Civil Engineering Reference
In-Depth Information
Development Length
It is necessary to examine the bond development length of the rein-
forcements when high bending stress is experienced near the end of a
concrete pole section. Examples of these conditions are at slip joints,
fl ange connections, long davit arms at the tip of a pole, or short embed-
ment conditions. Bond development length should be checked to ensure
adequate bond capacity when reinforcing steel is cut, such as by drilling
holes. ACI 318 (ACI 2008) covers the calculation for development length.
The Principal Assumptions of Ultimate Capacity
The ultimate moment capacity of a pole at any given cross section is a
function of the strains in the prestressing steel and concrete. The factored
design moment should not exceed the ultimate moment capacity.
The following assumptions are made in computing the ultimate
moment capacity of poles:
• Plane sections remain plane.
• The steel and concrete are adequately bonded.
• The steel and concrete are considered in the elastic and plastic
ranges, respectively.
• The concrete compressive stress at failure is 0.85
f
c
(as taken from
equivalent stress block methodology).
• The tensile concrete strength is neglected in fl exural computations.
• The ultimate concrete strain is 0.003 in.
• The conditions of compatibility and equilibrium are met.
While the fi rst two assumptions become somewhat less valid after the
section has cracked, the overall behavior of the member can still be pre-
dicted adequately.
Determination of Ultimate Capacity
Equilibrium of Section
Determination of the Compressive Stress Area:
Based on the afore-
mentioned assumptions and the provisions in ASCE/PCI (1997) and ACI
318 (ACI 2008), the assumed rectangular compressive stress distribution
in the concrete is used herein for simplifi cation and is represented by a
statically equivalent concentrated force, defi ned by the cylinder compres-
sive strength
f
′
c
, the parameter
β
1
, the distance from the extreme
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