Some engineering questions (1 Viewer)

[iatethesun]

Hey everyone,
I am doing the Engineering specimen paper from Board of Studies and it doesn't have answers so I ask for your help. Thank you.

A pre-tensioned concrete beam is to be used in the construction of a ferry wharf.The steel tendons used to pre-tension the beam are 18mm in diameter and 6m in length. A force of 30kN is to be applied to each tendon.(i)Young’s modulus for the steel used in the tendons is 210 GPa. Calculate the extension of each tendon.

My working out is: 210 000 000 Pa = (F/A) / (x/l)

x = 6(30 000/ pi*0.009^2)/210 000 000 = 3.368m but it seems dodgy but I don't know what I did wrong.

Also I was unable to answer:
What are TWO benefits of using pre-tensioned concrete beams in preference to using reinforced concrete beams?
Is a benefit that pre-tensioned concrete beams are prefabricated off site?

A timber-laminate beam is an alternative to a pre-tensioned concrete beam. For what reasons, other than strength and cost, would an engineer specify the timber laminate beam?

 

[Rathaen]

You've gotten the conversion from GPa to Pa wrong. Giga is 10⁹, not 10⁶. Nothing else seems wrong, so your answer should only be off by a factor of 1000.

Pre-tensioned reinforcement in concrete beams sets up compressive forces throughout the whole beam. These stresses will tend to close up any cracks or flaws in the beam, which both reduces the likelihood and speed of crack propagation and increases the strength of the beam.
Both types can be fabricated offsite, so I wouldn't consider that a valid point. Can't think of a second point off the top of my head, suggest a google search.

A timber laminate beam would be lighter than an equivalently sized reinforced concrete beam.

 

[spanky125]

Can someone please do the Q22 part b)
The one with the 3 tonne truck on the truss.

 

[Rathaen]

Hello, if you still need help could you please link to the relevant paper on the BoS website?