Post and Pole Foundation Design

ASAE EP486 DEC98

Post and Pole Foundation Design Develo Developed ped by the ASAE Post and Pole Pole Foundat Foundation ion Subcomm Subcommitt ittee;  ee;  approv approved ed by the Struct Structures ures and Enviro Environmen nmentt Divisio Divisionn Standar Standards  ds  Committee; Committee; adopted by ASAE March 1991; revised editorially editorially December  December  1992; reaffirmed December 1995, December 1996, December 1997;  reaffirmed for one year December 1998.

3.4.2  Soil design values should include a safety factor, convert raw soil test results to allowable design values using an adequate safety factor. The safety factor factor is recomme recommended nded to be between between 2.5, when small small movement is not detrimental, and 3.0. 3.4.3  Organic silt, soft clay, and peat are not adequate materials for foundation design.

1 Purpose and scope 1.1 Purpose: To Purpose: To present a design procedure for foundations that resist lateral and vertical forces acting on structural wooden post and pole members in buildings, space frames and plane frames. 1.2 Limitation:   Preced Precedence ence of applic applicable able buildi building ng codes codes or other other requirements.

2 Definitions 2.1 Post: A Post:  A post or a pole, except where specifically noted. 2.1.1 Post:  Post:   A rectangular primary structural member, usually vertical, and generally uniform in cross section along its length. Posts are partly embedded in the soil to provide lateral and vertical support for the structure. Posts may be sawn or laminated dimension lumber. 2.1.2 Pole: A Pole:  A round, unsawn, naturally tapered post. 2.1.3 2.1.3 Slabbed Slabbed pole: pole:   A pole pole modifi modified ed to provi provide de flat flat surf surfac aces es for attaching framing members. 2.2 Foundation design 2.2.1 Lateral: Foundation Lateral: Foundation design for post frame resistance to loads such as wind and stored granular material that tend to cause horizontal post displacement. support and is 2.2.1.1 2.2.1.1 Constrained Constrained case: The case: The post rotates about a rigid support laterally supported by reactive soil pressure on one side of the post. 2.2.1.2 Non-constrained case:  case:  The post rotates about an axis below the ground surface and is laterally supported by reactive soil pressure on both sides of the post. 2.2.2 Vertical: Foundation Vertical: Foundation design for post frame resistance to upward or downward loads such as wind, snow, and building mass that tend to cause vertical post displacement. displacement. 2.3 Collars: Components Collars:  Components that increase the bearing area of portions of the post foundation, and thus increase lateral or vertical resistance.

3 Material requirements 3.1 Wood. Design Wood. Design structural wood members and connections according to National National Forest Products Association Standard, Design Specifications Specifications for Wood Construction, or other appropriate design standards. 3.2 Posts.   Prote Protect ct post postss from from dete deteri riora oratition on acco accordi rding ng to ASAE ASAE Engineering Practice EP388, ‘‘Design Properties of Round, Sawn and Laminated Laminated Preservatively Preservatively Treated Construction Construction Poles and Posts’’. Posts’’. 3.3 Concrete.  Concrete.   Design plain concrete structural members according to Amer Americ ican an Conc Concre rete te Inst Instititut utee Stan Standa dard rd 318. 318.1, 1, Buil Buildi ding ng Code Code Requirements Requirements for Structural Structural Plain Concrete, Concrete, and reinforced reinforced concrete members according to ACI Standard 318, Building Code Requirements for Reinforced Concrete, or other appropriate design standards. 3.4 Soil. Variable Soil.  Variable characteristics, composition, and moisture condition require caution in evaluating soil strength properties. Lateral soil strength is assumed to increase hydrostatically (linearly with depth). 3.4.1  In the absence of satisfactory soil test data or specific building code requirements, requirements, presumptive soil characteristi characteristics cs listed herein may be used.

4 Lateral foundation design 4.1 General. Foundation General. Foundationss resist both rotational and horizontal effects of lateral lateral post loading. loading. The interaction interaction of loads and restraints on the post at the ground or floor floor surface surface results results in two lateral lateral foundat foundation ion design design cases—constrained and non-constrained. 4.1.1 Analysis.   Engine Engineerin eringg analysi analysiss of the post frame system system is required to determine the moment and shear resistance required of the lateral foundation at the surface of the soil or floor support. 4.1.2 Frictional Frictional resistance. A resistance. A component component of lateral resistance resistance resulting from sliding resistance of collars or footings occurs only with a downward axial post force. 4.1.3 Post-to-surface connection and frost heaving. For heaving. For lateral and vertical foundation design it is assumed that no vertical connection exists that ties the post to concrete surface slabs and collars. collars. The shrinkage shrinkage of the materials at the post-concrete interface is sufficient to provide the slip. It is assumed that frost heaving considerations are included in the design of slabs and collars. 4.2 Constrained case.  case.  The post rotates about a rigid surface or floor and the resisting moment is provided by reactive soil pressures that increase parabolically with embedment depth on one side of the post. When the post rotation is located a distance above soil level, as with an elevated floor support, the increase in the soil moment lever arm results in larger soil supporting moment, see paragraph 4.2.2. If a post is not encased in or anchored to a rigid support, the post is not constrained in both directions and requires embedment design for both constrained and non-constrained cases. A bearing plate support, as in paragraph 4.2.3.3, will will produce produce constr constraine ainedd conditi conditions. ons. Bottom Bottom collars collars and footing footingss are located in areas of high lateral soil bearing, and they may develop bearing and frictional resistance forces. Collar reinforcement (to maintain structural integrity) and connection to the post are required before collar forces are additive to the system. Structural connection to the collar is required required before footing forces are additive additive to the system. The soil forces on these collars collars and footings footings are additiv additivee to the post post values, values, see paragraph 4.2.1 or paragraph 4.2.2. Soil force calculations for collars and footings use the hydrostatic lateral soil bearing pressure at the mid-height of the collar or footing. An iterative solution is required to determine accept acceptable able embedment embedment depth depth for all constr constraine ainedd cases cases except except in paragraph paragraph 4.2.1. 4.2.1 Ground surface support. The support. The post rotates about a rigid support at ground surface. Fig. 1 illustrates the lateral forces on this embedded post post case. case. This case does not include include collar collar or footin footingg resista resistance nce comp compone onent nts, s, and fric frictition on on the the botto bottom m of the post post is negle neglect cted. ed. Calculate embedment depth, soil resisting moment, soil force resultant and support reaction with the following: (M s   Ph and Ph and Sd  S 3 were substit substitutedin utedin the America Americann Wood Wood Preserv Preserves es Institute/Uni Institute/Uniform form Building Building Code equation equation for constrained constrained embedment.) embedment.) d 3  4.25 M s  /  S b  M s  M r  0.7 d Q 1 Q 1  0.33 S bd 2

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Figure 1 – Post foundation free-body diagram for a surface supported post

R  V s  Q 1

Figure 2 – Post foundation free-body diagram for a surface supported post with bottom collar

k cc 



Q 2 Q 3



t c  w  P 



where b 



d  M r  M s 



Q 1 R  S 



V s 



 

 

effective width of the post in the soil, m (ft), see paragraph 4.4 post embedment depth, m (ft) moment resistance of the soil, kN·m (lbf·ft) moment at rigid surface from engineering analysis, kN·m (lbf·ft) resultant of soil forces on post, kN (lbf) reaction at support, kN (lbf) allowable lateral soil pressure per unit of depth including increases, kPa/m (lbf/ft2·ft), see paragraph 4.5 shear at rigid surface from engineering analysis, kN (lbf)

4.2.1.1 Ground surface support and bottom collar.  This case is applicable when the footing is not adequately connected to the collar and the sliding resistance, Q 3 , between the collar and the footing is less than the sum of the potential soil lateral bearing and sliding resistance of the footing. The normal force used to calculate sliding resistance is the minimum downward vertical force in the post. Fig. 2 illustrates the forces associated with a bottom collared post. The collar resisting moment and resultant are additive to the post, see paragraph 4.2.1. Assume d  and calculate the soil resisting moment, soil force resultants, and floor reaction with the following: M s  M r  0.7 dQ 1   d  0.5 t c  Q 2  dQ 3 Q 1  0.33 Sbd 2



 

plus applicable definitions in paragraph 4.2. 4.2.1.2 Ground surface support and bottom collar and footing. The footing is adequately connected to the collar, or the sliding resistance between the collar and footing is greater than the sum of the soil lateral bearing and sliding resistance of the footing. The embedment depth is increased by the footing thickness and the lateral sliding resistance, Q 5 , acts at the interface between the footing and the soil. Fig. 3 illustrates the collar and footing forces associated with this case. Assume d  and calculate the soil resisting moment, soil force resultants, and the total floor reaction with the following: M s  M r  0.7 dQ 1   d  0.5 t c  Q 2   d  0.5 t f  Q 4   d  t f  Q 5 Q 1  0.33 Sbd 2 Q 2  St c  d  0.5 t c  w  b  Q 4  Swt f  d  0.5 t f  Q 5  k cs P  R  V s  Q 1  Q 2  Q 4  Q 5 where

Q 2  St c  d  0.5 t c  w  b 

Q 4 Q5



Q 3  k cc P 

k cs 



R  V s  Q 1  Q 2  Q 3

t f  w 



where

sliding resistance coefficient, concrete on concrete, see paragraph 4.8 resultant of soil forces on the collar, kN (lbf) resultant of lateral sliding resistance of the collar, kN (lbf) thickness of collar, m (ft) width of collar, m (ft) post downward vertical force from engineering analysis, kN (lbf), see paragraph 4.7





resultant of soil forces on footing, kN (lbf) resultant of lateral sliding resistance of the footing on soil, kN (lbf) sliding resistance coefficient, concrete on soil, see paragraph 4.8 thickness of footing, m (ft) width of footing and collar, m (ft)

plus applicable definitions in paragraph 4.2.

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a  M f 



V f 





distance between ground and floor line, m (ft) moment at floor line, from engineering analysis, kN·m (lbf·ft) shear at floor line, from engineering analysis, kN (lbf)

plus applicable definitions in paragraph 4.2. 4.2.2.1 Elevated floor support and bottom collar.  This case is applicable when the footing is not adequately connected to the collar and the sliding resistance, Q 3 , between the collar and the footing is less than the sum of the potential soil lateral bearing and sliding resistance of the footing. The normal force used to calculate sliding resistance is the minimum downward vertical post force. Fig. 5 illustrates the forces associated with a bottom collared post. The bottom collar moment and resultant are additive to the post, see paragraph 4.2.2. Assume d  and calculate resisting moment, soil force resultants, and the total floor reaction with the following: M f  M  r   a  0.7d  Q 1   a  d  0.5 t c  Q 2   a  d  Q 3 Figure 3 – Post foundation free-body diagram for a surface supported post with bottom collar and footing combined

Q 1  0.33 Sbd 2 Q 2  St c  d  0.5 t c  w  b 

4.2.2 Elevated floor support. The post rotates about a floor surface a distance ‘‘a ’’ above the groundline. The longer lever arm increases the soil supporting moment. Fig. 4 illustrates the forces for this case. This case does not include collar or footing resistance components, and friction on the bottom of the post is neglected. Assume d  and calculate embedment depth, soil resisting moment, soil force resultant and floor reaction with the following: d 3  1.43ad 2  4.25 M f  /  Sb  M f  M r   a  0.7d  Q 1 Q 1  0.33 Sbd 2

Q 3  k cc P  R  V f  Q 1  Q 2  Q 3 where applicable definitions in paragraph 4.2 apply. 4.2.2.2 Elevated floor support and bottom collar and footing. The footing is adequately connected through the collar, or the sliding resistance between the collar and footing is greater than the sum of the soil lateral bearing and sliding resistance of the footing. The embedment depth is increased by the footing thickness and the lateral sliding resistance acts at the interface between the footing and the soil. Fig. 6 illustrates the forces associated with this case when the post is anchored by a combined collar and footing. Assume d   and calculate the soil resisting moment, soil force resultants and the total floor reactions with the following: M f  M r   a  0.7d  Q 1   a  d  0.5 t c  Q 2   a  d  0.5 t f  Q 4

R  V f  Q 1

  a  d  t f  Q 5

Q 1  0.33 Sbd 2

where

Figure 4 – Post foundation free-body diagram for an elevated floor supported post

Figure 5 – Post foundation free-body diagram for an elevated floor supported post with bottom collar

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Figure 7 – Concrete surface support to provide constrained conditions

Figure 6 – Post foundation free-body design for an elevated floor supported post with collar and footing combined

Q 2  St c  d  0.5 t c  w  b  Q 4  Swt f  d  0.5 t f 

Figure 7b – Wood surface support to provide constrained conditions

Q 5  k cs P  R  V f  Q 1  Q 2  Q 4  Q 5 where applicable definitions in paragraph 4.2 apply. 4.2.3 Surface support required to produce constrained condition. For a post to be constrained, the soil reaction on the post must be equal to or greater than the sum of lateral post forces. Cast rectangular or circular concrete slabs provide support through sliding resistance and lateral bearing. Preservative-treated wooden beams, located just below grade, fastened to the post provide support through lateral bearing only. Lateral soil resistance is assumed to increase linearly with depth because the support does not rotate. To prevent frost heaving of the post, do not connect the surface support to the post. In areas of deep frost penetration, these surface collars are not recommended. Figs. 7a and 7b illustrate surface support. Calculate the lateral resultant, R s  , with the following: 4.2.3.1 Concrete slab R  R s  k cs W d  0.5 Sw t 2 where R  R s  k cs 

  reaction, kN (lbf)   lateral soil resistance, kN (lbf)    lateral sliding coefficient, concrete on soil, see paragraph 4.8 W d     dead weight of concrete slab, kN (lbf) S    allowable lateral bearing soil pressure per unit of depth including increases, kPa/m (lbf/ft2·ft), see paragraph 4.5 t     slab or beam bearing depth, m (ft) w     slab or beam bearing width, m (ft)  

4.2.3.2. Wooden beam R  R s  0.5 Sw t 2

where applicable definitions in paragraph 4.2.3 apply. 4.2.3.3 Bearing plate support. A steel bearing plate on the outside of the post at the level of the rigid support may provide sufficient restraint to allow constrained case analysis. Design the steel plate with sufficient area to prevent post crushing under the reaction, R . Anchor the bearing plate to the concrete surface or floor with adequate concrete bond to sustain the reaction, R . This connector must allow independent vertical movement between the post and concrete surface or floor. 4.3 Non-constrained case.  The post rotates about an axis below the ground surface. Reactive soil pressures are on both sides of the post. An iterative solution is required for the non-constrained case. Vary d  until an acceptable embedment is obtained. The rotation axis location is sensitive to a balance between shear and moment forces. Some solutions for y ¯  exceed the valid boundaries of the rotation axis equation; therefore, y ¯  must be slightly less than d   and yield positive moment and shear resistance values. 4.3.1 Post.   Fig. 8 illustrates the post foundation free-body diagram forces on an embedded post. Sliding resistance on the post bottom is neglected. This case also applies to foundations with concrete backfilled post holes, see paragraph 4.4.3. In areas of deep frost penetration, concrete backfilled foundations require special care or frost heaving may be a problem, see paragraph 4.6. Calculate embedment depth, required soil resisting moment, and soil force resultants with the following: (M g    P h , Sd  /3  S 1  and  V g    P  were submitted in the AWPI/UBC equation for non-constrained embedment.) d 2  3.51V g  /  Sb  1   1   0.62 M g Sb d  / V 2g  1/2 ¯ Q 2  0.5y  ¯ Q 1 M g  M r  z  ¯  2bd 3 /  3  bd 2  2Vg  /  3S   y  4

3

¯  0.5 3d 4  y  ¯   4y  ¯ d 3  /  2d 3  y  ¯   3y  ¯ d 2  z 

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2

4

3

¯  0.5 3d 4  y  ¯   4y  ¯ d 3  /  2d 3  y  ¯   3y  ¯ d 2  z 

Q 1  0.5 Sby ¯ 

c  t u  / y¯  

3

¯   3y  ¯ d 2  / y¯   Q 2  0.5 Sb  2d 3  y 

2

Q 1  0.5 Sby ¯ 

V g  Q 1  Q 2

3

¯   3y  ¯ d 2  / y¯   Q 2  0.5 Sb  2d 3  y 

where b 



d  M g 



M r  Q 1



Q 2



S 



V g 



¯  y  ¯  z 









bearing width of the post in the soil, m (ft), see paragraph 4.4 post embedment depth, m (ft) moment at ground surface from engineering analysis, kN·m (lbf·ft) soil resistance moment, kN·m (lbf·ft) resultant of soil forces on post above the rotation axis, kN (lbf) resultant of soil forces on post below the rotation axis, kN (lbf) allowable lateral bearing soild pressure, per unit of depth including increases, kPa/m (lbf/ft2·ft), see paragraph 4.5 shear at ground surface, from engineering analysis, m (ft) location of the rotation axis, kN (lbf) location of centroid of Q 2 , m   ft 

4.3.2 Top collared post. Collars increase lateral support at the surface by providing a wider bearing area, generally post hole diameter in width. Top collars elevate the location of the rotation axis; y ¯  varies with collar size and post depth. Friction on the post bottom is neglected. In areas of deep frost penetration, top collars are susceptible to frost heaving; therefore, special care is required to backfill the post hole with sand or granular material, see paragraph 4.6. Fig. 9 illustrates the forces associated with a top collared post. Calculate the location of the rotation axis, soil resisting moment, and soil force resultants with the following:

q 3  St 2u  1.5 c  w  b  V g  Q 1  Q 3  Q 2 where c  m  Q 3 t u  w 

ratio of collar depth to depth of the rotation axis,  1 location of the mean of the collar force, Q 3 , m (ft) resultant of the soil forces on upper collar, kN (lbf) thickness of collar, m (ft) collar width, m (ft)

    

plus applicable definitions in paragraph 4.3 4.3.3 Bottom collared post. Bottom collars lower the rotation axis, yet increase resisting moment. Collars sustain lateral bearing and frictional ¯ . The rotation axis forces. Dimension changes require recalculating y  ¯  must be on or within the collar [t c   (d   y )], as less thickness is impractical. 4.3.3.1 Independent collar and footing.  Without connection to the footing and when the sliding resistance between the collar and footing is less than the sum of the soil lateral bearing and sliding resistance of the footing, the collar can slide on the footing surface. No lateral resistance is developed by the footing. Fig. 10 illustrates the soil forces on the post. Calculate the location of the rotation axis, soil resisting moment, and soil bearing force resultants with the following: ¯ Q 6  0.5y  ¯ Q 1   y  ¯  n  Q 5  dQ 4 M r  z  ¯  d  4  3c   /  6  4c   n   t c  y  

¯ Q 2  0.5y  ¯ Q 1  mQ 3 M g  M r  z 



¯  2  w  t 3c  3dt 2c  3d 2 t c   bd 3  3bd t 2c  bt 3c  3bd 2 t c  /  3  bd 2 y 

m  t u  4  3c  /  6  4c 

2

2

 bt c  2bdt c  w  t c  2dt c   2  k cc P  V g  /  3S 

¯  2  t 3u  w  b   bd 3  /  3  t 2u  w  b   bd 2  2V g  /  3S   y 

Figure 8 – Post foundation free-body diagram for a soil supported post



¯  0.5 3d 4  y  ¯ 4  4y  ¯ d 3  /  2d 3  y  ¯ 3  3y  ¯ d 2  z 

Figure 9 – Post foundation free-body diagram for a soil supported post with top collar

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

c 

¯  2  w  t 3f   3dt 2f   3d 2 t f  t 3c  3dt 2c  3d 2 t c   bt 3c  3bd t 2c  y 

¯  d  / y¯     t c  y 

2

2

 3bd 

Q 1  0.5 Sby ¯ 

t c  bd 3  /  3  w  t 2f   2dt f  t 2c  2dt c   bt 2c  2bdt c 

2

 bd   2  k cs P  V g  /  3S 

¯  d  2  1.5 c   w  b  Q 5  S  t c  y  



4

3

3

3

¯  0.5 3  d  t f  4  y  ¯   4y  ¯  d  t f  3  /  2  d  t f  3  y  ¯   3y  ¯  d  z 

2

¯   3y  ¯ d   / y¯   Q 6  0.5 Sw  2d   y 

 t f 

Q 4  k cc P 

c 



V g  Q 1  Q 5  Q 6  Q 4

2



¯  d  / y¯     t c  y  2

Q 1  0.5 Sby ¯ 

where

¯  d  2  1.5 c   w  b  Q 5  S  t c  y  

c 





k cc 



n 



Q 4



Q 5



Q 6



t c  P 

 

ratio of the collar section dimension above the rotation axis to the depth of the rotation axis sliding resistance coefficient, concrete on concrete footing, see paragraph 4.8 location of the mean of the upper section of the collar force, m (ft) resultant of lateral sliding resistance of collar on concrete, kN (lbf) resultant of soil forces on collar above the rotation axis, kN (lbf) resultant of soil forces on the collar and post below rotation axis, kN (lbf) thickness of lower collar, m (ft) downward post vertical force from engineering analysis, kN (lbf), see paragraph 4.7

plus applicable definitions in paragraph 4.3 4.3.3.2 Connected collar and footing.  When footings are mechanically connected to collars or when the sliding resistance between collar and footing is greater than the sum of the lateral bearing and sliding resistance of the footing, the sliding surface is the footing-soil interface. The effective depth and soil resistance is increased. Fig. 11 illustrates the soil force on post and footing. The lateral sliding resistance acts on the soil through the footing. Calculate the location of the rotation axis, soil resisting moment and soil force resultants with the following: ¯ Q 8  0.5y  ¯ Q 1   y  ¯  n  Q 5   d  t f  Q 4 M r  z  ¯  d  4  3c   /  6  4c   n   t c  y  



Figure 10 – Post foundation free-body diagram for a soil supported post with bottom collar

¯ 3  3y  ¯  d  t f  2  / y¯   Q 8  0.5 Sw  2  d  t f  3  y  Q 4  k cs P  V g  Q 1  Q 5  Q 8  Q 4 where k cs 



Q 4 Q 8



t f 





sliding resistance coefficient, concrete on soil, see paragraph 4.8 sliding resistance of footing on soil, kN (lbf) resultant of soil forces on collar and footing below rotation axis, kN (lbf) thickness of footing, m (ft)

plus applicable definitions in paragraph 4.3. 4.4 Post bearing. Effective width, b , for paragraphs 4.2 and 4.3: 4.4.1 Post. b   diagonal of post cross section, m (ft). 4.4.2 Pole. b   pole diameter [determined from circumference 1800 mm (6 ft) above butt], m (ft). 4.4.3 Concrete post hole backfill. b=diameter of concrete, m (ft). 4.5 Soil pressure. Lateral and vertical allowable soil pressure. 4.5.1 Source.   Site specific soil tests produce the most accurate allowable pressures when they include design load criteria and an acceptable safety factor. In the absence of tests or building code restrictions, basic soil lateral pressure values may be presumed from Table 1. 4.5.2 Allowable vertical soil pressure.  The presumed values from Table 1 may be increased for both depth and width adjustments. 4.5.3 Allowable lateral soil pressure. Lateral soil strength is assumed to increase hydrostatically with depth. The depth factor has been incorporated in all the embedment equations; therefore, the pressure,

Figure 11 – Post foundation free-body diagram for a soil supported post with combined bottom collar and footing

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S , should not be increased for depth. Presumed pressures from Table 1 may be adjusted for the conditions of design that include wind load, isolated post location and deflection tolerance. Some building codes may not permit a soil pressure increase for wind load, see paragraph 1.2. 4.5.3.1 Constrained case pressure increase.  Lateral soil pressure may be doubled for isolated posts that are spaced at least six times their width apart. The increase is due to the expanded volume of soil support for the post, as in deep footing cases. 4.5.3.2 Non-constrained case pressure increase.  Lateral soil pressure may be doubled for isolated posts which are not adversely affected by up to 13 mm (0.5 in.) lateral deflection in the soil due to short term load. 4.5.3.3 Wind loading pressure increases.  Lateral soil pressure may be increased one-third for wind forces acting alone or in combination with vertical loads. Wind increases are cumulative with other pressure increases for both constrained and non-constrained cases. 4.6 Backfill. Fill annular space around the post with one of the following: 4.6.1  Clean sand compacted by tamping layers not more than 200 mm (8 in.) deep. 4.6.2  Granular aggregate material compacted by tamping layers not more than 200 mm (8 in.) deep. 4.6.3  Excavated soil compacted to at least its undisturbed consistency, provided the soil is not susceptible to frost heaving in areas of deep frost penetration. 4.6.4 Placed concrete.  Concrete backfill increases effective width b . Concrete backfill is suitable for postholes except those in areas subject to deep frost penetration within soils prone to frost heave. Rough and/or upward tapered sides of postholes increase the heaving potential. 4.7 Vertical post force. The vertical reaction on the post foundation is

determined by engineering analysis of wind, snow, live and dead load combinations on the post-frame building. Consider uplift, sliding, and overturning forces from wind loads. 4.8 Foundation sliding resistance.  A downward post force provides lateral foundation sliding resistance from collar friction on concrete footing or footing friction on soil. If the post is rigidly anchored to the floor surface, sliding resistance is zero. The sliding resistance coefficient for concrete on concrete in a damp condition is 0.6. Coefficients for concrete on soil are in Table 1.

5 Vertical foundation design 5.1 General.   Post foundations resist vertical upward and downward forces, including overturning forces from wind and gravity loads from snow and building mass. 5.2 Gravity foundation designs. The vertical bearing area required to support gravity loads or vertical wind forces is: A  P  / S v  where A P  S v 

  

required footing area, m2 (ft2) vertical foundation load, kN (lbf) allowable vertical soil pressure, including increases, kPa (lbf/ft2), see paragraph 5.2.4

5.2.1 Minimum depth.  In addition to codes and other requirements, consider frost penetration. Special design may be needed when the minimum depth is impractical.

Table 1 – Presumed soil properties for post foundation design (for use in absence of codes or tests)

Class of materials 1. Massive crystalline bedrock 2. Sedimentary and foliated rock 3. Sandy gravel and/or gravel (GW and GP) 4. Sand, silty sand, clayey sand, silty gravel and clayey gravel (SW, SP, SM, SC, GM and GC) 5. Clay, sandy clay, silty clay and clayey silt (CL, ML, MH and CH)

Density or consistency*

Lateral† pressure per unit of depth

Vertical§ pressure

‡

lb/ft2·ft

Lateral sliding coefficient

kPa

180

(1200)

0.79

60

(400)

firm loose

45 30

(300) (200)

firm

30

(200)

22.5

(150)

20

(130)

15

(100)

loose

medium soft

kPa/m

 



Friction angle

lb/ft2

degree

200

(4000)

0.35

100

0.35

100

0.25

6(130)**

75

 

50

 

Density# kg/m3

lb/ft3

---

---

---

(2000)

---

---

---

(2000)

38 32

2000 1500

(120) (90)

30

1750

(105)

26

1400

(85)

15

2000

(120)

10

1500

(90)

(1500)

(1000)

 

*

Firm consistency of class 4 and the medium consistency of class 5 can be molded by strong finger pressure, and the firm consistency of class 3 is too compact to be excavated with a shovel. † The hydrostatic increase in lateral pressure per unit depth has been included in the equations of this Engineering Practice. A per unit depth increase is allowed up to 4.5 m (15 ft). The shallow depth foundations of post frame buildings are well within this limit. Source: Table 29-B UBC modified with the addition of firm and medium values from Hough. ‡ Sliding resistance source: Table 29-B UBC. § Allowable foundation pressures are for footings at least 300 mm (1 ft) wide and 300 mm (1 ft) deep into natural grade. Pressure may be increased 20% for each additional 300 mm (1 ft) of width and/or depth to a maximum of three times the tabulated value. Source: Table 29-B UBC.  Soil friction angle varies from soft to medium density for clay materials, and from loose to firm for sand and gravel materials. Source: Merritt. # Soil density varies from soft to medium density for clay materials, and from loose to firm for sand and gravel materials. Source: Hough. 2 **Multiply an assumed lateral sliding resistance of 6 kPa (130 lb/ft ) by the contact area. Use the lesser of the lateral sliding resistance and one-half the dead load. 756

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5.2.2 Minimum thickness of footings. For reinforced concrete footings, cover the reinforcement at least 150 mm (6 in.) above and 75 mm (3 in.) below. Provide sufficient depth to prevent diagonal failures in plain concrete footings. A minimum of 200 mm (8 in.) is required. 5.2.3 Post hole preparation. Assure that soil in the bottom of holes is level and has the density of undisturbed soil. 5.2.4 Presumed allowable foundation pressure. In the absence of soil test data or applicable building codes, allowable foundation pressures may be presumed from Table 1. 5.3 Uplift foundation design. Foundation mass, post uplift resistance, and anchorage to spread footings resist uplift loads on posts. 5.3.1 Post uplift design. Design the post to resist withdrawal from the soil under wind uplift forces. Below grade, use mechanical fasteners with durability equal to the service life of the building. 5.3.1.1 Friction. Do not include the resistance of soil-post skin friction. 5.3.1.2 Poles.  Tapered poles, when embedded large-end down, may have some resistance to vertical withdrawal from a wedging effect. 5.3.1.3 Concrete backfill.   Cast against undisturbed soil and mechanically fastened to the post, concrete adds vertical resistance of both the mass of concrete and the skin friction between concrete and soil. 5.3.1.4 Concrete paving.  When adequately mechanically fastened to posts, paving adds vertical resistance equal to the mass of concrete that remains connected to the post. Frost heaving considerations must be included in the concrete pavement design. 5.3.2 Enlarged post bottom.  The size of the soil cone above a foundation element depends on the soil friction angle. In the absence of soil test data or applicable building codes, allowable friction angles and soil density may be presumed from Table 1. 5.3.2.1 Concrete collars. Circular cast-in-place concrete collars, when mechanically anchored to the post, displace a conically shaped wedge of soil as illustrated in Fig. 12. Calculate the potential resistance of a circular collar, including soil and attached concrete, from the following. In practice, the mass is usually limited to the strength of the collar-to-post connection. U   G  0.33   d  t   0.5w  /tan    3  tan   2  0.125

w 3 /tan    A p  d  t    0.25C  w 2 hG 

where U 



 



C  G 



d  t  w 



 



A p 



where U 



 



G 



w  l  A p  d  t 



 



   

soil uplift resistance, kN (lbf) soil density, kg/m3 (lb/ft3), see Table 1 gravity acceleration constant, 9.810−3 kN·m/N·s2 (1.0 lbf/lb) width of collar, m (ft) length of collar, m (ft) post cross sectional area, m2 (ft2) embedment depth, m (ft) wood collar thickness, m (ft) soil friction angle, deg, see Table 1

5.3.3 Spread footing design.  Shallow bedrock depth or low strength soil that prevents the required lateral or vertical post embedment depth, may require spread footing design. Laterally fastening the post to bedrock with spread concrete footings may provide the required lateral resistance. These wide footings, connected to the post, may provide the uplift resistance.

6 Commentary 6.1 General.  This Engineering Practice has been developed to assist engineers improve the quality of post-frame buildings. These buildings are assumed to have shallow depth foundations usually less than 2.4 m (8 ft). Its basis is the Pole Building Design, American Wood Preserves Institute (AWPI); the current Uniform Building Code (UBC), International Conference of Building Officials; and the BOCA National Building Code, Building Officials and Code Administrators International (BOCA). This Engineering Practice expands the technology by including upper and lower post collar design. Assumptions involving soil, embedment conditions, and the performance of embedded posts and their structural collars and footings are part of this Engineering Practice, and are discussed in this commentary. Two basic design cases, constrained and non-constrained, have been expanded with the addition of collars, anchorage of footings to the post, and surface bearing supports. These additions offer the engineer latitude in meeting design requirements. Generally, the engineer will initially analyze the ‘‘post-only case’’ for required embedment depth and include the additional strength of collars and footings as required. The constrained lateral foundation design cases increase embedment strength as follows:

soil and foundation uplift resistance, kN (lbf) soil density, kg/m3 (lb/ft3), see Table 1 presumed concrete density, 90 kg/m3 (150 lb/ft3) gravity acceleration, constant 9.810−3 kN·m/N·s2 (1.0 lbf/lb) embedment depth, m (ft) collar thickness, m (ft) collar width, m (ft) soil friction angle, deg, see Table 1 post cross sectional area, m2 (ft2)



 

h  thickness of attached concrete, m (ft) (collar plus footing where footing is attached) 5.3.2.2 Wood collars. Rectangular pressure treated wood beams which are fastened to the post may displace a rounded corner truncated prismatic wedge of soil radiating above the wood surface as illustrated in Fig. 13. The mass of the soil which can be lifted is usually limited to the strength of the beam-to-post connection. Calculate the potential mass of the truncated prismatic volume with the following: U   G  wl  A p  d  t    w  l  d  t  2 tan  t 

3

tan2  

  0.33  d 

Figure 12 – Concrete uplift resistance

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1) ground surface support • embedded post only • bottom collar and its sliding resistance • collar and footing combined and their sliding resistance 2) elevated floor support • embedded post only • bottom collar and its sliding resistance • collar and footing combined and their sliding resistance The non-constrained lateral foundation increases in embedment strength as follows: • embedded post only • top collared post • bottom collared post with sliding resistance support • bottom collar, footing and sliding resistance support. 6.2 Soil pressures. Soil embedded posts are exposed to the elements; therefore, presumed basic soil pressures are conservative. The broad presumed soil classes limit specific soil selection and thus lower allowable pressure values. Soils are not considered ideal in density or moisture content. Soil tests may increase allowable soil pressure because they are more site specific. The presumed allowable pressures for two levels of soil condition are presented, and factors for stress increases are cited. The soil pressure increase factor for the constrained case isolated post results from the performance of soil behaving similar to the deep footing case. The expanded volume of soil which supports the post has been found to increase ultimate resistance 2.2 to 3.4 times that for a wall. Usually in the non-constrained design case, surface deflection of 13 mm (0.5 in.) does not present a restriction on post-frame building design. A soil pressure increase is allowable where this deflection is not a problem. 6.3 Non-constrained case. A basic assumption is that the soil bearing varies parabolically. The potential soil pressure values in Table 1 increased hydrostatically with depth. The equations of this Engineering Practice, used for the calculation of the soil resistance resultants, have included the proper depth value to convert the hydrostatic value to the design soil pressure value. In the non-constrained case, soil pressure is evaluated at the mean depth of the parabolic section above the rotation axis and at the post depth for the section below the rotation axis. The force volume of both these parabolic soil bearing resistances is equal to the uniformly applied soil pressure resistance used in AWPI Standard, Pole Building Design. In the case of collar segments, the resultant is the integral of the parabolic post function summed to the collar depth. The centroidal depth of this collar segment is used in determining the lever arm for the collar resistance moment.

The rotation axis for the post in the non-constrained case is sensitive to the shear force requirements of the design. Collars change the position of the rotation axis as they change the total bearing area. Sliding resistance has been neglected in determining the post-only case. 6.4 Constrained case.  In the constrained case, embedded collar resistance is calculated using the midpoint collar depth soil pressure acting on the collar over the entire equivalent rectangular area. Test data are referenced that illustrate collar performance. The rotation axis for the constrained post is at the surface support. 6.5 Vertical uplift.   Bottom collars attached to the post provide resistance to withdrawal of the post from the hole. A wedge of soil shearing along the soil friction angle is added to the mass.

7 Glossary 7.1 Constrained a  b 



d  k cc 



k cs 



M f 



M r  M s 



P 



Q 1 Q 2 Q 3



Q 4 Q 5



R  R s  S 



t  t c  t f  V f 



V s 



w 









 



 

  

 

W d 



distance between ground and floor line, m (ft) effective width of the post in the soil, m (ft), see paragraph 4.4 post embedment depth, m (ft) sliding resistance coefficient, concrete on concrete, see paragraph 4.8 sliding resistance coefficient, concrete on soil, see paragraph 4.8 moment at floor line, from engineering analysis, kN·m (lbf·ft) moment resistance of the soil, kN·m (lbf·ft) moment at rigid surface from engineering analysis, kN·m (lbf·ft) post downward vertical force from engineering analysis, kN (lbf), see paragraph 4.7 resultant of soil forces on post, kN (lbf) resultant of soil forces on the collar, kN (lbf) resultant of lateral sliding resistance of the collar, kN (lbf) resultant of soil forces on footing, kN (lbf) resultant of lateral sliding resistance of the footing on soil, kN (lbf) reaction at support, kN (lbf) lateral soil resistance, kN (lbf) allowable lateral soil pressure per unit of depth including increases, kPa/m (lbf/ft2·ft), see paragraph 4.5 slab or beam bearing depth, m (ft) thickness of collar, m (ft) thickness of footing, m (ft) shear at floor line, from engineering analysis, kN (lbf) shear at rigid surface, from engineering analysis, kN (lbf) width of collar, m (ft) width of footing and collar, m (ft) slab or beam bearing width, m (ft) dead weight of concrete slab, kN (lbf)

7.2 Non-constrained b 



c  c 





Figure 13 – Wood uplift resistance



bearing width of the post in the soil, m (ft), see paragraph 4.4 ratio of collar depth to depth of the rotation axis, 1 ratio of the collar section dimension above the rotation axis to the depth of the rotation axis

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d  k cc 



k cs 



m  M g 



M r  n 



P 



Q 1



Q 2



Q 3 Q 4



Q 5



Q 6



Q 8



S 



t c  t f  t u  V g  w  ¯  y  ¯  z 













     

post embedment depth, m (ft) h   thickness of attached concrete, m (ft) (collar plus footing sliding resistance coefficient, concrete on concrete footing, where footing is attached) see paragraph 4.8 l  length of collar, m (ft)  sliding resistance coefficient, concrete on soil, P   vertical foundation load, kN (lbf) see paragraph 4.8 S v   allowable vertical soil pressure, including increases, location of the mean of the collar force, Q 3 , m (ft) kPa (lbf/ft2), see paragraph 5.2.4 moment at ground surface from engineering analysis, t   collar thickness, m (ft) kN·m (lbf·ft)   soil friction angle, deg, see Table 1  soil resistance moment, kN·m (lbf·ft) U   soil and foundation uplift resistance, kN (lbf) location of the mean of the upper section of the collar w   collar width, m (ft) force, m (ft) downward post vertical force from engineering analysis, Cited Standards: kN (lbf), see paragraph 4.7 resultant of soil forces on post above the rotation axis, ACI 318, Building Code Requirements for Reinforced Concrete ACI 318.1, Building Code Requirements for Structural Plain Concrete kN (lbf) resultant of soil forces on post below the rotation axis, ASAE EP388, Design Properties of Round, Sawn and Laminated Preservatively Treated Construction Poles and Posts kN (lbf) resultant of the soil forces on upper collar, kN (lbf) AWPI, Pole Building Design, D. Patterson, 1969 resultant of lateral sliding resistance of collar on concrete, BOCA, National Building Code kN (lbf) ICBO, Uniform Building Code sliding resistance of footing on soil, kN (lbf) NFPA, National Design Specification for Wood Construction resultant of soil forces on collar above the rotation axis, kN (lbf) References resultant of soil forces on the collar and post below rotation axis, kN (lbf) 1. Carson, J. M. and Curtis, J. O. 1981. Improving the resistance of poles to resultant of soil forces on collar and footing below lateral loads. TRANSACTIONS of the ASAE 24(2):418–420. rotation axis, kN (lbf) 2. Davisson, M. T. and Prakesh, S. 1963. A review of soil-pole behavior. Highway Research Record #39, pp. 25–48. allowable lateral bearing soil pressure, per unit of depth 2 3. Ferguson, D. E. and Curtis, J. O. 1978. Strength alternative systems for including increases, kPa/m (lbf/ft ·ft), see paragraph 4.5 setting poles. TRANSACTIONS of the ASAE 21(5): 953–956 and 962. thickness of lower collar, m (ft) 4. Mahoney, G. W., Nelson, G. L. and Fryrear, J. I. 1966. Performance of pole thickness of footing, m (ft) anchorage under gravity and withdrawal loads. TRANSACTIONS of the thickness of collar, m (ft) ASAE, 9(2): 222–224. shear at ground surface, from engineering analysis, m (ft) 5. Nelson, G. L., Mahoney, G. W. and Fryrear, J. I. 1958. Stability of poles under tilting moments. AGRICULTURAL ENGINEERING 39(3):166–170. collar width, m (ft) 6. Riskowski, G. L. and Friday, W. H. 1991. Design equations for collaring post location of the rotation axis, kN (lbf) foundations. TRANSACTIONS of the ASAE (In review). location of centroid of Q 2 , m (ft) 7. Riskowski, G. L. and Friday, W. H. 1991. Design equation comparisons with

7.3 Vertical foundation design  



A A p  C  d  G 

    

oil density, kg/m3 (lb/ft3), see Table 1 required footing area, m2 (ft2) post cross sectional area, m2 (ft2) presumed concrete density, 90 kg/m3 (150 lb/ft3) embedment depth, m (ft) gravity acceleration, constant 9.810−3 kN·m/N·s2 (1.0 lbf/lb)

recent post foundation research. TRANSACTIONS of the ASAE (In review). 8. Walker, J. N. and Cox, E. H. 1966. Design of pier foundations for lateral loads. TRANSACTIONS of the ASAE 9(3):417–420 and 427. 9. Hough, B. K. 1969. Basic Soils Engineering, 2nd edition. Ronald Press Co. Table 7–2, p. 249. 10. Housel, W. S. 1943. Earth pressures on tunnels. TRANSACTIONS, ASCE 108:1037–1056. 11. Merritt, F. S. 1976. Standard Handbook for Civil Engineers, pp. 7–53. 12. Minikin, R. R. 1950. Winds, Waves and Maritime Structures. Charles Griffin and Co. Ltd. 13. Terzaghi, K. and Peck, R. B. 1948. Soil Mechanics in Engineering Practice. Wiley and Son.

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