EAA 455/2 – Reinforced Concrete Structural

Design II

Pile designTutorial

Eng. Majed Aldahdooh

1) Check the Column load criteria b(separate DL and LL or Total).2) Get the ultimate load using appropriate factor.3) Outline pile cap dimension, assume suitable depth.4) Check punching around column perimeter and its limit.5) Select analytical approach for tension reinforcement design:

a) Truss Analogy .b) Bending theory.

6) Design for Tension ( moment reinforcement).7) If bending theory is used , so the method to get the reinforcement is

similar to beam design.8) Check shear requirement at critical section.9) Enhanced shear (if necessary).10)Check clause 3.11.4.4 if the pile spacing is more than 3 x pile size.11) If the pile spacing more than 3 x pile size, check punching at required

perimeter.12)Provide detailing following appropriate clauses in BS 8110.

Topic 3: Steps in pile cap design

Given:

a) Column load ( service)= 1900kNb) Assumed self weight of pile cap= 50kNc) Allowable pile working load = 640kNd) Pile size =350mm Class A spun pile.e) Pile spacing = 3x pile diameter.

f) fcu (pile cape) = 35N/mg) Pile embedded length =75mmh) clear edge distance = 150 mmi) Column size= 300mm x 300mmj) Main rebar = 20mm

Required:1) Design and provide full detailing of a pile cap.

Example 1: Using Truss Theory

Solution:1) Pile group size = = = 3.05 so use 3 PG arrangement2) Ultimate load on pile cap, N = x Factor = 1900 x 1.5 = 2850 kN3) Pile spacing, L = 3 x pile size = 350 x3 = 1050 mm

Note : To select a pile cap size for a start, use d ≈ 12 , where d is the pile cap effective depth, ( this note not from code but it from experience).

4) Try pile cap thickness = 700 mm`5) Effective depth, d = 700 – 75 – 20 -10 = 595 mm.6) Column perimeter = 300 x 4= 1200mm7) Punching around column perimeter = = = 3.99 N/m< 4.73 N/m

8) Tension force , T = =

9) Area of steel , As = = = 1279 m10) Provide = 5T20/band = 1570 m

11) Note : To Calculate the As (min) 1) As (min) = 0.15 x b x h, for high tensile bar where h is the pile cap thickness,

and b is the width of pile cap. (recommended by reynold’s).2) As (min) = 0.13 x b x h, as in T.j. MacGinley. C.G. of pile cap must coincide

with C.G. of column ( in almost all cases especially for 3PG).

12) Check As (min) = .13 x b x h/100 = 0.13 x 1700 x 700/100 =1547 m13) 100 x 1570/(1700 x 595) = 0.15514) Max shear , V max = (N/ number of piles) x 2 = (2850/3) x 2= 1900kN15) v = V max/(bd) = (1900 x 1000)/ (1700 x 595) = 1.878 N/m16) design shear stress , vc = (0.79 x () x ()) / 1.25 = 0.79 x x /1.25 = 0.380 N/m

vc v……………………OKEnhanced shear ( Pile spacing not more than 3 x pile diameter)

16) Column size = 300 x 300 mm square 17) av = 300 – 150 -175 + (350/5) = 45mm18) Enhanced shear = vc x 2 x d / av = 0.380 x 2 x 595/ 45 = 10.05 , but limit to 4.73 v ,

1.878 ….OK

NO further check on punching shear is required because the pile spacing is not more than 3x pile diameter .

Detailing :

Given:

a) Column load ( DL)= 1600kNb) Column load ( LL)= 850kNc) Assumed self weight of pile cap= 100kNd) Allowable pile working load = 640kNe) Pile size =300 x 300 squaref) Pile spacing = 3x pile diameter.

g) fcu (pile cape) = 35N/mh) Pile embedded length =75mmi) Clear edge distance = 150mmj) Column size= 300mm x 300mmk) Main rebar = 20mm

Required:1) Design and provide full detailing of a pile cap.

Example 2: Using Truss Theory

Solution:1) Pile group size = = = 3.98 so use 4 PG arrangement2) Ultimate load on pile cap, N = = 1600 x 1.4+(850 x1.6) = 3600kN3) Pile spacing, L = 3 x pile size = 300 x3 = 900 mm

Note : To select a pile cap size for a start, use d ≈ 12 , where d is the pile cap effective depth, ( this note not from code but it from experience).

4) Try pile cap thickness = 800 mm`5) Effective depth 1, d1 = 800 – 75 -10 = 715mm.6) Effective depth 2, d2 = 800 – 75 – 20 -10 = 695mm.7) Use the effective depth = 695mm (conservative). You can use d = (d1+d2)/2

(average).8) Column perimeter = 300 x 4= 1200mm9) Punching around column perimeter = = = 4.32N/m< 4.73 N/m …. ok

8) Tension force , T = =

9) Area of steel , As = = = 1334m = 5T20/band 10) Provide = 10T20/width = 1570 m

11) Check As (min) = .13 x b x h/100 = 0.13 x 1500x 800/100 =1560m

12) 100 x 3140/(1500x 695) = 0.301

13) Max shear , V max = (N/ number of piles) x 2 = (3600/4) x 2= 1800kN

14) v = V max/(bd) = (1800x 1000)/ (1500x 695) = 1.73N/m

15) design shear stress , vc = (0.79 x () x ()) / 1.25

= 0.79 x x /1.25 = 0.473N/m

vc v……………………OK

Enhanced shear16) Column size = 300 x 300 mm square

17) av = (dis. Of pile center –center)- Clear edge distance –Column width/2 + pile diameter/5=450 – 150 -150 + (300/5) = 210mm

18) Enhanced shear = vc x 2 x d / av = 0.473 x 2 x 695/ 210= 3.131 ….OK

NO further check on punching shear is required because the pile spacing is not more than 3x pile diameter .

Detailing :

Given:a) Column load ( DL)= 2800kNb) Column load ( LL)= 1200kNc) Allowable pile working load = 750kNd) Pile size =300 x 300 squaree) Pile spacing = 3x pile diameter.

f) fcu (pile cape) = 35N/mg) Pile embedded length =75mmh) Clear edge distance = 150mmi) Column size= 400 mm x 600mm

Required:1) Design and provide full detailing of a pile cap.

Example 3: Bending Theory

Solution:1) Pile cap s/w (assuming 6 PG) = volume cap x = 2.4 x 1.5 x1 x 24 =86 kN

2) Pile group size = = = 5.448

so use 6 PG arrangement

2) Ultimate load on pile cap, N = = 2800 x 1.4+(1200x1.6) = 5840kN

3) Ultimate force per pile, F ult = 5840/6 = 974 kN

4) Pile spacing, L = 3 x pile size = 300 x3 = 900 mm

Note : To select a pile cap size for a start, use d ≈ 12 , where d is the pile cap effective depth, ( this note not from code but it from experience).

4) Try pile cap thickness = 1000 mm

5) Assume bar size = 25mm

6) Effective depth 1, d1, dx = 1000 – 75 – (25/2) = 912mm.

7) Effective depth 2, d2, dy = 1000 – 75 – 25 –(25/2) = 887mm.

8) Use the effective depth = 887mm (conservative). You can use d = (d1+d2)/2 (average).

9) Column perimeter = (400+600) x 2= 2000mm

10) Punching around column perimeter = =

= 3.29N/m< 4.73 N/m …. ok

Moment Design

11)Mxx = Number of pile x Fult x distance ( center of pile to center

of column) = 2 x 974 x 0.9 = 1753 kNm

12)Myy = 3x 974 x 0.45 = 1315 kNm

First : ( X- direction )

13)K = Mxx/(fcu x b x ) = 1753 x /(35 x 1500 x )= 0.04

14)Z = .95dx = 866 mm

15)As =Mxx/(0.95 x fy x z)= 1753 /(0.95 x 460 x 866)= 4632 m

16)Provide =10T25 = 4910 m

17)Assume thickness of cape = 1000mm

18)As min = 0.13 x b x h/100 = 0.13 x 1500 x 1000/100 = 1950 m

19) 100 x 4910/(1500x 912) = 0.3589

Second : ( y- direction )

20) Myy = 1315kNm

21)K = Myy/(fcu x b x ) = 1315 x /(35 x 2400 x )= 0.019

22)Z = .95dy = 842 mm

23)As =Myy/(0.95 x fy x z)= 1315 /(0.95 x 460 x842)= 3574 m

24)Provide =12 T20 = 3768 m

25)Assume thickness of cape = 1000mm

26)As min = 0.13 x bx h = 0.13 x 2400x 1000/100 = 3120 m

27) 100 x 3768/(2400 x 887) = 0.177

Shear Design

First : ( X- direction )

28) V xx, max = Number of pile x Fult= 2 x 974= 1948 kN

29) v xx = Vxx,max/(bdx) = 1948X1000/(1500 x 912) =1.453 N/ m

30) design shear stress , vc xx = (0.79 x () x ()) / 1.25

= 0.79 x x /1.25 = 0.502N/m

vc v……………………OK

31) Column size = 400 x 600 mm square

32) av = (d-pile center –center)- Clear edge distance –Column width/2 + pile

diameter/5=900 – (600/2) -150 + (300/5) = 510mm

16) Enhanced shear = vc x 2 x d / av = 0.502 x 2 x 912 /510= 11 ….OK

Second : ( y- direction )

28)Vyy, max = Number of pile x Fult = 3 x 974= 2922kN

29) v yy= Vyy,max/(bdy) = 2922X1000/(2400x 887) =1.37 N/ m

30) design shear stress , vcyy= (0.79 x () x ()) / 1.25

= 0.79 x x /1.25 = 0.397N/m

vc v……………………OK

33) Column size = 400 x 600 mm square

34) av = (dis. of pile center –center)- Clear edge distance –Column width/2 + pile

diameter/5= 450 – (400/2) -150 + (300/5) = 160mm

35) Enhanced shear = vc x 2 x d / av = 0.397x 2 x 887/160= 4.4011 ….OK

Full anchorage = 38 x bar diameter= 38 x 25 = 950 mmProjection of bar beyond centerline of pile =150+150-50 ( assume cover) = 250mmBent up = 1000-50-75=875mmTotal anchorage =875+250=1125 > 950mmDesign is satisfactory.

Detailing :