3107F.3 Timber Piles and Deck Components
The following parameters shall be established in order to assess component strength:
- Modulus of rupture
- Modulus of elasticity
- Type and grade of timber
- Original cross-section shape and physical dimensions
- Location and dimension of braced frames
- Current physical condition of members including visible deformation
- Degradation may include environmental effects (e.g., decay, splitting, fire damage, biological and chemical attack) including its effect on the moment of inertia, I
- Loading and displacement effects (e.g., overload, damage from earthquakes, crushing and twisting)
Section 3104F.2.2 discusses existing material properties. At a minimum, the type and grade of wood shall be established. The adjusted reference design values per Section 6 of ANSI/AWC NDS [7.11] may be used.
For deck components, the adjusted design stresses shall be limited to the values of ANSI/AWC NDS [7.11]. Piling deformation limits shall be calculated based on the strain limits in accordance with Section 3107F.3.3.3.
The values shown in the ANSI/AWC NDS [7.11] are not developed specifically for MOTs and can be used as default properties only if as-built information is not available, the member is not damaged and testing is not performed. To account for the inherent uncertainty in establishing component capacities for existing structures with limited knowledge about the actual material properties, a reduction (knowledge) factor of k = 0.75 shall be included in the component strength and deformation capacity analyses in accordance with Section 3107F.2.1.2.
The displacement demand and capacity of existing timber structures may be established per Section 3104F.2.
The soil spring requirements for the lateral pile analysis shall be in accordance with Section 3106F.
A linear curvature distribution may be assumed along the full length of a timber pile.
The displacement capacity of a timber pile can then be established per Section 3107F.3.3.2.
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:
|E||=||Modulus of elasticity|
|I||=||Moment of inertia|
|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:
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|
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:
r = radius of pile
The shear capacity must be greater than the maximum demand.