Why do we use 7 strands of wire in prestressed beams? 3

[bertoS]

is it about the normal force between the concrete and steel? and what the usual size of the strands?

 

[bridgebuster]

It works out geometrically. One in the core and six wires around.

 

[Lomarandil]

What BB said. The most common diameter of the entire strand is 0.6 inches (nominal), although other sizes exist.

 

[PEinc]

A 0.6" diameter, 7-wire strand has a cross-sectional area od 0.217 in².

http://www.PeirceEngineering.com

 

[Ingenuity]

The most common diameter of the entire strand is 0.6 inches (nominal), although other sizes exist.

For 7-wire strand, 1/2" (12.7mm) is more common in building structures and 0.6" (15.2mm) is more common in bridges, at least it is in the USA and Australia.

In the UK, the precast industry used to use (and some still do) 3-wire strands for prestressed planks.