Design Tips

Every year we see some pretty common design errors that we'd like to warn you about but, before we go too far, we need to warn you that this is FREE advise and might be worth what you pay for it. There is no substitute for THINKING about what you are doing in light of all that great training that you've been receiving in your engineering courses. Statics really is true as are the concepts learned in your structural mechanics, structural analysis, and materials courses. One of the great objectives of this competition is to help you connect what you learn in class with real life.

This year there are some substantial changes in the rules regarding connections. You will want to review the "Post Construction" page for more details on this.

So, that said, here are the tips.

Figure out which of the contest parameters are the most critical.
Avoid clearance problems.
Learn how to find steel that is economical.
For those who have participated in prior competitions, pay close attention to changes in the rules from prior years

Finding the Critical Contest Parameters

Scoring is based on three parameters: Lightness, Stiffness, and Speed. These are combined to determine Economy, Efficiency, and ultimately Overall Cost. Before getting too deep into your design you should run some scenarios to decide which is the critical parameter. Knowing this will have a big effect on which way you go with your design.

One way to do this easily is to download the scoring spreadsheet and create entries for your various designs. You will need to estimate things such as weight, time of construction, and stiffness and compare the resulting scores. By the way, if you do this and find an error in spreadsheet PLEASE report it ASAP so that the spreadsheet can be updated.

Avoiding Clearance Problems

Every year teams lose out on great rewards by a matter of a sixteenth of an inch or less. One year an apparent national champ lost out because the penalty associated with their base plate extending a sixteenth of a inch outside the footing knocked them out of the top ten. That REALLY hurt. It wasn't fun for the judges either.

Teams often push the clearance limits and then something happens during construction so that things don't line up too well and a big penalty is assessed. It is STRONGLY recommended that you think twice (or more) before you commit to building a bridge that is intended to precisely meet the spatial limits. It generally won't hurt your overall performance to leave a quarter inch or so to spare.

Finding Economical Steel

The Rules Committee comes under pressure regularly from competitors and sponsors to outlaw expensive steels, such a Chromalloy because there is a perception that somehow these steels give an unfair advantage to chapters with money to spend. When the rules committee can find a way to outlaw such material that is enforceable we probably will. Writing an enforceable rule restricting the type of steel has proven to be difficult.

These expensive steels seem to be used primarily because they can be obtained in lighter sections. They have generally the same stiffness (i.e. same Modulus of Elasticity, E) as other steels so they don't have a real impact on stiffness. In fact going to lighter sections may have a detrimental impact on stiffness (because of reduced AE or EI). They do come in higher tensile strengths than some of the other steels however the strength levels required by most members is minimal so there's not a huge savings here either. You might also note that if member strength is based on buckling then tensile strength is a non-issue and higher strength steels have no advantage over lower strength steels in this case. In the end, we find some very competitive bridges at the national competition that do not use these steels.

With all this in mind, the Rules Committee issued the following suggestion in August 2006:

Buying Steel Economically

You can save hundreds of dollars with smart strategies for buying steel. Here’s how:

Reuse parts of last year’s bridge and look for scrap steel from other student projects. If your university has a salvage yard or recycling program, you may be able to get scrap from research projects. Used steel retains its original strength and stiffness unless it has been distorted, heavily stressed thousands of times, or severely corroded (surface rust is harmless).

Some scrap metal dealers sell to the public. Selection is limited but prices are low. Check your local telephone yellow pages under “Scrap Metals.”

You probably won’t find everything you need in scrap yards but there are ways to save money on new steel.

Steel service centers may offer lower prices than building supply and hardware stores. You can find a service center in the yellow pages under “Steel Distributors” or at the Metals Service Center Institute web site, www.ssci.org. Navigate through “About MSCI” > “Directories” > “Member directory.” http://msci.org/memberDirectory/index.aspx Prices may vary from one service center to another so get quotations from several, if possible.

Service centers charge for cutting, so you may save by ordering full mill lengths. Order all your steel at the same time to minimize processing and delivery charges.

Service centers give big discounts to big customers. Therefore, you may get a lower price by ordering through your university’s purchasing department rather than directly from the service center. A local steel fabricator may be willing to order your steel and sell it to you at cost, passing on the company’s discount as a service to engineering education. The AISC web site, www.aisc.org, can direct you to fabricators. Navigate through “Find a company” > “AISC certified companies” > “Show all certified fabricators.”

Steel is available in various grades which differ in properties and cost. For example, AISI 4130 tubing is stronger (higher yield and ultimate stresses) but much more expensive than ASTM A513 tubing. However, the unit weight and stiffness (modulus of elasticity) are the same. If you design your bridge to minimize the structural cost score, C_s, you will find that the stress in most members is much less than the yield limit of the less expensive grades of steel. Therefore, a bridge made primarily from those grades can be a winner. However, there is a competitive advantage to using high-strength, expensive grades for members or parts of members that are highly stressed, if there are any. Judicious use of small amounts of expensive grades may improve performance without an excessive increase in cost.

Some sizes of tubing are available in AISI 4130 but not in ASTM A513. However, you can duplicate the weight and stiffness of a 4130 tube with a built-up or milled-out A513 section.

Changes to Rules

Each year the rules are modified by the rules committee. Frequently we encounter teams which have not pick up on nuances of changes found in the current year's rules and acquire significant penalties. Section 5 of the 2013 rules gives a list of major rule changes. Careful attention should be paid to these changes.