On Samson & Goliath

Samson and Goliath are a pair of gantry cranes at the Harland and Wolff shipyard in Belfast. They were built by the German engineering firm Krupp. Harland and Wolff was of course the firm made famous by their design and construction of the passenger liner RMS Titanic. That being said, Goliath was completed in 1969 and Samson in 1974, both long after the Titanic was lost at sea. 

The cranes are named after two characters found in the Bible. Goliath was a fearsome Giant, reported to be 9.5 feet tall and Samson was a man of enormous strength. Both names are surely appropriate, because of the cranes’ enormous size and great lifting power, however perhaps Samson is the more appropriate of the two. 

The Old Testament book of Judges tells us that there was a secret to Samson’s great strength. A secret that he had kept from Delilah; at least to begin with. This suggests to me that contrary to the popular image of Samson portrayed in artwork, he must have been a slight man. For if he were a large powerful man surely no one would have thought there was a secret to his great strength.

When observing both cranes one cannot help but notice the oddness of their form and wonder whether there is also a secret to their great strength. They are supported on one side be two slender tubular columns, which are attached to the large box girder that forms the gantry with a small connection. On the other side there is a large box column with a substantial connection to the gantry box girder. The question arises, why is this so?

The secret of Samson and Goliath is subtle. It relies on understanding a simple, yet at the same time complex, structural load path. If you are unfamiliar with engineering terminology then a couple terms need to be explained to understand what is going on.

When two structural members are joined together, but can rotate relative to each other a ‘pinned joint’ is formed. For example, the two parts of a pair of scissors are held together by a pin joint. If the same connection is formed, but the joints are instead held rigidly so that there can be no rotation, a ‘fixed joint’ is formed. This would make a hopeless pair of scissors. It also important to know that pin joints relieve bending forces by rotating instead of offering resistance while fixed joints attract them.

It is clear that one side of the crane gantry is connected to the supporting column with a rigid joint and the other side is connected with a pinned joint. Understanding the effect these joints have on the structure is the key to understanding the structure’s secret. 

Perhaps the best way to do this is to consider what would happen if the connections were jointed differently. Let us consider in the first instance the gantry box girder being fixed to the columns with pin joints, one at either side. 

If the columns on both sides of the crane could rotate relative to the gantry box girder then a mechanism would be formed and a horizontal load applied to the side of the crane would cause the columns to rotate thus allowing the gantry box girder to slide sideways. The crane is unstable and would topple.

Conversely the gantry could be supported by fixed joints, one at either side. In this instance the columns cannot rotate, because they are fixed rigidly to the gantry box girder. The structure is apparently stable, except that it isn’t.

Since the fixed joints do not allow rotation they attract bending forces out of the gantry girder and transfer them into the columns. This is a perfectly satisfactory situation where the supporting column is large, however the two slender tubes will simply buckle again causing the crane to topple.

The obvious solution to this problem is to use a large stiff column on both sides, so that the structure behaves like the portal frames found in a large supermarket or warehouse. There is of course a good reason why this has not been done. 

One of the immutable characteristics of bending forces is that they are always accompanied by shear forces. One cannot exist without the other. In the case of our imagined postalised frame bending forces at the connection between the columns and girder will necessarily co-exist with horizontal shear forces at the column heads and vertical shear forces at either end of the girder.

Horizontal shear forces at the column heads are a problem. The reason they are a problem is that equilibrium demands an equal and opposite shear force at the base of the columns. The two column bases are on wheels, which allow the crane to run back and forth along a set of rails. Horizontal shear forces at the base of the columns are therefore most unwelcome, because they will tend to press the wheels into the side of the rails causing either the rails to buckle or the wheels to jam or both.

It is now time to return to the actual cranes Samson and Goliath, which have one pin joint and one fixed connection at the column heads. It is of course very tempting to assume that when the crane is lifting the pinned connection is free from bending forces and the fixed connection attracts bending from the girder span. It follows that no bending forces are transferred into the slender tubular columns, which makes sense, and some bending forces are transferred into the box column. These after all are the rules and therefore, some engineers will make this assumption. Of course in this case they would be wrong.

Those who make these assumptions have forgotten about equilibrium. A bending force at the head of a column must, as we know, be accompanied by a horizontal shear force. The horizontal shear must in turn have an equal and opposite partner to maintain overall equilibrium, except in this case there can’t be an equal and opposite partner, because the opposite column has a pin joint i.e. if a shear is present the pin joint must carry a bending force which it cannot do.

The solution to this riddle is of course that the fixed connection does not carry any bending forces and behaves as if it was a pinned connection. How is this possible; as that is not the rule? This is the subtle part of the solution that is often missed. 

Since the large column is not infinitely stiff it will, when loaded, start to deflect ever so slightly due to the bending force its fixed joint would like to impart. This deflection is just sufficient to rotate the head of the column, but without changing the angle between the column and the girder i.e. rotation occurs in the column rather than at the joint, which causes the structure to behave as if there were a pinned connection. Only a very small movement is required for this effect to occur.

Now, since there are no bending forces at the head of either column there are no horizontal shears in the column heads and therefore equilibrium demands no shears at the base of the columns either. This means the cranes are free to travel up and down their rails without becoming stuck.

Conversely, if a horizontal load is applied, say the wind, the box column is sufficiently stiff, in combination with the fixed joint, to deflect only a small amount before resisting the horizontal load and keeping the crane stable. This is the reason for having a large box column on one side only. It is the secret of the Samson and Goliath Cranes.

Incidentally, the reason there are two rather than one slender column is to ensure that the cranes are stable out of plane. The keen observer will notice that the tubular columns rake in opposite directions to provide the required stable platform.