Bolt Group Analysis Program

I do a lot of work as a Stress Engineer and regularly need to calculate loads in bolts to ensure that they are not going to break. This is a heavyweight, industrial use program that may well be useful for a lot of people. If you have no idea what 'Bolt Group Analysis' is, I suggest you read the 'Beginner's Guide' below - it's always interesting to learn something new.


The Bolt Group Analysis Program is a Windows based program designed to model and analyse the loads in a group of bolts.

Beginners guide to Bolt Group Analysis

Unless you're a professional structural or mechanical engineer it's unlikely that you've come across bolt group theory, so here's a pretty non-technical beginners guide to what the program does and how it does it

The easiest way of explaining how the program works is to take an example. Imagine a post bolted to a flat concrete pan with shield anchors. This post is going to be an elephant's scratching post in a zoo, so there will be a large force applied to the top of the post. Where there are engineering terms with specific engineering meanings, I have put them in capitals - and on the first time they're encountered an explanation of their meaning.

To begin with, assume a horizontal leaning load from the elephant at the top of the post, and one bolt at the bottom of the post. This situation is dreadful, as the bolt takes the force at the top of the post (called the DIRECT LOAD) in shear and is also bent by the load. The amount of bending in the bolt is determined by the force at the top of the post multiplied by the post height, this is called the BENDING MOMENT. Bending bolts is one of the best ways of breaking them, so we don't want to do this.

Now we've two bolts. The DIRECT LOAD is spread between two bolts, and if the base of the post is rigid, the bolts will not be in bending. The way the BENDING MOMENT is reacted is by tension in one bolt and compression in the other. This tension or compression is called a DIFFERENTIAL INDUCED LOAD or a DIFFERENTIAL LOAD. It is also dependant on the distance between the two bolts - the greater the distance, the lower the DIFFERENTIAL LOAD. This is intuitively correct too, as the wider the base of the post, the harder it will be to push the post over.

Now there are three bolts in the base of the post, one is halfway between the other two. How does this help? To begin with, the extra bolt means that the DIRECT LOAD is spread between three bolts, that's advantageous. Now, what about the DIFFERENTIAL LOAD? In fact it's not changed in the outer bolts at all, and there is no DIFFERENTIAL LOAD on the central bolt.
This is where you have to imagine the bolt group working together. The elephant's leaning load causes a MOMENT (force multiplied by distance) around the centre of the group of bolts (the CENTROID). This has to be withstood (REACTED) by the load in the bolts. The load is proportional to their distance from the bolt group CENTROID (This is one of the defining assumptions of Bolt Group Theory). Now, the MOMENT induced by the bolt loads must be equal and opposite to the MOMENT applied by the elephant (otherwise both the post and the elephant would fall over). Also, with three equally spaced bolts, the bolt group centroid will always fall at the central bolt, so it's contribution to reacting the applied MOMENT is nil. This means that the DIFFERENTIAL LOAD hasn't changed in the outer bolts at all.

Four bolts, all equally spaced. All bolts take DIRECT LOAD, and all take DIFFERENTIAL LOAD, because none are on the bolt group centroid. However, when you calculate the loads, the central two have only one third of the DIFFERENTIAL LOAD of the outer two, that's because they are only one third the distance from the bolt group CENTROID of the outer two.

Now extend the idea to the elephant pushing the post across at the same time as pulling up on it. There are four bolts at the base of the post. Well. The elephant's load can be broken down into components at right angles to each other - a horizontal component and a vertical component (the loads are said to be RESOLVED into ORTHOGONAL directions).

Now, even though we've got a vertical load to think about too, the horizontal component will still load the four bolts in exactly the same way as it did in figure 4. What about the vertical load?. Well, that applies a DIRECT LOAD to all the bolts in the same manner as the horizontal load. This vertical load adds to the load in the bolts due to the horizontal DIFFERENTIAL LOAD (direction signs on the loads must be rigorously adhered to though).

There is now a cross piece on the top of the post, and the elephant is scratching behind it's ear. The load can be RESOLVED as before, and the horizontal DIRECT and DIFFERENTIAL LOADS calculated as before. The vertical DIRECT LOAD is also as calculated before. However there is something new too. The vertical up load is not acting through the bolt group CENTROID, so the vertical load also induces DIFFERENTIAL LOADS which must be added to the loads already calculated. All this takes time to calculate by hand. Thank goodness there's a program available!

OK, now to extend the idea further:-

a) So far we've been working in 2 dimensions. Now add a third dimension to the applied load and bolt group, once again the loads act around the bolt group centroid causing DIRECT and DIFFERENTIAL LOADS.

b) Make the base of the post some other shape than flat. When you do this the DIFFERENTIAL LOADS stray away from vertical, as they are really loads acting tangentially about the bolt group CENTROID. The DIFFERENTIAL LOADS have seemed to be vertical so far because the bolt group has been flat up until this point - so the tangential direction is vertical. These DIFFERENTIAL LOADS will then put the bolt into shear and tension/ compression.

c) Put some of the bolts in slots so that they can't take load in some directions, but can in other directions. This means that the bolt group CENTROID is different in different directions. For this a bolt is given a relative stiffness, 1.0 for a bolt offering restraint in a direction and 0.0 for a bolt in a slot in that direction. With care this idea can also be used to represent bolts with different diameters in the group. For example an M10 coarse bolt has a minimum minor area of 49.49mm^2, and an M8 coarse has an area of 30.90mm^2. Thus the relative stiffnesses used in the program would be 1.00 and 0.62 respectively.

d) The whole bolt group theory is really independant of the thing causing the load (the elephant) and the item connecting the load to the bolts (the post). So let's get rid of these mental props and only consider the load application point and the geometry of the bolts.

Now you're ready for the full three dimensional bolt group program. Specify the bolt position in space, stiffness in each direction, the load in each direction and the point of application of the load. Hey presto, you get the bolt loads out.

This type of program has been used to check rivet loads in aircraft, bolt loads in loft conversion strengthening beams, attachment loads in camper van conversions. It is very versatile.

This program is really just one step down from the accuracy of Finite Element Stress analysis (FE). FE takes into account the stiffness of the structure in addition to the geometry, but with very stiff structures relative to the bolts (a majority of structures fall into this category) the inaccuracy is going to be fairly small (in the region of 1-2%).

Do read the program limitations section of the help file though! It is just as important to know when the theory cannot be used as it is to know when it can.


There are two downloads for this program, a Windows 32 bit graphical program and a faster running DOS program. As is the case with DOS programs, it's much harder to use, but a great deal smaller in download size. I leave it up to you to decide which to download. The Windows version is included as one of the ancilliary tools in the 3D Modelling Studio, so there is no need to download this program if you have that one already.

Download the Windows Installshield Installation package here (Works on Win 95, Win98, WinME, WinNT, Win2000 and Win XP BUT not a later version):

Windows Bolt Group Analysis Installation, zipped (2.3Mb)

The installation works on Windows 95, 98, ME, NT, 2000 and XP provided you have administrator rights.

For loading on Win7 or above, the installation program doesn't work even in compatibility mode, so the zip file below just has the files you need to run it in a single directory. No Icons or any other 'fripperies' are created, and the built in text editor's spell checker is missing its dictionary

Windows 7 Bolt Group Analysis, zipped (795 kb)

Download the DOS package here:

DOS Bolt Group Analysis version, zipped (270 kb)

The DOS version works in DOS and in a DOS window in Windows 95, 98, ME, NT, 2000 and XP. I have not tried it on Vista or Windows 7 at all.

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