# Is it practical to use tensile cables in a composite beam?

Many composite beams combine material that has a good tensile strength with material that has a good compression strength (like concrete and rebar).

I was wondering if there has ever been a beam that used tension cables on the side in tension. It seems to me that this might be a cheaper solution than using I beams in certain cases, though it would be more involved construction since cabling pieces would be needed (like turnbuckles).

Could this ever be practical? It seems to me that it could be in very specific circumstances, but I'd like to hear from more knowledgable people because I feel like I'm overlooking something.

• What have you searched for? We don't do opinion questions on here. – Solar Mike Oct 9 '20 at 4:53
• All I found was information on cablenet structures, nothing on beams. – Klik Oct 9 '20 at 4:56
• Look up "prestressed concrete" . Of course much of the info predates the internet. – blacksmith37 Oct 9 '20 at 14:40

Steel beams and columns can and have been prestressed by steel tendons in major buildings. Because steel tendons can be applied post-construction many times they are employed in refurbishing bridges and distressed structures.

Following is a steel beam with a post-tensioned tendon.

If $$X$$ is the amount of force in the tendon, and $$c$$ is the distance to the exterior edge of the flange from the neutral axis, and $$I$$ is the moment of inertia for a given section, then total stress at any point on the beam is:

$$f_{total}= -\frac{X}{A} \pm \frac{Xec}{I} \pm \frac{Mc}{I}$$

• That's interesting, I've heard of tractor axles being pre-stressed to handle higher torque, but I didn't know they did this with beams. This question came up because I was looking for a supporting member but I found the cost of I-beams relatively high and was wondering why I don't see any beams that use cables on the side under the neutral axis to reduce material used. Thanks for the thoughtful answer. – Klik Oct 9 '20 at 21:33

Pretty standard old technology; A moot point whether prestressed cable or rebar is used. However ; cold drawn cable can be stronger. The problem is civil engineers don't understand the possible problem of hydrogen stress cracking of high strength steels. There have been several TV mockumentaries ( eg. Engineering Disasters) of serious cable failures. Usually on a sea coast with more corrosion potential . The explanations are often poor as the liberal arts majors who write the dialogue do not understand the principles. A recent TV program comes to mind regarding the "cheese grater" building in London; However in that case it is high strength bolts , not cables , failed because of hydrogen stress cracking. Most highway bridges are built with prestressed concrete. When these beams are being hauled on trucks ,you can often see a bow because of the prestress.

• Excellent point, this might explain why it is not more common to see. – Klik Oct 9 '20 at 21:35
• You can hear-treat rebar to very high strength levels but it is lower cost to cold draw wire and make cables. – blacksmith37 Oct 9 '20 at 23:50