light load & 3000 psi

[ticas]

3000 psi means concrete has compressive strength of 3000 psi. What if the load is very light, like using only the concrete 200 psi. As the load gets heavier, would the column compresses more (and shorten in length in say very tiny 0.001 mm)? Or is there no change in concrete column length due to the light or heavier compression depending on axial load?

 

[BAretired]

Change in length of a compression member is PL/AE where P is load, L is length, A is area and E is modulus of elasticity.

A spider standing on top of a concrete column causes a change in length of the column which can be calculated precisely if you know the weight of the spider and the modulus of elasticity of the column (assuming the spider is standing still).

BA

 

[JAE]

3000 psi means concrete has compressive strength of 3000 psi

I'm not sure that is true.

 

[ticas]

Imagine you have a vertical column and all the loads (beam) are in the right side (eccentric column), the vertical half of the column facing the right side is in compression, the vertical half of the column facing (or left) outside is in tension, does anyone here design column with more reinforcement at one side (the tension side) on the left? This is assuming the compression side at right of column can take the compression without much reinforcement bars?

 

[hokie66]

There would occasionally be a case for eccentric reinforcement of a column, but in the main, we reinforce them concentrically to avoid site errors. The builders don't know which side is in tension. It is different with horizontal elements. When we show reinforcement in slabs and beams, most builders know the meanings of top and bottom. Another thing...with lateral loading, reversal has to be considered.

 

[Ron]

Further to JAE's comment....

3000 psi concrete generally means the design compressive strength (f'c, the value upon which the design is based). The strength you get in the field can be either below (that's not good) or above that value(generally ok within reason, but there are exceptions) . In addition to the strength consideration for the concrete, thought has to be given to constructability and durability. Constructability and durability often control the recipe of the mix such as selecting the coarse aggregate size for clearances between rebar and forms and for pumping, determining the slump at placement, determining the water-cement ratio outside of strength considerations and several other issues.

Good concrete includes a proper consideration of its properties in the strength design, the constructability and its performance for the life of the structure. It doesn't happen by accident.

Keep in mind that good concrete and bad concrete contain the same materials. It is what we do to it and with it in the field that often determines its success.

 

[JAE]

Ron, thanks for pointing that out and clarifying. I was also trying to be a bit snarky as well.

 

[Ron]

JAE...Snarkiness deserved considering the other threads! I guess I can't resist looking at the train wreck.

 

[racookpe1978]

But, what if that spider has only 6 legs in contact with the concrete column? Or 4 legs??????

Do you not have to calculate the changing average stress risers as each leg lifts, moves forward, then goes back down again?

Now, the total length of the column compression is correctly based on "average spider weight/column area" but the local stresses become important under each foot and should be considered as well.

Now, about that resulting increase in the cross-section area at the midpoint of the column as it compresses under the average spider weight.......

 

[dhengr]

ticas:
Do you own a text book on concrete design? You might want to do some reading on some of your questions. That type of learning will stick with you better/longer than being spoon fed here and arguing about it. Have you ever had a college course in Concrete Design, Structural Analysis and Design or Strength of Materials. We really don’t offer college credit or courses here a E-Tips. Are you a student, or what is you work position? Finally, since BA brought it us, you might want to get a text book on spider dynamics and column hopping, since I hear tell you got some big spiders over there; may need more than 3000psi conc. for some of them.