Design of Piles and Cylinders for the Supports of Docks

continuation for Design of Pier:

General Design

- Determine support condition at both the top and bottom

- In dock construction - piles are designed as LONG columns

- The piles may be considered "fixed" if its ends are prevented from rotating (meaning that for a vertical pile, the axis of the pile must remain vertical at its ends

For Fixity at top of pile:

1. Deck must be of heavy construction and that the pile be rigidly fastened to the deck either by embedding it (if H-Pile) or by extending the reinforcement into the supporting cap or girder(if a concrete pile)

For fixity at bottom at a point not too far below bottom:

1. The soil must be a firm material such as compact sand or hard clay into which the pile is driven a substantial distance between the point of fixity (assumed to be 10 to 15 ft below the bottom)

2. For soft bottom such as silt, point of fixity at 20 to 25 ft below bottom; assume supported from buckling at 5 to 10 ft below bottom (most soils even though of relatively low strength will provide sufficient support to prevent pile from buckling

3. For firm materials and bearing depth is not deep enough to attain the assumed point of fixity, the pile may be assumed to be supported from 0 to 5 ft below the bottom

4. For soft rock, hardpan, or hardclay, the support may be taken at the surface

5. For other materials taht are less dense and may be eroded or disturbed near the sruface, point of support should be assumed 5 ft below the bottom

Various Conditions of Fixity (see fig. 5.21)

- with both ends fixed, the effective length of the pile is taken to be 0.5 of its lenghts between the points of fixity

- if piles are fixed at the top and not at the bottom (for hard materials where point of fixity was not reached), effective length =0 .75 of length figured from the point of fixity at top to teh point of support at the bottom

- if the deck is light construction (wood or light steel) the top of the pile cannot be considered as fixed: effective length = distance between the point of support at the top (underside of the deck) and the point of support at the bottom. Once the effective length is determined, the allowable load may be figured.

Precast-Concrete Piles

- provide a very prmanent type of construction, even in salt water, even without maintenance (if properly constructed and driven)

- limitations in length : handling stresses and weight becomes excessive for very long piles

(prestressing provide longer lengths)

- not recommended for clay soils which lose strength when remolded because a solid concrete pile will displace an equal volume of soil it will disturb and remold clay soils which may result in a considerable loss in shearing strength and frictional resistance to support pile

Design of Axially Loaded RC columns

SHORT COLUMNS

- for effective length, l <> , maximum allowable pile load

P = 0.8 Ag (0.225 f'c + fs pg)

where:

p = max allo pile laod in lb

Ag = gross area of tied column in sq.in.

f'c = comp strength at 28 days, psi

fs = nominal allo stress in vert column reinf. psi

- to be taken at 40 per cent of min spec value of yield point (ex. 16000 psi for 20000 psi int grade steel

- nominal allo stresses for higher yield point may be taken at 40 percent but not less than 30000 psi

pg = ratio of effective c.s.a. of vert reinf in sq.in. to the gross area Ag

LONG COLUMNS

-effective length of pile usually exceed 10 times the least lateral dimension, maximum allowable load P will be reduced to P', max allo load in lb

P' = P (1.3 - (0.03l/t)) where l = effective length or height in inches and t = least dimension

Table 5.2 > Maximum loads in piles varying in size, f'c, and reinf ration from 0.02 to 0.04.

Minimum 0.02 was used eventhough ACI permits 0.01. (reference only. verify with design)

Additional factors to consider in design of piles and thus use lower loads than would be allowed:

1. Piles have to withstand the impact stress from driving

Driving will greatly excede the static stress for which the max pile load is figured; Pile should be driven t oa resistance of three times the design load to provide an adeqaute factor of safety;

Impact stress = Ru/Ag ≤ 0.85 f'c (ultimate strength of concrete

2. There is a maximum resistance to which a pile of a certain weight can be driven with a hammer without damage to the pile.

3. Handling of pile from the casing bed to upright position may cause cracking

reinforcing/handling stress must be kept low to 12000 psi. otherwise, fine cracks may appear; piles must be designed with crack control

Pick-up Points of Piles

two-, three-, or four-point pickup as shown in fig. 22

-steel pipe sleeves are cast in the pile at the pickup points so that the pins cn be inserted to which the wire rope is attached when picking up the piles

-to keep handling stress low to below 12000 psi, long piles must have additional rods at pickup points (weight of pile = 1.25W to consider impact)

- cover = 3 in.

- all corners = 1" chamfer

- bands should be spaced 12in. o.c. except at ends and at pickup points where a closer spacing is required- ---l--long piles will require splicing the main rods (by butt welding or using short splice bars with full tension laps at each side of the splice)

-corner rods shall not be lapped as this requires offsetting the rods

See design example:

* In designing batter piles, the bending stress from the weight of the pile must be combined with the direct stress in determining the safe pile load

fa/Fa + fb/Fb ≤ 1

fa = axial unit sress in lb (actual axial load divided by gross area)

fb = actual unit bending stress due to weight of pile

Fa= allo axial stress P/Ag or P'/Ag

Fb - allo bending unit stress in steel if bending only existing (20000 psi for int grade and 18000psi for structural grade)

Steel Pipe Piles and Cylinders

-Pipes above 24" in diameter is are referred as cylinders

- specified as seamless or longitudinally welded ASTM designation A252 grade 2 (min etnsile strength 60000 psi) or grade 3 (min tensile strength 75000 psi)

- Electric-fusuion-welded spipral-seamed steel pipe, ASTM designation A252, grade 2, not recommended for hard driving conditions

-can be driven either with closed or open ends depending upon soil conditions

- closed ends predominates in marine work and dock construction

- open end piles - for rock or other hard stratum; Skin friction is ignored in figuring the load-carrying capacity of the pile

-close end more economical unless 1) a dispalcement type of pile is not desirable because of loss of shearing strength due to remolding of the soil and 2) a boulder formation exist in which it may be necessary to drill inside the pipe to btain the proper penetration

- pipe piles are filled with concrete except for temporary installations; aside from additional laod caryying capcity, it protects the shell form corrosion from the inside

-for docks, pipe piles are spliced by butt welding using a back-up plat or ring essentially if the pipes are to take uplift or bending

- the allowable closed-end friction loads may be determined as fr H piles (page 324)

Pipe piles or cylinders for dock construction have to be designed as long columns

Allowable load on columns consiting of steel pipe filled with concrete

P' = 0.25 f'c (1-0.000025 * l^2 / Kc^2) Ac + fr' As

where:

P' = allo axial load in lbs

f'c = compressive strength at 28 days

l = effective length of column

Kc^2 = radius of gyration of concrete

Ac = area of concrete

As = area of steel pipe

fr' = allo unit stress i steel pipe (17000-0.485 l^2 / Ks^2 when the pipe has a yield strength of at least 33ksi and an l/Ks ratio equal or less than 120

Ks = radius of gyration of metal pipe section

table 5.9 = max loads at diff effective lengths of pipe piles filled with 3000 psi concrete

Reference: The above are taken from this book unless otherwised referenced to other sources.
DeFQuinn, A. Design and Construction of Ports and Marine Structures. 1972. McGrawHill. p293-298