2016 NJIT Steel Bridge

Report

Top 10: Take IICE 490 – 203

Written by:Devin Berniz

Introduction

We all know what bridges are and what they look like, but what makes a bridge

successful? A successful bridge will not only get you from point A to point B, but it will also

meet code requirements along with budget requirements and side goals which will vary from

client to client such as aesthetics. In order to design a successful bridge, the first step is to come

up with goals. A bridge can only be as successful as what is trying to be achieved.

Competition

I. Background

The student steel bridge competition is designed to provide a project experience that

replicates issues, concerns and challenges that civil engineers face on a day by day basis in

practice. In order to approach these challenges successfully, students must apply engineering

principles and theory acquired from class and work together as a team effectively. In doing so,

each student will ultimately gain the vital experience of what it is like to complete a project

from start to finish. Steps taken to achieve these goals will consist of designing conceptually

before coming up with a final bridge design that will reach its goal expectations, running several

tests, and then finally bringing the bridge to life through fabrication, erection and running more

tests. As a result, this project experience should give students great insight of the common

issues that engineers face in practice such as spatial restrictions, material properties

(compressive and tensile strength, etc.), management and cost.

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II. Categories

Overall, there are 7 separate categories in which we get scored on. There’s construction

speed, lightness, stiffness, construction economy, structural efficiency, display, and overall

performance. Construction Speed is solely how long it takes to construct the bridge. Number of

builders is irrelevant and the team with the shortest time would place first in this category. The

winner of the lightness category will be the team that designs the lightest bridge without

failing. The next category coincides with lightness, which is stiffness. Stiffness is judged by how

much the bridge deflects or in simpler terms, how much it bends. For this category, it is

imperative to stay as far away as possible from a smiling bridge. The reason why this category is

so tricky is because it clashes with lightness as well. In some cases, a lighter bridge will have

more deflection than a heavier bridge depending on how it is designed. By combining these two

categories, the goal would be to design the lightest bridge with the least amount of deflection,

which is what many schools will end up trying to achieve. The next category on the list is

construction economy. This is one of the two most important categories when it comes to

which school will design the winning bridge. It is determined by who can build a bridge with the

lowest construction cost (Cc) based on an empirical equation. The other equally as important

category is structural efficiency. This particular category is based on who can design a bridge

with the lowest structural cost (Cs). These two categories combined make up the overall

performance category (Cc + Cs), which determines who designs the winning bridge. The bridge

achieving the lowest value of the two categories combined wins the overall competition.

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III. Changes

There are some pretty big alterations to this year’s competition that will influence how

we design our bridge. Although there are a few minor changes, the major changes that will

impact our design the most are the fact that there are six load cases that are asymmetrical

including a constant point load that is slightly offset from the center (Table 7.1). The rules also

indicate that no one will know from which side of the bridge the loads will be measured from,

therefore it’s not as simple as supporting the bridge from where the determination of D is

because we won’t know. However, we were left with the choice of taking a chance and

guessing from which side they will take the measurements and designing a bridge with best

results, or taking the safe route and making a symmetrical bridge that will give the same results

no matter what side is chosen for the measurements of the determination of D.

Another major change that made an impact our design is the fact that the weight of the

bridge has a lesser effect on the final cost of the bridge compared to last year. For this year’s

competition, the structural Efficiency “Cs” = Total weight (pounds) x 10,000 ($/pound) +

Aggregate deflection (inches) x 1,000,000 ($/inch) + Load test penalties (6.2.6), whereas last

year was 20,000 ($/pound). This makes it possible to play with all sorts of tube diameters and

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thicknesses without making too much of an impact on final cost. Many if not all schools will

take advantage of this.

As for the construction side of the competition, there are a few changes that will

strongly impact the speed of construction and we all know that construction speed is always

king and queen of the competition. Therefore we need to come up with a strategy that will

ultimately give us the best results. The changes from last year include the fact that the river

now extended using the whole width of the construction site boundary lines (Figure 1 drawn by

ASCE-AISC). It is because of this reason that there are now two separate starting locations for

the builders as show in the site plan below. This makes it a bit complicated as to how many

builders should be used to give us the best results. As of now, we are thinking two builders on

each side for a total of four builders, but we won’t know what will give us the results we’re

hoping for until practice starts.

Another reason why were are strongly considering 4 builders is because of the fact that

now two members can be preassembled as long as the assembled pieces get carried by two

builders. There is a lot of trial and error to be done with this in order to figure out what the best

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(Figure 1)

approach is. Lastly, they brought the idea of using a pier back into the competition, which will

greatly impact how we construct our bridge, but again, this is all something to be figured out

with trial and error.

NJIT History

NJIT has come a long way since competing in the ASCE-AISC Steel Bridge competition for

roughly 20 years now. It wasn’t always as easy as it has been to earn a spot at the national level

of competing. The Metropolitan Regional Competition was more than enough struggle to make

it a goal just to reach the national level. Throughout the years we have progressed through

setting higher standards, expectations and goals, but most importantly we have learned to

grow as a team.

It has been a decade now that we have been winning 1st place in the Metropolitan

Regional Competition without a sweat. Not only that, but previously our best ranking nationally

was 12th place overall and now we have improved earning ourselves a spot in the top ten

finishing 6th place overall.

Goals & Schedule

Considering that we have won regionals for a decade straight, we definitely want to

continue that tradition. As for competing at the national level, we would like to continue the

path of reaching the very top. The goal is always to progress. We placed 12th place overall

nationally 2 years ago and 6th place last year. Although the ultimate goal is always to get first, it

is our expectation and one of our goals to at least reach a spot in top 5.

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In order to achieve our goals, we must be determined and follow a strict schedule as

follows:

SCHEDULEHave top designs ready to show Schiavone by October

Choose the final design by November

Start working in SolidWorks by November

Get orders for materials sent out before January

Complete Jig and Fabrication Drawings by February

Start of fabrication of the bridge by early February

Finish fabrication by the end of February

Begin Practicing by early March

Design Process

I. Training

There is no time to waste when it comes to designing a winning bridge; and being

determined as we are, we went straight to the lab as soon as we got back from the 2015

national steel bridge competition. Although the rules for the upcoming year’s competition

didn’t come out until August, there was still a lot of work to catch up on in order to get ahead of

the game. Our first step as captains was to familiarize ourselves with drawing in 3D Wireframe

in AutoCAD and designing our own bridges using the previous competition’s rules. Then in

order to analyze the bridges that we designed in AutoCAD, we had to import our CAD drawings

into a program called SAP2000. SAP2000 makes it easier for us to input all load cases and

desired tube sizes for our wireframe drawing of our bridge. We then took all data from the

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tables that SAP2000 provided and imported it into a formulated excel file that converts all of

the data into a final cost of our bridge design. We were able to create this excel file using

formulas listed in the rules for construction cost and structural cost, and then we created our

own empirical formula for building time to be included in the cost to give us an accurate overall

cost of our bridge the way it would be judged at the competition.

II. New Rules

As soon as the rules came out for the upcoming year’s competition, the first thing we

did was point out all of the obvious big changes from the previous year’s competition that

would influence the way we design for this year’s competition. We then took a solid week of

clarifying each word precisely so that we knew exactly the kinds of things that we can get away

to use to our advantage that would either improve our bridge structurally, economically, or

building time. If there were any difficulties interpreting the rules for any reason, we are to

report them, which is why it is vital to check for clarifications of the rules on the “website” on a

day by day basis since it can potentially influence change to our design.

III. Final Bridge Design Process

Once we had a good understanding of the rules and all of our constraints, it was time to

design. Our first approach was to analyze the moment diagram for each load case (Figure 2),

and to overlap them to see what kind of overall shape it would provide.

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0 50 100 150 200 250 3000

10000

20000

30000

40000

50000

60000

70000

Load Case 1Load Case 2Load Case 3Load Case 4Load Case 5Load Case 6

We then took the outermost line of each moment diagram and extended each line so that they

would intersect and provide an overall shape for minimal material use without failure (Figure

2). The next step was to scale the provided shape to the necessary dimensions that would

satisfy our span and height restrictions of our bridge. Since all of the values that we got in the

process of designing our bridge only occur in a perfect world, the only way to take that in

consideration is to manipulate certain values by a fraction of on an inch where necessary, which

serves as a minor factor of safety. These precautions need to be taken so that we don’t get a

failing bridge when competition day comes. After obtaining the overall shape that we would

analyze with further investigations, we came up with various versions of a simple suspension,

Howe, Pratt and Warren truss to see what kind of structure would provide the best results.

After observing that the Pratt and Warren truss provided the best result, we then came up with

preliminary truss designs to do further investigating on (Figure 3).

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(Figure 2)

From our preliminary ideas, design (c) of a warren truss kept providing the kind of results that

we were looking for, but we still knew that there was room for improvement. Since there is a

lot more wiggle room to play with the weight of the bridge compared to last year’s rules, that

left us with a lot for options to play with bigger tubes and the boundaries of our shape, while

still keeping in mind not to add to many more members since construction speed is king and

queen of the overall competition. Our first thoughts to adding weight in a beneficial way were

to extend the outermost diagonal members to a flat top, thus creating a trapezoidal shape

(Figure 4). On top of that, we also toggled with the angles of the outermost diagonals. We

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(Figure 3)

(a)

(b)

(c)

(d)

analyzed results using a 30, 35, 40 and 45 degree angle from the connection at the abutment. A

45 degree angle ultimately gave us our best results while only adding an additional 4 members.

IV. Deck Truss

After being satisfied with the design of our bridge in 2-D, it was time to give it some

volume and add 3 dimensional components such as the deck truss and abutments. Although

the main components of the bridge that influences how much weight a bridge can uphold is the

truss design of the top chord, it wouldn’t be practical to use single tubes for the deck and

abutments. Not only that, but we also had to meet certain template restrictions drawn by

ASCE-AISC (Vehicle Clearance Template/A Section provided on the next page), which gave us no

choice but to make our decking three dimensional. The trick isn’t just to abide by the

restrictions given to us but how to optimize our design in the best ways possible given our

restrictions. For example, we could have made our decking consist of completely horizontal

double tubed members, but why would we do that when we know that the more vertical we

make it, the better. Provided in Figures 5.1 and 5.2 below is the decking design that we feel

comfortable with providing numbers that we are happy with.

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(Figure 4)

(Figure 5.1) (Figure 5.2)

VI. Tubing

Once we were confident with our overall shape and design of our bridge that would give

us the results we were looking for, the next step was to come up with tube sizes that would

optimize our bridge. It isn’t always a guarantee that our supplier has all the sized tubes that we

need in stock, therefore we had to first come up with a list of tube sizes and total lengths

needed to create our bridge and from there we are given a response of which tubes cannot be

provided. In this case we simply just bumped up the diameter and thickness values to a tube

sizing that they can provide just because of the fact that weight doesn’t make as big as an

impact on final cost as it did last year. Although there were cases where we knew we could go

down in size, which we did in order to compensate for all of the other tubes that went up in

diameter and thickness. The thing about going down in size however is followed by more tests

to make sure we don’t go to the point of failure.

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V. Abutments

Once we had a full understanding of our entire bridge in 3-D from the deck up, it was

time to come up with a design for the abutments. Most of the design was influenced by how

the rest of the bridge was designed since the decking and top chord both need to connect to

the abutment. Where we were able to be more so creative instead of depending on the decking

or top chord was the design of the lacing as shown below in Figures 6.1, 6.2 and 6.3.

VII. Fittings

The style of fittings for the bridge we used for last year’s competition seemed to work

really well. We plan on using the same style of fittings for the decking, diagonal, and lateral

members, while also incorporating a different style of fittings that has been used in previous

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(Figure 6.1) (Figure 6.2) (Figure 6.3)

years for the thicker tubing of the top chord (all styles of fittings provided below and on the

following page).

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Top Chord Female

Top Chord Male

Decking Male

VIII. Final Drawings

As soon as our bridge was complete in design form, the only thing left before fabrication

was to come up with a shop drawing of every different member and to define it in a way that

its’ almost like giving instructions on how to create the member. Important information to

provide in these drawing are things such as specific angles, lengths, diameters, thicknesses, etc.

of every component of that member as provided below and on the following page.

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Decking Female

Conclusion

If there is one thing I have learned about the designing aspect of making a bridge is that

there is always room for improvement, but the challenge is to do the best that you can with the

time given to you, while simultaneously making sure to abide by rules/code requirements.

Lucky enough we have three co-captains this year to share all of the responsibilities and tasks

needed to reach our goals, which brings me to another valuable lesson. I have learned and

lived through understanding the difference between having slackers for lab partners in class

and what true teamwork is. I am so fortunate to have the same co-captains that I was a builder

with last year. They understand the kind of hard work that it takes to reach the very top and

what it takes to design a successful bridge. We can proudly say that we have given our design

our best efforts with the time given to us and the only thing left to do is to bring our design to

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life. We understand that many schools will design successful bridges, but who will design the

most successful bridge?

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