assessment of 3 pilecap

512 International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp 512-515

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Assessment of Current Design Practices of 3-Pile Cap: An Analytical Approach

Abhilash Thakur Undergraduate student, Department of Civil Engineering, National Institute of Technology, Durgapur-713209 Email: [email protected] Amiya K. Samanta Faculty Member, Department of Civil Engineering, National Institute of Technology, Durgapur-713209 Email: [email protected] ABSTRACT: Pile Caps are concrete structural element in the category of rigid foundation, which are used to transfer the load from column to pile/pile groups resting on subsoil. Current design practices in regard to the design of pile cap depend upon the provisions of code of practice governing the design and construction of reinforced concrete (RC) structures in the respective countries throughout the world. The deviation in approach towards the design of RC structural members is varying widely. This investigation presents a comparative study in design of pile cap supported on 3-pile group subjected to both axial load as well as moment in both directions. The stresses in pile cap have been computed with the help of a 3-D elastic analysis of the same using finite element software Abaqus. Although pile cap consists of both concrete as well as reinforcement, the FE analysis has been carried out considering only the concrete as homogenous medium and the effect of reinforcement has been neglected. Eight noded 3-D hexahedral elements with incompatible modes have been considered to model the medium/concrete. The load deformation pattern and stress contours have been presented to understand the behaviour of the same. Here the said pile cap is designed as per the provisions of code of practice laid down in IS 456:2000, BS 8110 and ACI 318:2008 separately. This also includes the design of piles when only loads and soil conditions are provided and subsequently the pile cap. Also another design of pile cap is made as per three codes when the dimension of the pile cap is fixed along with the imposed load. It has been found that American code practice (ACI) is most conservative of the three procedures and the Indian style being the most economical one for a given combination of soil condition and load. Whereas American style is most economical when load and dimension of pile cap is provided. British Style in this case being conservative compared to the other two. KEYWORDS: Reinforced Concrete, Rigid pile cap, Structural design, Current design practices, Abaqus INTRODUCTION Pile caps are concrete structural elements that come under the category of rigid foundations. They are used to transmit loads from column to pile or a group of piles. Load transfer occurs by bending and shear of pile cap. Currently the design of pile caps is governed by codes of practice for reinforced concrete structures of respective countries. Also the diameter of the pile plays an important role in choosing the planar dimensions of the pile cap, which in turn guides the depth of pile cap. Here 3-pile caps have been designed as per three codes of practice- IS 456:2000 (Indian Standard), BS 8110 (British Standard), ACI 318:2008 (American Standard). The designing has been done for two cases: Case I – For a given soil condition and axial compressive load of 170 ton, moments of 5.1 ton-m and 3.4 ton-m about orthogonal transverse axis is applied. Soil conditions: Backfill from GL to a depth of 1.5m. Soil below backfill to a depth of 7m is soft brownish clay. The cohesion of soil is 3.5 ton/m2. The layer below is of fairer clay and extends upto a depth of 9 m below ground level. SPT value of the layer is 10 and cohesion of 5 ton/m2. Next layer is of medium dense silty sand which extends 6m below the previous layer. SPT value of this layer is 25 and angle of internal friction Φ is 330. Next layer is that of medium dense silty sand which extends from 15m below ground level and beyond. SPT value of this layer is 40 and angle of internal friction Φ is 330.

As the soil layer at a depth of 15m below ground level is suitable getting requisite end bearing, hence the length of pile is chosen as 16m. In this case the ultimate bearing capacity of soil is found using the empirical formulae in IS 2911:1979 (Indian Standard), BS 8004:1986 (British standard) and ACI543R:2000 (American Standard). Hence the net bearing capacity of the soil is found. Thereafter accordingly the pile diameter is chosen and the planar dimension of the pile cap is accordingly fixed. Then the depth of the pile cap is found out. Case II – For a fixed dimension of the pile cap and a given load as in Case-I Case II – A finite element model has been developed in commercial software Abaqus to check the maxumum value of stresses to check the economy of the foundation designed using the three codes. The dimension of the pile cap is the same as shown in Figure-1 with an effective and overall depth of 615mm and 700mm respectively. The pile cap is designed using M20 grade concrete and Fe 415 grade steel. The pile cap supports a column of 300mm x 400mm and is supported on 3 piles. For simplicity the column is considered to be resting directly on the pile cap i.e. pedestal has not been considered. During designing, the maximum bending moment at the face of the column has been found out. The designing has been as per the moment found at face of the column.

513 Assessment of Current Design Practices of 3-Pile Cap: An Analytical Approach

International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp 512-515

Figure 1: View of a typical 3-pile cap

ANALYSIS & DESIGN Case I : As per IS standard, the minimum depth is evaluated for balanced section to carry the applied load. A clear cover of 75mm is provided and considering main bars of 20mm diameter, the effective cover becomes 85mm. Then the overall depth is increased to nearest multiple of 50mm. The design is done for this overall depth. Also the design has been done for every 50mm increase in overall depth to a depth of 1000mm. The main reinforcement in either of transverse direction is found out. A check for shear stress has also been done. To the close vicinity of the supports, the enhanced shear stress capacity of concrete is found out. Corresponding shear carrying capacity of the pile cap is found out. Next the pile cap is checked for one way shear (beam shear) at critical section (at a distance equal to effective depth from the support). Also it is checked for two way shear (punching shear) at the circumference equal to half of effective depth from the face of the column for punching

under column and at the circumference equal to half of effective depth from the face of the pile for punching over pile. The total reinforcement and concrete requirement is then tabulated against varying depths of the pile cap. It is noteworthy to mention here that whereas in case of Indian Standard designing of pile cap is restricted to flexural design, the design of same pile could be done as per flexure (beam design) or as per truss analogy (strut & tie model) in case of British and American Standards. The choice of the model depends upon the depth the pile cap with respect the spacing of the piles. Generally for effective depth of pile equal to or greater than half of the spacing between piles, Truss analogy method (Strut & Tie model) is adapted, else design for flexure is done. For designing as per BS standard, for the maximum bending moment assuming the balanced section the minimum depth is found out. Then as in previous case considering effective depth of 85mm and rounding the depth found to nearest higher multiple of 50mm the reinforcement requirements are found out. Similarly for consecutive 50mm rise in depth of pile cap the reinforcement requirement in both directions are found out upto an overall depth of 1000mm. The shear stress capacity of the concrete is found. A check for the enhanced shear capacity of concrete is performed along with one way shear and two way shear. It may be mentioned here that the minimum depth of pile cap is found to be 538.13mm whereas the spacing between the piles is 2250mm. Hence the designing has been done as if the pile cap is a flexural member. For designing as per ACI standard at first the bending moment at the face the column is found. Then the minimum pile depth is found out for wide beam action of pile cap and two way shear depending upon the number of piles falling within the tributary area. Then the adequacy of the pile cap is checked for punching shear at the circumference equal to half of effective depth from the face of the column for punching under column and at the circumference equal to half of effective depth from the face of the pile for punching over pile. Then the reinforcement to carry bending moment is found out in both directions along with the shear reinforcements. It may be noted that the spacing of piles is 3600mm whereas the minimum depth required to satisfy the shear criteria is 688.5mm. Hence Strut & Tie model could not be applied, so flexure design has been done. Case II: In this case the dimension of the pile cap has been fixed as shown in Figure-I. The effective depth of the pile cap is 615mm with an effective cover of 85mm and the overall depth becomes 700mm. Since the plan dimensions are fixed, design procedure for the said pilecap has been carried out as described in Case-I. In this case, while designing as per ACI standard it was found that the section of the pile cap was not adequate for wide beam action. However the same was adequate for the rest of the checks.

514 Abhilash Thakur, Amiya K. Samanta


RESULTS & DISCUSSION Case I The reinforcement requirement decreases with increase in depth of the pile cap for design as per Indian and British standards whereas for design as per American standard the percentage requirement of reinforcement remains almost constant. The minimum depth of pile cap required increases from BS code (538mm) to ACI code (688.5mm) with IS code coming in between (551mm). Looking at the comparison table below it can be inference that ACI method is the most conservative method of designing whereas IS method is the most economical way.

Figure 2

Figure 3

Comparison of Estimates for Pile Cap of Minimum Depth is shown below:

Properties IS BS ACI Overall Depth (in mm) 700 650 700

Gross Volume (m3) 1.235 3.596 12.187

Vol of Concrete (m3) 1.222 3.556 12.162

Total Reinf. (kg) 98.98 314.71 194.27

Case II In this case although the ACI design could not satisfy the wide beam action criteria, but the rest of the criteria were satisfied. It has been found out that the ACI method is the most economical method of design whereas BS method is a bit on the conservative side. Although IS and BS

methods do not vary much in results, the variation of ACI with respect to the other two is significant which is clearly visible in the comparison chart being followed.

Figure 4

Figure 5

Comparison of Estimates for Pile Cap of Fixed Dimension

Properties IS BS ACI

Overall Depth (in mm) 700 700 700

Gross Volume (m3) 1.235 1.235 1.235

Vol. of Concrete (m3) 1.222 1.220 1.229

Total Reinf. (kg) 98.98 114.36 45.71 Case III A finite element model of the pile cap with dimensions as in case II has been generated. The modelling has been done as if it was a PCC structure neglecting the effect of reinforcement. Concrete of characteristic Strength 20MPa has been used in modelling. Loads have been applied as uniform pressure over the area of the column on the surface of Pile Cap. The position of pile on the bottom face is made as the boundary. Eight noded 3-D hexahedral elements with incompatible modes have been considered to model the concrete.

515 Assessment of Current Design Practices of 3-Pile Cap: An Analytical Approach


It was found that the maximum stress arising is 10.6 MPa in vertical direction in the element just beside one of the edges of the column where the stress is maximum due to the effect of moments. The stress is compressive in nature. From the above discussion it can be said that whereas in case of Indian standard pile cap is designed by bending theory only, for the other two it may be designed by Truss analogy also. For Case I the depth of pile cap as obtained from Indian & British codes are close by (551mm & 538mm respectively) whereas that obtained from ACI code is on the higher side (688.5mm). The reinforcement requirement (in percentage) increases with decrease in depth in case of BS & IS Codes, whereas it is almost constant while designing with ACI code. In case of IS and BS codes, bending moment guides the depth requirement whereas in case of ACI code it is the shear which guides the depth requirement. American code practice (ACI) is most conservative of the three procedures and the Indian style being the most economical one for a given combination of soil condition and load. This is due to lower bearing capacity of soil as calculated from the empirical formula from NAVFAC DM 7.02 :Naval Facilities Maintenance Code for Foundation and Earth structures: 1986. For Pile Cap of same dimension, ACI is the most economical compared to the other two. BS method gives about 20% higher value of reinforcement required that for IS method. Finite element Analysis shows that the maximum stress that could generate under the given loading would be 10.6 MPa. Hence for a pile cap made of M20 grade concrete, factor of safety is 2, which will go up if the effect of reinforcement is considered.

CONCLUSION From the above discussion it can be said that for design according to codal provisions the soil bearing capacity and the pile capacity should be found out by Indian method and then the designing could be done as per American standard. The result of finite element analysis also supports the findings. As the maximum stress is 10.6MPa and the nature being compressive, hence the pile cap is well safe under the given load.

REFERENCES

[1] IS 456-2000 : “ Plain and Reinforced Concrete - Code of Practice”, Bureau of Indian Standards, New Delhi.

[2] IS 2911-1979 Part 1 / Sec 1: “Code of practice for design and construction of pile foundations”: Part 1 “Concrete piles”, Section 1 “Driven cast in-situ concrete piles”, Bureau of Indian Standards.

[3] IS 2911-1979 Part 1 / Sec 2: “Code of practice for design and construction of pile foundations”, Part 1 “Concrete piles”, Section 2 “Bored cast-in-situ piles”, Bureau of Indian Standards.

[4] BS 8004-1986 : ‘Code of practice for Foundations’, British Standards Institution, U.K.

[5] BS 8110-1997, Part I: “Structural use of concrete. Code of practice for design and construction”, British Standards Institution:1997

[6] BS 8110-1985, Part II : “Code of practice for special circumstances”, British Standards Instution: 1985

[7] ACI 318 “Building Code Requirements for Structural Concrete”, American Concrete Institute, 2008.

[8] ACI 543R-“ Design, Manufacture, and Installation of Concrete Piles”, American Concrete Institute:2000

[9] NAVFAC- “Naval Facilities Maintenance Code”, DM 7.02 – “Design Manual for Foundation and Earth Structures”, Naval Facilities Engineering Command: 1986

[10] Terzaghi K., Peck R. B. and Mesri G., Soil Mech. Engineering Practice, 3rd Ed. Wiley-Interscience (1996)

[11] Prakash, S. and Sharma, H.D. “Pile Foundations in Engineering Practice”. John Wiley and Sons, NY, 1990.

[12] V.N.S. Murthy: ‘Geotechnical Engineering: Principles and Practices of Soil Mechanics.’ CRC Press.

assessment of 3 pilecap

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