Load  Paths

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What is a load path?  It is quite a simple idea.  Any structure touches the ground in certain areas, which are the only places from which it can derive support.  It will also experience forces from loads, such as traffic and wind pressure.  The load path is the route by which forces applied by the ground can exert the forces necessary to balance the loads.

The forces from the ground and elsewhere will normally be different from the final ones which oppose the loads.  The "flow" of forces is not at all like the flow of water or electricity.  Although flowing water or electric currents may split up and join, the total flow does not change.  That is not the case with forces.  An applied force may generate forces in a structure which are very much greater than itself.  The idea of flow is useful only if we do not misapply it.  Forces within a solid body are often along a particular direction, but they do not point one way or the other.  In that respect they differ from lines of laminar flow, and from electric and magnetic lines of force.

That the forces are not the same throughout a system is apparent when we consider simple machines such as levers and rope-pulley systems. If we consider a heavy load at the centre of a beam, we see that the forces within the structure do not even have to be of the same kind, for the top of the beam is in compression, while the bottom is in tension. The forces within the beam will generally be far greater than the load. We could say that not only do we require a load path - we require one that minimizes the forces at any point in the structure. But the minimal solution is not always the ideal one. The minimum force to support a heavy weight in an upper room would be obtained by filling the room below with solid material, but that is extremely inconvenient.

Even if we are not sure what a load path is, we can definitely say that wherever the forces are found, at no point must the stresses exceed the values which are safe for the material, not forgetting resistance to buckling, which so often dominates in the design of compression members.

Perhaps a few examples will help to explain how a load path works.

StreetLamp3.jpg (28152 bytes)LampPostBaseEA.jpg (53032 bytes)BareTree2.jpg (85488 bytes)TVTowerAX.jpg (58080 bytes)GuyedMast.gif (18112 bytes)The first picture shows a very tall lamppost.  In principle it could stand on an accurately levelled base with no fixtures, but even a fairly feeble wind would blow it over.  It would have a load path into the ground for its own weight, but it would not have a load path that could prevent it from tipping.  The second picture shows a solution.  The base has many bolts that are embedded in the concrete base.  When the wind blows, some of these bolts are thrown into tension, which is carried into the concrete and there dispersed until the forces are weak.  Thus there is now a load path for the effect of the wind.  The roots of a tree fulfill a similar function.  The fourth picture shows a tall mast for TV transmission.  Again, it could rest on the ground, but it is unlikely that anyone would build it without fixing the legs firmly to a large and strong foundation.  On the other hand, the legs of the Tour Eiffel slope so much, and are so widely spaced that they are never in tension.  In the fifth picture we see another solution to the load path for wind pressure - the provision of wires.

The next diagram shows a pinned frame comprising seven members, connected by loose pinned joints.  It cannot sustain its own weight, let alone a load.

HingedFrame.gif (3893 bytes)

Two possible solutions are shown below.  Can you think of others?

HingedFrame2.gif (5072 bytes)

If we try to imagine the forces in these frames we see that there isn't a unique path from the load to the supports.  All the members come into play.  In the second example, the forces in the sloping struts and the horizontal ties are larger than half the load, illustrating the fact that forces in a structure may be larger - sometimes much larger - than the load and self weight.  It is possible to design a structure with too many parts, perhaps "to make it stronger".  In such a case it may be impossible to calculate some of the forces.  This is explained in the page about indeterminacy.

See also truss calculations and truss calculations two.

Click here to download a program simulating a kingpost truss.

Click here to download a program simulating a kingpost truss, a Warren truss, and a third type of truss.

Another page about load paths.

 

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