3 ms·
It is pre-stressed concrete, it is working in compression against the cable tension. This seems strange to me. My understanding is that pre-stressed concrete
by jmts 8y ago
It is pre-stressed concrete, it is working in compression against the cable tension.
This seems strange to me.
My understanding is that pre-stressed concrete is concrete with reinforcements inside that apply a compressive force on the concrete. As mentioned by GP, this is to mitigate the effect of tensile forces within the concrete since concrete performs poorly in tension.
Are you suggesting that the cables within the concrete are not just stays, but also the component of the pre-stressed concrete applying a compressive force? Or is the reinforcement some other component?
I would assume that if the cables themselves were applying compressive force to the concrete, then their use as stays releases some of this force since the stays are in tension. Since concrete itself performs poorly in tension, it's going to provide a negligible counter to reinforce the cables even if it is still compressed, since the compression forces are provided by the cables. It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place.
Or is concrete in some compression able to respond better in tension by weight than say, a thicker cable at the same weight as the given cable and concrete combined?
- deleted 8y ago[deleted]
- vlehto 8y ago>It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place. I think you are correct there. But it looks like the stays were in separate tubes with little allowance around them to separate them from the pre-stressed concrete. That airspace around the stays then would prevent the concrete from providing any chemical protection against corrosion. And concrete as physical protection against rainwater is often only as good as the paint in the surface of the concrete. It's often way too porous even without cracks.
- geezerjay 8y ago> I would assume that if the cables themselves were applying compressive force to the concrete, then their use as stays releases some of this force since the stays are in tension. Since concrete itself performs poorly in tension, it's going to provide a negligible counter to reinforce the cables even if it is still compressed, since the compression forces are provided by the cables. It seems to me that putting pre-stressed concrete into tension via its reinforcements is defeating the purpose of having pre-stressed concrete in the first place. Not quite. High resistance steel is used in prestress applications, which provides a considerable headroom to determine what cables are used (number of strands per cable group and number of cable groups per cable) and how to design a structure (target prestress values, dynamic/vibration properties, load variation/fatigue design, etc). Therefore, just because a designer selects a specific prestress cable that doesn't mean the cable will be designed to be stressed closed o yield or even fatigue limits. The designer can pretty much design a structure that uses prestress cables loaded at a fraction of their design limits but subjected to higher loads at construction to be able to unload the initial prestress load as compression. This strategy is often used in bridges, as is designing a bridge to e intentionally heavier so that variable loads are a fraction of the dead load to increase fatigue life.