SES 2015 - T5.4.2 Rule, What do you undesrtand from it ?

[Tiago-IFS]

Hello everyone, since I've haven't posted anything for a while, I will present myself again. My name is Tiago and I'm part of the chassis group from ISEL Formula Student Team, from the Lisbon Higher Institute of Engineering.
This is our second year as a team and we are now building our first car to participate in the Formula Student Spain competition.

This time my issue is about the Structural equivalency spreadsheet, referring to tab T 5.4.2. , I don't know if I've understood it wrongly, but Is it just me or this rule and respective section in SES is just about confusing and ambiguous?

First:

The rules states: "If the harness is mounted to a tube that is not straight, the joints between this tube and the structure to which it is mounted must be reinforced in side view by triangulation tubes to prevent torsional rotation of the harness mounting tube. Supporting calculations are required. Analysis Method: Use 7kN load per attachment and the range of angles in T5.4.4 calculate that the bent Shoulder Harness Bar triangulation stresses are less than As Welded Yield Strength (T3.4.1 note 4) for combined bending and shear and does not fail in column buckling. If the team chooses not to perform the strength analysis rule T3.5.5 will apply."

Referring to the SES interpretation and approach:

ses1.jpg

So if the design corresponds to the first point everything is ok and no calculations are required, if the design corresponds to the second point, this means a CONTINOUS TUBE connected to the main hoop with additional supports due to the existing offset ex:case3.jpg extra calculation are required. And then starts a nightmare for my understanding.... for the third point my interpretation tells me that any design which has NO continuous tube has connected to the main hoop should include extra calculation in 3D. From this, I can conclude that any other shoulder harness bar whether or not continuous as well as NOT connected to the main hoop also needs extra calculations, well also this covers the rule right before this one ! Which is T5.4.1.

resuming all of this:

We have three generic types of situations (presented on the next images):


case 2.jpgcase 4.jpgcase 5.jpg

My doubts are right here, Is this section of SES only applying to the case where cases are not covered by T5.4.1 or does this include it ?????? Normally according to the excel file I would think that the first statement is the right one and no extra calculation are required when the shoulder harness bar is mounted in the main hoop bracing and proper triangulation is done, because the tab is only referring to rule T5.4.2. Since many teams have this type of design and haven't got problems with it, being able to participate in all events, and adding the fact that this has been working for a while and these rules are fresh new I’m a little bit unsecure with them.

Can anyone help with that, whether with their experience and/or interpretation?

Tiago
ISEL Formula Student

[Claude Rouelle]

Tiago,

You will probably have a few useful comments from your post but ultimately the best answer you can get is from the rules committee. Did you try to ask them your question?

[Tiago-IFS]

Rouelle, Claude

Thank you for your quick answer,

Yes I've already sent one email to the rules committee, I'm now waiting for an answer. I decided to try it here as well to see the general opinion about it.

Hope someone brings their interpretation too.

Tiago

Ps: As soon as I receive an answer I'll share it here.

[jd74914]

I'm having a hard time understanding your exact question (probably too much Easter food!), but the first frame image you show (the one in the body of the text) should be ok.

We have a similar design which is SES and rules committee-approved except that our frame is missing the cross-bracing which runs from the bottom of the roll hoop support to the hoop at the shoulder bar. The calculations we very easy to do since they only must be done in 2D side-view to show the harness bar cannot rotate around the hoop. All of the calculations are even done for you in the SES spreadsheet.

Until our SES we accepted we were also a little worried...

[Tiago-IFS]

jd,

Thank you for your answer, I'm sorry for my explanation, maybe I was not so cle
Summarizing what is above: The main issue is to know if the case where the harness bar is mounted bewteen the main hoop braces needs the calculations for SES Section T 5.4.2 or not. Since the case represented at the most left picture of three pictures that are together is direclty covered by the rule T 5.4.1, supposedly that type of design wouldn't require the fullfilment of the SES T5.4.2. ( This is my understanding ). At end, it's an ambiguosity.

And since this is our first frame, and therefore our first SES, we really don't want to mess it up because of this.

Tiago
ISEL Formula Student

[Jonny Rochester]

"If the team chooses not to perform the strength analysis rule T3.5.5 will apply."

I looked up T3.5.5 and it doesn't exist. [Edit: it does exist, I was using the wrong method to navigate the rules, sorry]

[MCoach]

Jonny, T3.5.5 used to be the 'bent tube' rule that was removed. Intended?

Here it is from the 2014 ruleset:

"T3.5.5 If a bent tube is used anywhere in the primary structure, other than the front and main roll hoops, an additional tube must be attached to support it. The attachment point must be the position along the tube where it deviates farthest from a straight line connecting both ends. The support tube must have the same diameter and thickness as the bent tube. The support tube must terminate at a node of the
chassis."

[jd74914]

Summarizing what is above: The main issue is to know if the case where the harness bar is mounted bewteen the main hoop braces needs the calculations for SES Section T 5.4.2 or not. Since the case represented at the most left picture of three pictures that are together is direclty covered by the rule T 5.4.1, supposedly that type of design wouldn't require the fullfilment of the SES T5.4.2. ( This is my understanding ). At end, it's an ambiguosity.

And since this is our first frame, and therefore our first SES, we really don't want to mess it up because of this.

I understand now; thanks for clarifying. I too am not sure as to what you should do. My understanding is that you do not need the braces to prevent twisting as this is a non-issue when mounted there. You likely will need braces connecting to the main hoop as you are not supposed to be putting bending loads on the bracing. Unfortunately, you really need to wait for the rules committee response before going forward since none of us are officials.

I'm not sure how FSS works, but at the FSAE competitions you can submit your SES pretty early and receive feedback too, rather than just going through the rules clarification process.

Good luck!

[John_Burford]

Tiago

The image in the SES guidance represents a half-car. Having a section on the left and a section on the right is NOT which was intended by the image in the guidance, so "case4" and "case5" are not acceptable.

"Case3" falls under the 2D analysis approach which is coded into SES.

"Case2" is a multi-segment harness bar and requires 3-dimensional analysis. This design relies on highly loaded welded joints which puts you on a path to verify strength that is difficult satisfy. The rules discourage multi-segment shoulder harness bar designs. You will have to change the design either to a continuous 1"x0.095" bar attached to the main hoop or compete the analysis requirement using As-Welded material properties.

John Burford

[Tiago-IFS]

Tiago

"Case2" is a multi-segment harness bar and requires 3-dimensional analysis. This design relies on highly loaded welded joints which puts you on a path to verify strength that is difficult satisfy. The rules discourage multi-segment shoulder harness bar designs. You will have to change the design either to a continuous 1"x0.095" bar attached to the main hoop or compete the analysis requirement using As-Welded material properties.

John Burford

John,

Thank you for your attention, as well to the others.
John what do you mean with a multi-segment harness bar ? I'am asking this question because the image that you see in case2 has two lines that seem to cut the tube in three parts (The Harness bar in RED color). But this is NOT the case, the image is not clear enough at a point that some may think that the tubes consists in three tubes welded together, again this is NOT what a pretended to show. Could you please tell if this is what you were talking about ?

A clear image. Without the lines ( which could induce some to think that the tube was segmented):

case2_modif.JPG