Size of a pier

[Robbiee]

A 600mm (2ft) long reinforced concrete pier is required to carry 2150kN (482kips) axial load applied on 250x250x50(10"x10"x2") plate. Your concrete f'c=35MPa(5000psi). What is the pier size and required rebars? This is not a school problem. I will provide more info after getting some feedback. Thanks

 

[mudflaps]

482kips on 100 sq. in. bearing plate. Must have dropped a decimal somewhere!

 

[asixth]

What code are you using?

50mm is a very thick baseplate.

I would have a 450mm or 600mm diameter pier with 1% reinforcement.

 

[frv]

Generally speaking, a pier is considered fully braced laterally (i.e. Lb=0). Consequently, you would have to use bearing provisions to design the pier for downward axial load. In the US, ACI 318 does NOT cover piers.

In my neck of the woods, the geotech engineer would usually specify the minimum reinforcing required in the pier as a function of cross sectional area (usually 1 to 2 percent).

If you have to resist axial tension, then you must ensure you have sufficient steel present (and developed) to do so. The mechanism is that the base plate transfers load to the pier, which in turn transfers the load to the longitudinal steel in the piers and, finally, the steel transfers this load through friction to the subgrade.

 

[Lion06]

aren't piers typically just designed as short columns?

 

[Robbiee]

The pier was for rock anchors for a foundation raft. The anchors were 600mm long. We required 400mmx400mm pier reinforced with 4-25mm diameter bars and 11mm dia. closed ties every 300 (total of 3). The steel plate was required to be 305x305x50. The anchor design load was 1650kN. Some of them were needed be tested until 2150kN. Contractor used 250x250x50 plate and only 2 stirrups, the first was 150mm below the top. When tested, the concrete crushed at 1700kN.

 

[civilperson]

Bearing of plate on concrete is failing. (ACI 318-08 10.14.1)

 

[csd72]

Ailmar,

In summary:

Base plate in accordance with design guide 1 (including bearing stress check)
Pier soil interaction in accordance with geotechnical principles
Check pier for combined bending/axial force as a short circular column.
I would normally allow for a misalignment of up to 150mm in my calculations.