Code issues (Eurocode and other building codes)

Code issues (Eurocode and other building codes)

Code issues (Eurocode and other building codes) 28 28.1 Eurocode An overview of the Eurocode will be provided in this chapter. The following codes a...

Code issues (Eurocode and other building codes)

28

28.1 Eurocode An overview of the Eurocode will be provided in this chapter. The following codes are applicable for different types of piles: • • • • • •

EN 1993-5: Eurocode 3, part 5: design of steel structures: piling EN 1536:1999: bored piles EN 12063:1999: sheet pile walls EN 12699:2000: displacement piles EN 14199:2005: micropiles EN 12794:2005: precast concrete products. Foundation piles.

Section 7 (1997-1) discusses the following subjects: 7.1 General (3 paragraphs) 7.2 Limit states (1) 7.3 Actions and design situations (18) 7.4 Design methods and design considerations (8) 7.5 Pile load tests (20) 7.6 Axially loaded piles (89) 7.7 Transversely loaded piles (15) 7.8 Structural design of piles (5) 7.9 Supervision of construction (8)

28.1.1  Approaches to design Eurocode allows three approaches: Approach 1: design using results of static load tests. Approach 2: empirical or analytical calculation methods. But the code adds that methods have to be verified through pile load tests. Approach 3: the results of dynamic load tests. As in approach 2, the code states that the results have to be verified using static load tests.

28.2  Design using static load tests  (R ) (R )  Rck = min  cm mean ; cm min  ζ2   ζ1

(28.1)

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Table 28.1  Correlation

factors

n 1

1 1.4

2 1.3

3 1.2

4 1.1

>5 1.0

2

1.4

1.2

1.05

1.0

1.0

where Rck, characteristic pile resistance; Rcm, measured pile resistance; (Rcm)mean, mean value of pile load tests; (Rcm)min, minimum value of pile load test data; and 1 and 2, correlation factors. Correlation factors are obtained using Table 28.1 “n” is the number of load tests completed. Design pile resistance is calculated using characteristic pile resistance (Rck). Let us see how to compute the characteristic pile resistance. Step 1: write down all the measured pile resistances using static load tests. Step 2: find the mean value of load test data. This is (Rcm)mean. Step 3: find the minimum value of load test data. This is (Rcm)min. Step 4: find correlation factors using Table 28.1. Step 5: use Equation (28.1) to find Rck.

Design example Three static load tests were done and the following values were obtained: 2.4 MN, 2.7 MN, 3.2 MN Find the characteristic pile resistance. Solution Step 1: find the mean value of load test data. [Rcm]mean = (2.4 + 2.7 + 3.2)/3 = 2.77 MN Step 2: find the minimum value of load test data. [Rcm]min = 2.4 MN Step 3: find correlation factors. For three tests, 1 = 1.2 and 2 = 1.05. Step 4: use Equation (28.1) to find Rck.  (R ) (R )  Using Equation (28.1) R ck = min  cm mean ; cm min  , ζ2   ζ1 Rck = min (2.77/1.2; 2.4/1.05) Rck = min (2.31; 2.29) The minimum of the two preceding values is selected: Rck = 2.29.

Code issues (Eurocode and other building codes)

331

28.3  Compute characteristic axial compression load using ground tests Ground tests, such as CPT and SPT, can be used to find the characteristic pile resistance.  (R ) (R )  Rck = min  c.cal mean ; c.cal min  ζ ζ4   3

(28.2)

This equation is similar to Equation (28.1); where Rck, characteristic pile resistance; Rc.cal, calculated pile capacity; (Rc.cal)mean, mean value of calculated pile capacity; (Rc.cal)min, minimum value of calculated pile capacity; and 3 and 4 are obtained from Table 28.2. Let us look at an example. Design example Seven CPT cone tests were conducted in seven different holes. CPT test values were converted to calculated pile capacities for seven CPT tests. 2.4 MN, 2.7 MN, 3.2 MN, 2.8 MN, 2.1 MN, 3.0 MN, and 2.9M N Find the characteristic pile capacity. Solution Step 1: find the mean value of the calculated pile capacities. Mean value = (2.4 + 2.7 + 3.2 + 2.8 + 2.1 + 3.0 + 2.9)/7 = 2.73 MN (Rc.cal)mean = mean value of calculated pile capacity = 2.73 MN Step 2: find the minimum value of calculated capacities. [Rc.cal]min = minimum value of calculated pile capacity = 2.1 MN Step 3: obtain correlation factors from Table 28.2 for the seven tests. 3 = 1.27 4 = 1.12 Step 4: Apply Equation (28.2).  (R ) (R )  Using Equation (28.2) R ck = min  c.cal mean ; c.cal min  , ζ ζ4  3  Rck = min (2.73/1.27; 2.1/1.12) Rck = min (2.15; 1.875) The minimum of the two values is 1.875 MN. Hence, Rck = 1.875. Characteristic capacity is used to find the design capacity. The procedure is not provided in this book.

Table 28.2  Correlation n 3 4

1 1.4 1.4

2 1.35 1.27

factors for ground testing 3 1.33 1.23

4 1.31 1.20

5 1.29 1.15

7 1.27 1.12

10 1.25 1.08

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28.4  NYC building code 28.4.1  Pile capacity: dynamic equations Many building codes around the world depend on dynamic formulas for guidance. Table 28.3, based on the engineering news formula, is adopted by the NYC Building Code. Formula used P =

2 Wh H ; 2000 (s + 0.1)

Wh H = E = hammer energy

Table 28.3  Minimum

driving resistance and minimum hammer energy for steel H-piles, pipe piles, precast and cast-in-place concrete piles, and composite piles (other than timber) Piles bearing on cemented sand free of lenses of silt, clay, and Pile Hammer Friction soft rock capacity energy piles (hardpan, (tons) (ft. lb) (blows/ft.) blows/ft.) Up to 20 15,000 19,000 24,000 30 15,000 19,000 24,000 40 15,000 19,000 24,000 50 15,000 19,000 24,000 32,000 60 15,000 19,000 24,000 32,000 70 and 19,000 80 24,000 32,000

19 15 11 30 23 18 44 32 24 72 49 35 24 96 63 44 30

19 15 11 30 23 18 50 36 30 96 54 37 25

Nondisplacement piles bearing on decomposed rock (blows/ft.)

48 27 16 72 40 26 96 53 34 120 80 60 40 240 150 100 50 Five blows Five blows per 1/4 in.) per 1/4 in) Minimum Minimum hammer hammer energy energy of of 15,000 19,000 ft.lb ft.lb

Displacement piles bearing on decom­ posed rock Piles bear(blows/ft.) ing on rock 48 27 16 72 40 26 96 53 34 120 80 60 40 240 150 100 50

Five blows per 1/4 in. minimum hammer energy of 15,000 ft.lb

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Here P, allowable pile load in tons; Wh, weight of striking part of hammer in pounds; H, actual height of fall of striking part of hammer in feet; E, rated energy of the hammer; and s, penetration of pile per blow in inches = 12/blows per foot. Wp/Wh should not exceed 3 (Wp is the weight of the pile). For timber piles, allowable pile load is measured using the preceding driving formula. For timber piles in soft rock and hardpan (cemented sand free of silt, clay or soft rock lenses): • Driving resistance given by the formula should be maintained for last 6 in. • For timber piles in gravel, sand, silt and clay soils. • Driving resistance given by the formula should be maintained for last 12 in. •

Design example A pile is designed to carry 40 tons. The pile hammer selected has an energy rating of 24,000 ft. lb. Find the number of blows/ft. required to achieve a pile capacity of 40 tons. Solution: 24 blows/ft. When the pile reaches 24 blows/ft., the pile could be ended. The engineer needs to make sure that the pile is in the soil type that it is supposed to be in.

28.4.2  Driving criteria (International Building Code) The International Building Code (IBC) states that the allowable compressive load on any pile should not be more than 40 tons, if driving formulas were used to compute the allowable pile capacity. According to the IBC, if an engineer were to specify an allowable pile capacity of more than 40 tons, wave equation analysis and pile load tests should be conducted.