Structural Equivalency form data

[URracing]

(Sorry if this post is a bit long, but it's hard to get info across in a simple paragraph. Like they say a 20 pg essay is easier to write than a 5 pg one).

I would have to say we are a new team although we did have a car that last competed in Pontiac in 2001. I say we are new "because we are". Since 2002, several students tried to contribute to a new car but over the years the organization just couldn't get re-established. After some hard work in recruiting and additional fund-raising, I have been able to re-establish the team in hopes of finishing a design that was started in 2002 in hopes of getting to Michigan in 2009.

I'm only in my second year of engineering and probably the only one (out of seven students) with the most FSAE competition experience, which I gained from attending the Michigan 2007 competition.

I searched the topic and came up with only one (1) unuseful post so that is why I'm here now. In 2004 our cockpit space frame was built using 1" x 1" square tubing for EVERYTHING besides the the main hoop and the diagnal brace on the side-impact structure, which are made from round tubing.
Now from what I can tell, square tubing is weaker and heavier in alot of ways over round but the other thing is that they used square tubing with the minumum wall thicknesses as outlined in the rules for ROUND tubing. Now I know your moment of inertia would be higher (at the corners) of the square tubing but round tubing has a higher rigidity in most cases from what I can tell.

So from what I can see, the square tubing is not compliant since they used the minimum wall thicknesses for ROUND tubing, so should I even worry about a SEF or should I start designing a new frame?

BTW, they used square tubing because it was cheaper than round for us to buy at the time and it was easier to model square joints into CAD as opposed to round notches. I know they did FEA testing and found the frame assembly had good resistance to torsional loads and indicated it had the potential of standing up to a front-end impact. I just want to make sure I know what I'm talking about before we decide to use this frame in our design. I would include a photo of the frame on the table but haven't found how to attach photos to these threads yet.

Cheers and hope to hear from somebody.

Brett

[URracing]

(Sorry if this post is a bit long, but it's hard to get info across in a simple paragraph. Like they say a 20 pg essay is easier to write than a 5 pg one).

I would have to say we are a new team although we did have a car that last competed in Pontiac in 2001. I say we are new "because we are". Since 2002, several students tried to contribute to a new car but over the years the organization just couldn't get re-established. After some hard work in recruiting and additional fund-raising, I have been able to re-establish the team in hopes of finishing a design that was started in 2002 in hopes of getting to Michigan in 2009.

I'm only in my second year of engineering and probably the only one (out of seven students) with the most FSAE competition experience, which I gained from attending the Michigan 2007 competition.

I searched the topic and came up with only one (1) unuseful post so that is why I'm here now. In 2004 our cockpit space frame was built using 1" x 1" square tubing for EVERYTHING besides the the main hoop and the diagnal brace on the side-impact structure, which are made from round tubing.
Now from what I can tell, square tubing is weaker and heavier in alot of ways over round but the other thing is that they used square tubing with the minumum wall thicknesses as outlined in the rules for ROUND tubing. Now I know your moment of inertia would be higher (at the corners) of the square tubing but round tubing has a higher rigidity in most cases from what I can tell.

So from what I can see, the square tubing is not compliant since they used the minimum wall thicknesses for ROUND tubing, so should I even worry about a SEF or should I start designing a new frame?

BTW, they used square tubing because it was cheaper than round for us to buy at the time and it was easier to model square joints into CAD as opposed to round notches. I know they did FEA testing and found the frame assembly had good resistance to torsional loads and indicated it had the potential of standing up to a front-end impact. I just want to make sure I know what I'm talking about before we decide to use this frame in our design. I would include a photo of the frame on the table but haven't found how to attach photos to these threads yet.

Cheers and hope to hear from somebody.

Brett

[moose]

square makes some mounting slightly easier, and can be easier for some fab - but just read the rules carefully and make sure that for all parts it will even be eligible to be considered for the structural equivalency.
Its going to be an awfully odd frame to start with having all square tube. I would reccommend, as you have about a year and a half, that you work on getting a bunch of the other design sorted out - and then you can go decide if you want to redesign the chassis to better fit that - aka engine, suspension, drivetrain, driver; in which it would probably serve you best to switch to mostly round tubes.

One thing to be aware of, is if you do keep the square tube and mount anything inside the tube by cutting out a chunk from the side and putting a bolt hole in - make sure to not knock off those corners as the tube will be severely weakened.

Chassis construction is certainly not the most difficult or time consuming part of the build if well planned - just make a justifyable decision, and you should be fine.

-Matt

[t21jj]

For what little we have used square tubing for in the past, a-arm mounting and lower front frame rail(from the front hoop foward) we have always had to do a structural equivalency form. Although we will not be using any this year.

3.3.2 Structural Equivalency and Structural Equivalency Form (SEF)
The use of alternative materials or tubing sizes to those specified in Section 3.3.3.1
"Baseline Steel Material,"� is allowed, provided they have been judged by a technical
review to have equal or superior properties to those specified in Section 3.3.3.1.
Approval of alternative material or tubing sizes will be based upon the engineering
judgment and experience of the chief technical inspector or his appointee.
The technical review is initiated by completing the "Structural Equivalency Form"�
(SEF) using the format given in Appendix A-1.

[Brian Perry]

During tech at Michigan last year I was told that they would like to see square tube included in the SEF even if it meets the wall thicknesses given in the rules, even though they know it's fine.

Are you planning on using a chassis built in 2004 in the 2009 competition? You might have to build a new one anyway with all the rules changes.

[RawePower]

I was directed to this thread by a fellow FSAE alumni. Per our experience at the University of Cincinnati in 2007, complete an SEF for every tube in your chassis that deviates from what is spelled out in the rules. Note that if there is a minimum wall thickness for any tube, they will not approve an SEF for thinner wall tubing (i.e. bront bulkhead supports made of less than .047 wall tubing) regardless of cross sectional area or second moment of area for the alternate cross section.

UC had to build two frames last year, due to an SEF dissagreement. We have one bare frame sitting in the shop that had its SEF rejected for two reasons. First, we used tubing cross sections that had lower cross sectional area than the baseline tubing reqs. This failed b/c they don't recognize the added strength of using superior steel alloys (i.e. anything other than 1010). Secondly, our front bulkhead support tubing had a .035 wall thickness, which is ess than the .047 minimum. Even though we met the cross sectional area requirements with this tubing, it was still rejected.

Also, if you cut any windows in your tubing to mount anything, you must prove that your tube is still superior at that location. We used windows in square tubing to mount front A-arms last year. All we did to meet the minimum area was weld a .035 x .250 piece of strap to the backside of the tube. In all practicallity, this does little to nothing for the strength of the joint... but approved it.

As you may notice, I'm still bitter about constructing a complete second frame for the 2007 car, less than a month before competition, nearly single-handedly. Of course, I must give credit where credit is due. The master welder did notch the main hoop and do about 50-75% of the welding for me. Apparently, I can work miracles with an angle grinder.

[Brian Perry]

Ben,

I can relate. Our last car took a few years longer than desired to make it to competition for various reasons. I joined after it missed its first competition. That team wasn't very good at reading the rules apparently, and decided to build everthing forward of the front roll hoop out of .035 wall tube. We spent a weekend in February rebuilding everything.

[URracing]

Well, basically the entire cockpit (forward of the front role hoop) is made from 1" sq tubing. From what I can tell, the tubes are the minimum wall thicknesses as specified in the rules so to me this makes them equivalent if not stronger than the 1" round tubing. (moment of inertia is higher since cross-sectional area is larger and more material is located further from the centre of the square tube)

As far as info that needs to be provided to the judges on the SEF, I guess I'll just do a cross-sectional/moment of inertia calculation for both the 1" round and 1" square tubing using the same specified minumum wall thicknesses. Hopefully we shouldn't need more than this. Wherever components are attached, we'll most likely re-enforce the area with plate (or internal welded inserts where holes go right through the frame member). We most certainly will not cut through the corners since of course, that is where the strength is in the tube. Mostly just wanted to make sure I am submitting the right info into the SEF for the square tubing vs. round tubing equivalency. (The square tubing should surpass the round tubing requirement by far anyway).

-I may have to start a new discussion on rod-end chassis mounts if I don't find what I'm looking for on that either. That's my next step in questions I need answered.

Cheers and thanks for the replies.
-Brett