Why thrust blocks are needed
Water pressure in an underground fire main produces unbalanced thrust at bends, tees, dead ends and reducers. Except for welded and approved restrained joints, gasketed push-on and mechanical joints rely on the surrounding soil to stay in place. NFPA 13-2025 section 6.7 (extracted from NFPA 24) does not mandate a restraint method; the choice rests with the designer or owner. There are two routes: concrete thrust blocks (6.7.1) and restrained joint systems (6.7.2).
Thrust blocks are permitted where the soil is stable and able to resist the anticipated thrust (6.7.1.1). The block sits between undisturbed earth and the fitting and must resist the calculated thrust (6.7.1.3); the concrete mix must be no leaner than 1 part cement, 2½ parts sand and 5 parts stone (6.7.1.2).
Thrust force
Per Annex A.6.7.1, the resultant thrust at a bend of angle θ is T = 2 · P · A · sin(θ / 2), and at a dead end T = P · A, where P is the water pressure and A the cross-sectional area based on the outside diameter of the pipe. Table A.6.7.1(a) lists the thrust at 100 psi (6.9 bar) for ductile-iron and PVC pipe; at another pressure, multiply by the pressure ratio. A 12 in (300 mm) 90° bend, for example, carries 19,353 lbf at 100 psi and 24,191 lbf at 125 psi.
| Nominal size | Dead end | 90° bend | 45° bend |
|---|---|---|---|
| 4 in · DN100 | 1,810 lbf | 2,559 lbf | 1,385 lbf |
| 6 in · DN150 | 3,739 lbf | 5,288 lbf | 2,862 lbf |
| 8 in · DN200 | 6,433 lbf | 9,097 lbf | 4,923 lbf |
| 10 in · DN250 | 9,677 lbf | 13,685 lbf | 7,406 lbf |
| 12 in · DN300 | 13,685 lbf | 19,353 lbf | 10,474 lbf |
Bearing block area and size
A bearing block passes the thrust to the soil through a larger area; the design finds the face area at which the pressure on the soil stays within its horizontal bearing strength:
A_b = h · b = T · S_f / S_b and, for a horizontal bend, b = 2 · S_f · P · A · sin(θ / 2) / (h · S_b)
S_f is the safety factor (usually 1.5 for thrust blocks), S_b the horizontal bearing strength of the soil, h the block height and b the block width. Annex A gives these sizing rules:
- The bearing face should be placed against undisturbed soil; where it cannot, compact the fill between them to at least 90 percent Standard Proctor density.
- Block height h should not exceed half the total depth to the bottom of the block (H_t) and should not be less than the pipe diameter.
- Choose h so that the calculated width b lies between one and two times the height.
The same approach applies to blocks at tees and dead ends. At a tee the run thrusts balance; the P·A at the branch remains and pushes the block away from the branch.
Soil bearing strength
| Soil | S_b, lb/ft² | S_b, kN/m² |
|---|---|---|
| Muck | 0 | 0 |
| Soft clay | 1000 | 48 |
| Silt | 1500 | 72 |
| Sandy silt | 3000 | 145 |
| Sand | 4000 | 190 |
| Sandy clay | 6000 | 285 |
| Hard clay | 9000 | 430 |
These Table A.6.7.1(c) values are considered conservative, but their accuracy depends entirely on correct soil identification, and the design engineer is responsible for the value chosen. A designer may instead use a Rankine passive pressure calculated from actual soil properties (the “value from soil report” option).
Which pressure to use
The note to Table A.6.7.1(b) says to size the block area with the hydrostatic test pressure. Underground piping is tested at 200 psi (13.8 bar) or 50 psi (3.4 bar) above the system working pressure, whichever is greater (6.11.2.2.1). Thrust blocks should be sufficiently hardened before hydrostatic testing begins (A.6.11.2.2.4).
Gravity block at a vertical down bend
At a vertical down bend the vertical thrust component T_y = P · A · sin θ acts upwards and is balanced by the weight of the block: V_g = S_f · P · A · sin θ / W_m, where W_m is the unit weight of the block material. The horizontal component T_x = P · A · (1 − cos θ) is resisted by the side of the block bearing on the soil and is sized like a bearing block.
Worked example
The example under Table A.6.7.1(b): a 6 in (150 mm) 45° bend, 150 psi (10.3 bar) internal pressure, soil of 3000 lb/ft² (145 kN/m²) and a safety factor of 1.5. The table’s base value for a 90° bend at 100 psi is 7.9 ft²; with the 45° coefficient 0.541, the pressure ratio 150/100 and the soil ratio 1000/3000 the result is 2.1 ft². The calculator gives 2.15 ft² (T = 4,293 lbf); at a height of 1.2 ft the block is 1.79 ft wide.
Do HDPE and welded mains need blocks?
Under NFPA 13 6.7.3, a private fire service main built with threaded, grooved, welded, heat-fused, or chemical or solvent-cemented connections needs no additional restraint, provided the joints pass the hydrostatic test of 6.11.2.2 without the piping shifting. HDPE butt fusion and electrofusion are heat-fused connections, so a fully fused HDPE main does not need thrust blocks at its bends and tees.
Frequently Asked Questions
What is a thrust block?
A concrete block that passes the thrust produced by water pressure at bends, tees and dead ends of an underground fire main into undisturbed soil. NFPA 13-2025 6.7.1 permits thrust blocks where the soil is stable and able to resist the thrust; the concrete must be no leaner than 1 part cement, 2½ parts sand and 5 parts stone.
Which pressure is a thrust block sized for?
The hydrostatic test pressure (note to Table A.6.7.1(b)). Underground piping is tested at 200 psi or 50 psi above the working pressure, whichever is greater (6.11.2.2.1). At a working pressure of 150 psi the block is sized for 200 psi; at 175 psi, for 225 psi.
Where does the soil bearing strength come from?
NFPA 13 Table A.6.7.1(c) gives conservative values: soft clay 1000, silt 1500, sandy silt 3000, sand 4000, sandy clay 6000 and hard clay 9000 lb/ft²; muck is zero. Their accuracy depends on correct soil identification; with a soil report, a calculated passive pressure may be used instead.
Does a butt-fused HDPE main need thrust blocks?
No, if the main is heat-fused throughout (butt fusion or electrofusion): NFPA 13 6.7.3 requires no additional restraint, provided the joints pass the 6.11.2.2 hydrostatic test without the piping shifting. Where the line meets a gasketed or mechanical-joint valve, hydrant or gasketed PVC main, a thrust block or a 6.7.2 restraint method is needed.
MEP Calc — 110+ Engineering Calculators
MEP Calc bundles 110+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
Download MEP Calc on the App Store
SprinkCalc — Fire Sprinkler Design in Three Standards
Run these calculations on site, offline and with PDF reports: NFPA 13, FM Global and BS EN 12845 in one iOS app.
Get SprinkCalc on the App Store