Fastener Load Distribution 6

The importance of fastener load distribution depends on the application of the joint. If the joint is to be statically loaded, then an ‘average’ load distribution is satisfactory. The maximum running load of a riveted or bolted lap joint being equal to the number of fasteners multiplied by the joint strength of the fastener multiplied by the number of rows, divided by the pitch. If, on the other hand, the joint is subjected to regular cyclic stress, load distribution becomes very important from a fatigue point of view. This is particularly important under loading conditions in which the fasteners behave as linear elastic members. In a standard lap joint fastening together two plates, the critically and heighest loaded fastener is the one on each end. In a fatigue environment the end fasteners will fail first, increasing the burden on their neighbour and significantly weakening the joint.<br><br>In order to determine the load distribution, a finite element based analysis can be used. The joint is broken down into a series of spring elements. The plates are divided into a number of springs lying between each fastener, which is its self, portrayed as a spring. The spring constant of the plate is a function of cross sectional area and E (Youngs modulous), the spring constant of the fastener (C) is calculated using the NACA document. Once the equations describing deformations of these springs has been derived they can be solved simply in Excel using simple martix inversion methods (which Excel does very well). You don’t need to spend vast sums of dollars and time with expensive FEA software to do this.<br><br>The whole point of this exercise is to determine what variation in the plate thickness (spring stiffness) will give an even load distribution. The results of this detailed design and analysis can be seen in joints that are tapered or stepped. Boeing has a manual titled Structural Design for Durability which contains guidance on critical end fastener loads and fatigue resistant joints.<br><br>The next stage in the joint analysis is the assessment of the severity factor. This accounts for the effects of the fastener type, method of installation, interference, hole preparation and so on.<br><br>Once the severity factor has been determined the fatigue life of the joint can be predicted. This is very important in new designs and also in repair and modification. Fatigue life of repairs and mods can be compared to the original structure, which forms a sound basis for assessing its suitability.<br><br>Fastener load distribution is important and can be calculated with relative ease. If anyone would like to know more about this or would like some assistance feel free to contact me.<br><br> <p>Nigel Waterhouse<br><a href=mailto:n_a_waterhouse@hotmail.com>n_a_waterhouse@hotmail.com</a><br><a href= > </a><br>A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

Check out the FAQ, if the post and FAQ are intersting don't forgrt to vote for them....please. <p>Nigel Waterhouse<br><a href=mailto:n_a_waterhouse@hotmail.com>n_a_waterhouse@hotmail.com</a><br><a href= > </a><br>A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

Nigel,

I note that you stated that the stiffness of the fastener could be determined using the NACA reports. I assume that you are referring to NACA TN 1051 (and to its follow up TN 1458).

If so, I have encountered a limitation in this approach that renders it unusable for my requirements. The NACA approach assumes that the bolt will be loaded in a double shear arrangement, with the outer plates carrying equal load.

Beyond this, I have looked at ESDU data sheet 85034, in which its calculations I have little faith. This method can analyse asymmetric cases, but there is an unexplainable discontinuity in its approach, and a double shear case with an outer plate equal to (close to) zero, does not approach a single shear case at all.

Are you aware of any other appraoches that can deal with a variety of materials and assymetric design.

Get your hands on a Peery or Bruhn aircraft structures textbook, or an Advanced Mechanics of Materials textbook by Den Hartog or Boresi and Sidebottom. All of these cover this topic very well. This is a fundamental problem that any aircraft structures engineer worth the title must be able to work out without the finite element method, which can produce very strange results in the hands of an inexperienced analyst, especially one with inadequate training in the theory.

Apkohn,

I am not familiar with the reference you quote. Could you tell me where I can find it, please. Also, I am interested in your problem; sorry I didn't reply to your e-mail. Could you give me a few more details regarding the problem.

Nigel

Nigel Waterhouse
n_a_waterhouse@hotmail.com

A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

Regarding locating the ESDU Data Sheets that I quoted, I don't actually know how to get them. We had this particular one in the office. You can find out more about ESDU at
Now as a follow up to the original post. My work has involved attempting to write a simple program to calculate the load distribution. It was originally planned as a 4 week task. Now 5 months later, no definitive, acurate, reliable method has been determined to calculate the fastener stiffness. 3 theoretical and 2 empirical methods have been analysed, andnone has been able to be verified. This includes the above NACA TN 1051 method (used in a simplified way in M Niu's &quot;Airframe Structural Design&quot a couple of propriety methods, and the ESDU method.

None of these methods consider different fastener head properties. Only the ESDU method supports assymetric double shear.

Now for the disturbing aspect. The variation between any of the above methods can be as high as 100%. Using stiffness values that differ by 100&amp; can result in Fastener Loads that differ by up to 50%... obviously resulting in an unreliable application.

As for the Peery/Bruhn suggestion. These do not provide sufficient ingormation to create a generalised calculation methodology, and dot sufficiently allow for differences in fastener stiffness.

The textbook from Micheal Niu has a relatively easy methods utilizing FEM to derive the load distribution of fasteners, but, only contains a limited numbers of fasteners and configurations.

Jeffs,
Realize it's a little late but I just read your inquiry.
Try the website: &quot;Design and Analysis of Mechanically Fastened Joints and Bolted Patch Repairs&quot; at
I found that just by going onto Google.

Also the FAA Rapid Assessment and Integrated Design manual at gives load distribution procedures.

Finally, although its dated, you might try &quot;Analytical Design Methods for Aircraft Structural Joints&quot; by McCombs, McQueen, and Perry, AFFDL-TR-67-184

In regards to bolted/riveted joints all. go here

the URL for FAA RAPID

Can anybody tell me where I can purchase a copy of Report No AFFDL-TR-67-184 &quot;Analyitical Design Methods for Aircraft Structural Joints.&quot;

Thank you
Graham Murphy

As of a couple of years ago, you could order one directly from Bill McCombs, including his corrections and comments for $19.95 plus $3.00 shipping.

He can be reached at:

W.F. McCombs
2106 Siesta Dr.
Dallas, TX 75224

He also sells a supplement to the Bruhn textbook which is worthwhile.

Randal Heller

Thanks SuperStress.
I have a copy of the Bruhn &quot;supplement&quot; and it is a very good addition to the basic text.

FYI I finally purchased a copy of the report from NTIS, their No is AD831711.

The report is somewhat dated as it is written pre PC's but there is some interesting data on bolt stiffnesses, and it &quot;filled in some gaps&quot; in my background knowledge.


Rgds
Graham Murphy


Graham Murphy

From what I have seen, all design of bolted and rivetted joints is based on testthat involve the group and material thicknesses and material properties. Analyses is self delusional in that the errors can be so large. Stick to standards that are test proven and joint configurations that are test proven.

I am concerned about your statement. For an engineer with 20 years experience it appears to be naive. What is your experience that qualifies you to make such a rash, generalised and demeaning judgment?

Nigel Waterhouse & Associates
Aeronautical Consulting Engineers

Transport Canada and F.A.A approval of fixed and rotor wing aircraft alterations: Structures, Systems, Powerplants and electrical.
n_a_waterhouse@hotmail.com

The judgement I make should be taken at face value only- and it is basically a caution against calculating something that has so many influences and variables, including those introduced in manufacture. I base that on design manuals given by a number of aerospace companies, where the joint allowable is the determining factor and invariably it is read from a table that is developed by testing. From my experience, calculations play a part only in shear analysis where the loading on each bolt is calculated using a cg/centroid of bolts method, but the bolt allowable capacity is test derived and is compared to the calculated load to determine the reserve factor.

Yakov Polyakov, P.E. has a bolt group program that may interest the forum.

To address jeffs's original query, free analysis software capable of joint analysis, you can try a general FE code such as MYSTRAN ( ). This is similar-ish to NASTRAN in input format, but quite different internally and less capable (e.g., you have to explicitly remove stray drilling freedoms manually - not much hassle if you're modeling a flat joint). You'll still need a method of estimating fastener stiffness (see above controversy). See also , which has a LOT of info on analysis software available, including free stuff.

I don't know of a free or commercial dedicated joint modeller. You tend to find larger companies have in-house methods which have some experimental verification.

[I realise this is way too late to be of use to the original post, but I followed the thread in the hopes of finding more info on free stuff.]

Some other references:

Huth, H., The influence of Fastener Flexibility ont he Predictions of Load Transfer and Fatigue Life for Multi-Row Joints,&quot; ASTM Conference Presentation

Tate, M.B. Rosenfield, S.J. &quot;Preliminary Investigation on Loads Carried by Individual Bolts in Bolted Joints,&quot; NACA TN-1051, Washington, 1946.

Swift, T., &quot;Development of the Fail-Safe Design Features of the DC-10,&quot; ASTM STP 486, 1971 pp 164-214.

The were presented in:

Muller, R.P.G. &quot;An Experimental and Analytical Investigation on the Fatigue Behavior of Fuselage Riveted Lap Joints&quot;, 1995 Delft University, p 154.