CHAPTER 5 ALLOWABLE LOADS ON A SINGLE PILE

1 FM 5-134 CHAPTER 5 ALLOWABLE LOADS ON A SINGLE PILES ection I. BASICS5-1. safe, economical pile foundations inmilitary construction, it is necessary todetermine the ALLOWABLE load capacity of asingle pile under various load structure is designed on pile supports if thesoil is inadequate for other types offoundations. The basic principles for pilefoundations are that they must be safeagainst breaking (bearing capacity and shearfailure) and buckling and that they must notsettle excessively or exceed the soil s bearingcapacity.

2 There can be other factors, such asthe need to protect a bridge pile from requirements for the preliminary designof a pile foundation Studying soil. Obtain a soil profileresulting from subsurface explorations. Thisanalysis will determine whether the piles willbe friction (sand or clay), end bearing(stratum of firm earth), or a combination ofboth. Local experience can be a useful guide,and sufficient laboratory test data to estimatestrength and compressibility of major strataare Determining pile length.

3 The mostaccurate method for determining the lengthof friction piles is to drive and load test the time factor in a theater of operationsrarely permits driving and load testing offiction piles, lengths may be calculated fromanalysis of the soil profile. Uniformity of soilconditions will determine the number of testpiles driven at selected locations to verify thecomputed lengths. On small projects and forhasty construction, dynamic formulas maybe used to assess the ALLOWABLE pile load.

4 Ifsoil conditions are nonuniform and esti-mating pile lengths accurately is difficult, apile with an easily adjustable length (timberor steel) should be used. The driving resistancein blows per inch should be used to establishallowable LOADS by comparing results ofnearby piles in similar soil conditions. Fordeliberate construction, where little ex-perience is available, load tests on selectedpiles should be performed and interpreted. Aminimum of three driving tests (and more ifsubsurface conditions are erratic) should be 5-1 FM 5-134performed.

5 Record driving resistance of testpiles and all piles installed. Compare theresistances of the test piles to insure againstlocalized weak subsurface II. STRUCTURAL DESIGNS5-4. Structural ALLOWABLE pile stresses. Overstress intimber piles under design LOADS should notexceed the values given in table 2-2. Theallowable stress in steel piles should notexceed 12,000 psi. Estimate possible re-ductions in steel cross section for corrosivelocation or provide protection from ALLOWABLE stress in precast or cast-in-place piles should not exceed 33 percent of theconcrete cylinder strength at 28 Driving stresses.

6 Do not damage thepiles by overdriving. Final driving resis-tances for various pile types should be limitedto the values indicated in CHAPTER Buckling failures. There is no danger ofbuckling a fully-embedded, axially-loaded,point-bearing pile of conventional dimensionsbecause of inadequate lateral support,provided it is surrounded by even the softestsoils. The ultimate load for buckling of slendersteel piles in soft clay is discussed later in thischapter. Buckling may be a problem whenpiles project appreciably above groundsurfaces.

7 In such cases, the unsupportedlengths of the pile should be used in column-stress formulas to determine the safe loadcapacity. The unsupported length, 1 , iscomputed assuming the pile is laterallysupported at 10 feet below ground surface insoft soils and 5 feet in sands and firm laterally supported point is commonlyreferred to as a fixed point (see figure 5-l).d. Lateral LOADS . Lateral forces on embeddedpiles may produce high bending stresses anddeflections. The behavior of short, rigid pilesunder lateral LOADS is discussed later in thischapter.

8 The behavior of relatively flexiblepiles extending appreciably above groundsurfaces and subjected to lateral LOADS isbeyond the scope of this Column-stress Slenderness ratio.(1) Timber piles. The slenderness ratio(lu/d) is the ratio of the effective un-supported length (1u) to the average pilediameter (d). The average pile diameter ismeasured at a point one-third the distancefrom the butt of the pile. For the effectiveunsupported length of a SINGLE row of pilesunbraced in the longitudinal direction, lu of the distance from the fixed point tothe top of the piles, as shown in figure a SINGLE row of piles adequately bracedin the longitudinal direction, lu is one-halfthe distance from the fixed point to thelowest bracing.

9 For piles arranged in twoor more rows and adequately bracedbetween rows, the unbraced length is the fixed point to the lowest bracing asshown in figure 5-1. If the ratio (lu/d) is lessthan 11, then a buckling capacity need notbe determined. If the ratio exceeds a valueof 11, the buckling capacity must bedetermined. To avoid use of extremelyslender piles, the value of lu/d should notexceed 40. When the slenderness ratio isless than 11, the ALLOWABLE load is based ontable 2-2. When the slenderness ratioexceeds a value of 11, the allowableconcentric axial load is computed as thelesser of the 5-134(2) Steel piles.

10 The slenderness ratio ofsteel piles is the ratio of the unsupportedlength (1.) as described to the least radiusof gyration(r). Tables and formulas for theradius of gyration are given in TM buckling capacity of steel piles mustalways be determined regardless of thenumerical value of lu/r. For the most econom-ical design, the value of lu/r should notexceed 120. For steel piles the allowablebuckling load is calculated as 5-134 Section III. DYNAMIC FORMULAS5-6. pile formulas are based on thetheory that the ALLOWABLE load on a pile isclosely related to the resistance encounteredduring driving.