3107F.3.3 Timber Piles
Section 3107F.2.5.2 shall apply to timber piles.
A distinction shall be made between a pier-type pile, with a long unsupported length and a wharf-landside-type pile with a short unsupported length between the deck and soil. The effective length, L, is the distance between the pinned deck/pile connection and in-ground fixity as shown in Figure 31F-7-15. For pier-type (long unsupported length) vertical piles, three simplified procedures to determine fixity or displacement capacity are described in UFC 4-151-10 [7.12], UFC 3-220-01A [7.13] and Chai [7.14].
In order to determine fixity in soft soils, another alternative is to use Table 31F-7-8.
The displacement capacity, Δ, for a pile pinned at the top, with effective length, L, (see Table 31F-7-8 and UFC 4-151-10 [7.12]), and moment, M, is:
where:
| E | = | Modulus of elasticity |
| I | = | Moment of inertia |
ASSUMED IN-GROUND FIXITY
| PILE EIg | SOFT CLAYS | LOOSE GRANULAR & MEDIUM CLAYS |
| < 1010 lb in2 | 10 feet | 8 feet |
| > 1010 lb in2 | 12 feet | 10 feet |
Assuming linear curvature distribution along the pile, the allowable curvature, ϕa, can be established fromml:
where:
εa = allowable strain limit according to Section 3107F.3.3.3
c = distance to neutral axis which can be taken as Dp/2, where Dp is the diameter of the pile
The curvature is defined as:
The maximum allowable moment therefore becomes:
The displacement capacity is therefore given by:
The following limiting strain values apply for each seismic performance level for existing structures:
| EARTHQUAKE LEVEL | MAX. TIMBER STRAIN |
| Level 1 | 0.002 |
| Level 2 | 0.004 |
For new and alternatively, for existing structures ANSI/AWC NDS [7.11] may be used.
Timber components for all non-seismic loading combinations shall be designed in accordance with ANSI/AWC NDS [7.11].
To account for material strength uncertainties, the maximum shear demand, Vmax, established from the single pile lateral analysis shall be multiplied by 1.2:
The factored maximum shear stress demand τmax, in a circular pile can then be determined:
where:
r = radius of pile
For the seismic load combinations, the maximum allowable shear stress, τcapacity, is the design shear strength, τdesign, from the ANSI/AWC NDS [7.11] multiplied by a factor of 2.8.
The shear capacity must be greater than the maximum demand.