as it did before when we plugged it at the top?
To this knowledge, we kept all chambers plugged on the initial plunge, then once it hit the ground, we opened one chamber, and kept the other two plugged. (See Figure 11)
(Figure 12) Pressurization Hypothesis Source: Grace, Xiong
(Figure 10a) Original Caisson Drawings (https://www.jamesmaherphotography.com/new-york-historical-articles/brooklyn-bridge/)
We were hopeful that if we were able to keep the bottom of the caisson covered by the soil/grade, then it would essentially be the same idea as plugging up the top of the chamber. Therefore, this would keep the caisson stable (Or mostly stable). Additional air pressure would then be pumped into the caisson to further stabilize the cabin.
(Figure 11) Failure. Once we let go, we saw water slowly start to penetrate the caisson. Source: Grace, Xiong
Back to the drawing board....
Took a step back to re-examine photos and notes. At a closer look we noticed the bottom of the excavation chamber was just about plugged into the bottom of the soil/grade.
(Figure 10b) Original Caisson Drawings (https://www.jamesmaherphotography.com/new-york-historical-articles/brooklyn-bridge/)
(Figure 13) Exploring the relationship between the excavating chamber and the soil/grade. Source: Grace, Xiong
Alterations to the Caisson and Environment:
Redo!
Figure 17. Current Configuration in Action
Source: Grace, Xiong
-Increased the length of the excavation chamber -Added sand to the tank to help -Added air pump to the air pressure chamber (Figure 14) Current model configuration Source: Grace, Xiong
Methodology -Keep all chambers closed until it hits the bottom -Then release the excavation chamber once it is closed into the sand -Pump air into the caisson with air pump to stabilize lose of air
Failed Immediately:
Air pump has openings in the ‘pump’ portion of the unit, therefore it acts as if there is nothing inserted into the chamber and it is wide open. Similar to Figure 8a.
17a. Plugged the excavation chamber until it was secured in the sand.
(Figure 16) Current model configuration
17c. Once the excavation chamber was released, we monitored the water level. Once the water level started to raise, we let air out of the balloon to counteract that loss of air pressure.
17b. Then released at the top of the chamber
(Figure 15) Photo of air pump used Source: Grace, Xiong
Balloon for Air pump:
Swap out Air pump for a balloon since it is a source of forced air and it is always closed. (For the sake of the project, we pretend it was a never ending balloon)
Source: Grace, Xiong
Success! We were able to successfully keep the water out of the caisson by slightly pumping air into it with the balloon. See the results live through the link below:
https://www.youtube.com/watch?v=_ZT3CddWFlI&feature=youtu.be
The Brooklyn Bridge:
Conclusion
For the relatively lack of available technology at the time, the construction and completion of the Brooklyn bridge truly was a wonder. It is satisfying to know that it is still standing.
Ingenuity + Courage = Lasting Results!
Additional Observation:
The previous drawing we have been referencing has been showing water in the chamber. We tested our own model to see if they were similar. See Figure 18 (Source: Grace, Xiong) Below:
18a. Marked the paper to see how much it went into the chamber
18b. Then placed in water to allow the paper to get wet, if possible.
Same results as the original drawings!
18c. Pulled out paper and unrolled it where ever water soaked it. (Figure 19) Original Caisson Drawings (https://www.jamesmaherphotography.com/ new-york-historical-articles/brooklyn-bridge/)