Download - piled raft
Second Seminar on
Analysis and Design of Piled Raft Foundation
Submitted by
Contents
1. Overview
2. Literature Review
3. Numerical Studies
4. Work to be done
1. Overview
1. Literature Review
Noh et al (2008) carried out analysis of un-piled and piled raft
foundations. They observed
• The raft thickness affects diff. settlement and bending moments,
but has little effect on load sharing or maximum settlement.
• Piles spacing plays an important role on the performance of
piled raft foundation
• Differential settlement, the maximum bending moment and
the load sharing are not affected much by increasing the pile
lengths.
“ Finite element modeling for piled raft foundation in sand.” Proc., 11th East Asia-Pacific Conference on Structural Engineering and Construction (11EASEC-2008), Taipei,Taiwan,19-21.
Gowri (2011) analyzed raft foundation using FEM.
• load carrying capacity increased with the increase
in thickness up to 1.25m only
• Also the load carrying capacity increased up to 3m
x 3m size raft and later load carrying capacity
decreased with increase in raft size.
“Analysis of Mat foundation using finite element method”, J. Earth Science and Eng., 04, 113-115.
Shukla et al (2011)
• Parametric study of piled raft foundations
• Increase in raft thickness increases settlement
• Increase in pile diameter reduces the settlement
“Behavioral study of pled raft foundation in layered soil deposits.” J. Advanced Eng. Tech., 2(4), 191-195.
3. Numerical Studies
• The foundation is designed for a storage 5 story building. The bearing stress assumed around 100 kN/m2
.
• The raft is modeled in SAFE software. All analysis and design are based on the ACI code.
• “Conventional Rigid Method” is used for design.
Parameter Notation Value
Yield strength of steel Fy 400 MPa
Strength of concrete fc 30 MPa
Young modules of elasticity E E 2000000 kN/m2
Dear load factor D.L.F D.L.F 1.2
Live load factor L.L .F L.L .F 1.6
Soil Unit weight γ soil γ soil 15 kN/m3
Allowable Bearing stress qa qa 100 kN/m2
Concrete Unit weight γ concrete γ concrete 25 kN/m3
Design Parameters
Raft Modeling in SAFE
Columns loads on Raft
Load type Load case Load value (kN/m2)
Services Dead 2.5 kN/m2
Slab own weight assumed
Dead (25kN/m3)(0.2m) =5 kN/m2
Flooring Dead 1 kN/m2
Live loads Live 7 kN/m2
Punching Shear Factors
All Punching shear factors are less than 1
Soil Pressure Check
< 100 kN/m2
q1 = 95 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2
q2 = 75 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2
q3 = 71 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2
q4 = 92 /𝑘𝑁 𝑚2 < = 100 /𝑞𝑛𝑒𝑡 𝑘𝑁 𝑚2
In SAFE analysis stresses are less than 100kN/m2 at
all the points in raft .
SAFE Settlement Analysis
Maximum settlement for 800mm Raft width ∆ manual = 30 mm. ∆ SAFE = 12 mm
Raft thickness effect
Thickness(mm) Settlement (mm)200 30300 19400 16500 14600 13800 12
1000 111200 10.81400 10.6
020
040
060
080
010
0012
0014
0016
000
5
10
15
20
25
30
Thickness v/s Settlement
Settlement
Raft Thickness V/s Settlement
Conclusion
• Increase in raft thickness beyond a particular limit has little or no effect on settlement of foundation.• In such a case, alternative method such as introducing piles in raft could prove effective.
. Work to be done
• To study the effect of introduction of piles in a raft foundation settlement
• Parametric studies of piled raft foundations.
References
Gowri, S. (2011) “Analysis of Mat foundation using finite element method”, J. Earth Science and Eng., 04, 113-115.
IS 2950. (Part I) (1981). Code of practice for design and construction of raft foundations (second revision), BIS, New Delhi, India.
Noh, E. Y., Huang, M., Surarak, C., Adamec, R. and Balasurbamaniam, A. S. (2008). “ Finite element modeling for piled raft foundation in sand.” Proc., 11th East Asia-Pacific Conference on Structural Engineering and Construction (11EASEC-2008), Taipei,Taiwan,19-21.
Shukla, S. J., Desai, A. K. and Solanki, C. H. (2011). “Behavioral study of pled raft foundation in layered soil deposits.” J. Advanced Eng. Tech., 2(4), 191-195.
THANK YOU