Pole Foundation Design (IBC 1807.3)

Jul 16, 2024

3 min read

Updated: Oct 25

Introduction

Designing a pole foundation involves meticulous calculations to ensure stability and compliance with building codes such as the International Building Code (IBC) 1807.3. This blog post provides a comprehensive step-by-step example of an iterative design process to determine the required foundation depth. The process includes: 1) Determining Design Inputs and Demands, 2) Calculating Vertical Bearing Pressure, 3) Calculating Horizontal Bearing Pressure, and 4) Determining Required Depth of Foundation. By following these detailed steps, designers can achieve a robust and compliant pole foundation design, balancing structural integrity with practical construction considerations.

Pole foundation: lateral load V applied at height h above grade and axial load P on a post embedded to depth d in a footing of diameter b or width W.

Try CalcBook for Free

Factors to Consider when Designing Pole Foundations

When designing pole foundations, several factors control the required embedment depth. The first is whether the footing is laterally constrained at grade, for example by a rigid floor or pavement: IBC Equation 18-1 applies to nonconstrained footings and Equation 18-2 to constrained ones. IBC Section 1806.3.4 permits lateral bearing pressures of twice the tabulated values for isolated poles, such as flagpoles and signs, and for poles supporting buildings that are not adversely affected by 1/2 in. of motion at the ground surface under short-term lateral loads. IBC Section 1806.1 permits a one-third increase in the presumptive bearing values of Table 1806.2 when they are used with the alternative allowable stress design load combinations that include wind or earthquake loads. The size and shape of the footing also matter: the diameter of a round footing, or the diagonal of a square one, enters the depth equations directly.

Example Problem (Solved with CalcBook CalcBook): Problem Statement and Design Inputs:

Example problem: a bollard for vehicle impact per 2021 IBC 1607.10 and ASCE 7-16 4.5.3, 6,000 lb applied 1 ft 6 in. to 2 ft 3 in. above grade, 24 in. diameter footing, constrained by pavement at grade, 1/2 in. movement not acceptable, clay soil.

Try CalcBook for Free

Step 1: Determine Design Inputs & Demands

The 6 kip vehicle load is applied 2.25 ft above grade, the top of the 1'-6" to 2'-3" range, which gives the largest moment. The pavement restrains the footing at grade, so the footing is constrained. Because 1/2 in. of movement is not acceptable, the tabulated lateral bearing pressure is used without the IBC 1806.3.4 increase. The vehicle load is a live load, not wind or seismic, so the one-third increase of IBC 1806.1 does not apply either.

For clay (IBC Table 1806.2, class 5), the presumptive allowable vertical pressure is 1,500 psf and the lateral bearing pressure is 100 psf per foot of depth, which IBC 1806.3.3 caps at 15 times the tabular value, or 1,500 psf.

CalcBook Pole Foundation inputs: constrained at grade, 1/2 in. movement not OK, lateral load not wind or seismic, h = 2.25 ft, 2 ft round footing, depth 6.596 ft from the Solver, clay soil bearing pressures.

The vehicle load is entered as a 6 kip lateral live load, and the footing weight is the only dead load:

D = W_ftg = 3.11 kips (2 ft diameter, 6.6 ft deep, 150 pcf concrete)

ASD LC1 (D) governs the vertical bearing check, and LC2 (D + L) governs the depth.

CalcBook load combination table for the bollard: ASD LC2 (D + L) with V = 6 kips governs the depth, and the dead load is the 3.11 kip footing weight.

Step 2: Calculate Vertical Bearing Pressure

A_ftg = π(b/2)² = π(2 ft/2)² = 3.14 ft²

q = P / A_ftg = 3.11 kips / 3.14 ft² = 0.99 ksf ≤ 1.5 ksf, OK

CalcBook show-your-work for vertical bearing: footing area 3.14 ft² and bearing pressure q = 3.11 kips / 3.14 ft² = 0.99 ksf.

Step 3: Calculate Lateral Bearing Pressure

The tabulated lateral bearing pressure applies, q_lat = 0.1 ksf/ft. For a constrained footing, S₃ is the allowable lateral bearing pressure at the full embedment depth (IBC 1807.3.2.2):

S₃ = min(q_lat d, q_lat,max) = min(0.1 ksf/ft × 6.6 ft, 1.5 ksf) = 0.66 ksf

CalcBook show-your-work for lateral bearing: qlat = 0.1 ksf/ft without the 1/2 in. movement increase, and S3 = min(0.1 ksf/ft × 6.6 ft, 1.5 ksf) = 0.66 ksf at full depth.

Step 4: Determine Required Depth of Foundation

IBC Equation 18-2 gives the required depth of a constrained footing:

d_req = √(4.25 V h / (S₃ b)) = √(4.25 × 6 kips × 2.25 ft / (0.66 ksf × 2 ft)) = 6.59 ft

S₃ depends on the depth, so the equation is solved by trial and error. CalcBook's Solver button runs that iteration and returns the depth at which the required and provided depths match: d = 6.60 ft (D/C = 1.00). Round up to a practical embedment, such as 7 ft.

CalcBook show-your-work for IBC Equation 18-2: dreq = √(4.25 × 6 kips × 2.25 ft / (0.66 ksf × 2 ft)) = 6.59 ft.

CalcBook pole foundation design summary: vertical bearing 0.99 ksf vs 1.50 ksf, and required depth 6.59 ft vs 6.60 ft provided, all OK.

Conclusion:

Designing a pole foundation per IBC 1807.3 requires a detailed and methodical approach to ensure compliance and structural integrity. By following the outlined steps and considering key factors such as footing constraints, allowable movement, and lateral loads, engineers can achieve a robust foundation design. With careful calculation and adherence to code requirements, the foundation will provide the necessary support and stability for the pole.

Try CalcBook for Free

Related design examples: foundation sliding and eccentric spread footing design.