Design of Single Steel Pile under Steel Column Foundation System

Transcription

1 Design of Single Steel Pile under Steel Column Foundation System Mandeep Singh Kohli * and Naim Hossain ** * Structural Engineer, Fluor Daniel India Pvt. Ltd, Gurgaon, Haryana, India mandeep.singh.kohli@fluor.com ** Lead Structural Engineer, Fluor Daniel India Pvt. Ltd, Gurgaon, Haryana, India naim.hossain@fluor.com Abstract: Steel has now overtaken concrete in most refinery and industrial plants as a preferred material for construction of superstructures and concrete is only being used in case of foundations. However in deep foundations with increasing use of steel piles the pile cap remains the only component where concrete is still used. This paper presents the concept and design of structural steel column directly rested on single pile eliminating concrete foundations (Pedestal, Anchor Bolt, Pile Cap) completely. This alternative proves to be very cost effective for North American regions, (e.g. Canada) where field labor costs are very high, cold weather concreting is expensive and complex, projects have a limited construction schedule and deep foundations become necessary to counter frost effects. With the elimination of concrete, construction becomes easy, fast and more reliable. It also reduces the site labor work and provides schedule advantages both ultimately resulting in reduced project TIC. Keywords: Steel Cap Plates, Plate design, Modularization, Steel Pipe Piles, Modularization, Fast-track Construction. Introduction Projects in the North American sub continent face several challenges. One of the fore most challenges is the short construction cycle. The region experiences sub zero temperature for a major part of the year, which makes site work a big problem as the sites for oil and gas projects can be accessed only for a few months in a year. Moreover the labor cost in these countries is very high because of which it makes much sense to reduce site work as much as possible. Modularization concept is widely used here where the entire structure along with its various piping, cable trays, equipment and other attachments are constructed in shop and are then transported to the site. This usually increases the steel quantity but offsets the site work, which is 2 to 3 times as costly as shop fabrication. Modularization thus helps clients in achieving lower TIC and also shortens the construction schedules. Plants can become operational faster. It also allows the option of offloading construction to countries that have more competitive labor costs. One bottleneck that still remains is the foundation, which usually relies on concrete and hence is a major contributor to over all cost. The preferred foundation system in these regions is pile foundation, as the ground suffers from frost effects and top layers of soil cannot offer reliable resistance. Pile can be either Concrete or Steel sections. Steel sections offer better reliability and can be easily purchased in various sizes. These can be pipe sections or H sections, both of which are easily available. Concrete piles however offer higher lateral capacities. The typical foundation system with a pile cap usually consists of the steel column with base plate, Anchor bolts to tie the column to the pedestal, the pedestal supported on the pile cap, the pile anchorage system to transfer the compression, tension and lateral forces from the pile cap to the pile and finally the piles. Here we see that between the column and the pile there are 4 intermediary components, if we directly join the columns to the piles these components will not be required, hence saving not only material and construction costs but also engineering costs. The overall problem now becomes very simple with only one connection between the columns and the piles. This connection can be achieved by providing a Pile Cap-Plate and welding it with the column base plate. In the subsequent sections we will review the various options available for the design of the Pile cap Plate. Design philosophy The design of the foundation system is broken down into the following sub parts a) Pile selection b) Pile cap plate sizing c) Base Plate to Pile cap Plate connection d) Pile cap plate design.

2 212 Advances in Engineering and Technology Pile selection The pile selection basically depends on the outcome of the geotech Investigations presented to the client in the form of Geotech Recommendations. These recommendations depend on the following factors: Soil / Sub-Surface conditions present in the area: The soil type and the depth of refusal strata greatly decides the type of piles to be used. Cases where the refusal can be reached at shallow depths end bearing piles are preferred, for other cases friction piles are preferred. Steel pile are a preferred for friction piles due to ease of manufacturing piles of lengths till 20-30metre with better quality control. Construction Philosophy: Construction philosophy also influences the type of foundation best suited for a particular site. For shorter construction time driven steel piles are preferred. Ease of procurement: Sometimes easy access to manufacturing plant or presence of nearby shore can help in procuring piles at cheaper rates and can influence the foundation type. Once the pile type has been finalized the design can be performed based on that particular type. Steel piles can be used in almost all cases. There are predominantly two types of steel piles: H Piles: These are like Wide flange H section and are primarily used for piled foundation. These are normally used in areas where the loads are not very huge. Pipe Piles: These are Circular Hollow sections and are most widely used. These offer good friction and end bearing properties and can be produced in big diameters and lengths. Typical pile sizes of this type are Larger diameters can also be purchased but usually these are sufficient for vertical loads up to 250 Ton and lateral loads up to 25 Ton. Cases where lateral loads govern the pile diameter can be increased and where the vertical loads govern the pile embedment lengths can be modified, thereby offering a very customizable foundation design. Table 1: Types of Hollow circular piles Pile Diameter(mm) Cross-section Thickness (mm) Installation Tolerance Steel driven piles are often liable to be offset from their actual location due the nature of pile driving. This is usually governed by the construction tolerances agreed with the pile contractor. The piles should be designed for the worst case where the pile is offset from its original location by a minimum of 50mm or construction tolerance. The pile thickness depends on the structural capacity of the pile and the energy delivered by hammer blows during pile driving. The structural capacity of the pile has to be large enough so that the pile itself is not the weak link in the foundation capacity. Pile cap plate sizing The pile cap plate size will depend on the pile diameter and the pile eccentricity in any direction. For the sizing of the pile cap following formula generally works Plate size = Pile diameter + 2*75mm + 25mm This size will work for almost all cases where the column size is smaller than the pile diameter. For cases where the column size is nearly equal to the pile size, the plate size shall be increased accordingly. Base plate to Pile cap plate connection The Column base can be connected to the base plate by a variety of means depending upon the lad conditions. Stud welding: Steels studs can be welded on top of the pile cap plate and then can be bolted to the column base plate. This can emulate the pile cap to Base plate connection correctly, although it will be a bit costly when compared to other alternative. The studs will require to be welded according to the manufactures criteria and can be very costly as this will have to be performed at site.

3 Design of Single Steel Pile under Steel Column Foundation System 213 Shear: From the anchor rods to the cap plate & Friction between the base plate and cap plate. Tension: From the anchor rods to the cap plate Compression: Bearing of the column base plate on the Cap plate Simple Rest: For cases where the columns are always in some compression the column base plates can be simply made to rest on the pile cap plate. The transfer of shear will be through friction between the Base plate and the Pile cap plate. This kind of connections is very easy to construct. These also have the added benefit of less restraint to Thermal expansion. Simple rest connections can absorb this expansion without any damage. Guides can be provided along the column to hold the column in place. Welding Base plate to Cap Plate: This is the most versatile connection type as it can tackle all the three forces with equal ease. This is also suitable for cases where we have tension in piles. The weld can be provided along the entire perimeter or only along the web of the column. (refer figure 1) Pile Cap Plate Design (thickness) The plate thickness will depend on the following parameters a) Pile diameter b) Column & Column base plate size c) Nature of the loads (Tension/ Compression) d) Plate end conditions The design of Pile cap plates is fairly complex, as the traditional formulae do not directly apply to the case of a column base plate resting on pile cap plate. The second issue is that the load distribution under the column base plate will not be uniform as the base plate itself is flexible. The nature of loads will also influence the plate design as compression will flow as an area load where as tension will flow through the circumference of the weld of pile cap plate with the base plate. This issue can be resolved through FEM model where we can model the column cross-section, the base plate and the pile cap plate, but that model will be complex and would require specialized software to do that. Figure 1

4 214 Advances in Engineering and Technology The following discussion will present a method of design that will utilize the Timoshenko s formulas for design of circular plates and cross check them against FEM Modeling. In our example the two give nearly same results. Timoshenko s Theory of steel plate When the plate is under compression from the column the compression can be assumed to be symmetrical about the center of the pile, this can be achieved by making the column to pile connection as pinned and assuming there is no eccentricity between the column and the pile. The diametrical section should alone be sufficient for the design due to symmetry. The radius of curvature of the plate along this diameter can be assumed to be proportional to the bending moment, just like a prismatic beam, Hence the assumption of plane sections remain plains is used here to derive the expression for bending moment. As the circular plate will have a two way behavior, two moments one about the diameter and the other along the circumference, that is radial moment and tangential moment respectively, need to be calculated. The end conditions for the plate can be pinned or fixed depending on the type of connection of the cap plate with the pile. If the plate is rigidly welded with stiffeners to the pile along its circumference, then the fixity at the ends can be assumed to restrict the rotation. This will reduce the moment at the center of the plate and consequently will reduce the cap plate thickness. A pinned end can also be provided for cases where the loads are not very high and for small diameter piles. Pinned end can be provided by means of intermittent welds. Using these assumptions and principles of statics based on Timoshenko s theory the plate thickness can be calculated as follows. Design Basis: 1.) The load from the Column will be transferred to the base plate as uniform pressure. 2.) This pressure is assumed to be applied on a concentric circular area at the centre of the plate of equivalent area as that of the base plate of the column ( r = ) 3.) The value of moments is taken from Timoshenko s Strength of Material chapter on Thin plates and shells 4.) The edge condition is assumed to be fixed. 5.) Column to Pile connection is assumed to be pinned and no moment is transferred to the pile cap plate. Pile Cap Plate Pile 410 Radius of circular plate supported on pile edge "a" Base Plate 410 Steel Column Radius of equivalent circle for load transfer "c" The Formula for maximum moment is M 1 Figure a c M 2. q. c log n 2 4 c 4. a

5 Where: M 1 = moment at Center M 2 = Moment at edge µ= Poisons ratio q = Pressure over assumed area of load c = radius of circular plate supported on pile edge (Radius of pile) a = radius of equivalent circular area of base plate. Design of Single Steel Pile under Steel Column Foundation System 215 FEM Analysis for the Pile-Column Connection Single piles supporting a single column on a Cap plate may not be rigid enough to transmit the load uniformly over the entire base plate. The Column flanges at its corners are stiffer than the inner portions and column web due to deflection in the plate under the load. Column Size: W310X107 Base Plate: 345mmX355mmX32mm thk Pile Size: 610X12.7 HSS mm*mm Pile Cap Plate: 675mmX675mm X38mm thk Plate Weld b/w Base Pl. and Pile Cap Pl.: 200mm long X 12mm thk (Along the column web) Figure 3 : The above figure shows the analysis model complete with the column, pile, plates and the weld Hence a major portion of the load will flow from the column flanges and there is a possibility that it may cause local overstress in the column flange corners. The pile cap plate may also see regions of overstress due to this non uniform distribution of load. To assess the impact on the column and pile cap plate the connection was modeled in RISA using plate elements the plate elements were given the properties as per the column, plate and pile cross-section. Compression only links were modeled under the base plate. The base plate was modeled under the column cross-section. The weld was modeled along the length of the base plate using rigid links with full force & moments transfer at the edges. At low loads the behaviors will be more like fixed, and as the load increases the behaviors will ultimately become fixed. Initially the connection of pile cap plate to pile was assumed as fixed; later on it was modified to pin to reduce the local stresses in the pile. It is the view of the author that the connection will be somewhere between fully fixed and pinned as a single fillet weld can offer only limited flexibility. At low loads the behaviors will be more like fixed, and as the load increases the behaviors will ultimately become pinned. Non Linear analysis are required to take care of the local pre-mature yielding that is expected to happen around the corner columns and around the pile circumference for cases when the pile and the column are of nearly same size and the column corners approach the pile edges. Fig. 4 presents the results of one such Non linear Analysis. It was observed that certain elements were taking excessive load and the remaining were very much below their limits Since RISA does not have non linear analysis capabilities, a "mocked up" version was devised. Plates which reached their yield limits were softened by reducing their Modulus of elasticity iteratively till all plates reach safe stress limits. This ensures a

6 216 Advances in Engineering and Technology Figure 4: The above figure shows the stress distribution as per FEM analysis for a compression of 1000 KN positive load path. The elastic modulus was reduced on the elements that were yielding to approximate the redistribution of stresses that will take place with the design loading condition. Alternatively the modeling can be done in software which is capable of Non-Linear analysis. It was an iterative process to reach equilibrium with the stress level in no element exceeding yield. When the modulus of the yielding elements was reduced the stress of adjacent elements that share the load would increase. If the stress in the adjacent elements exceeded yield, their elastic modulus was reduced also. Ultimately, the model reaches equilibrium with all stress levels at yield or less with minimal distortion. With this approximation of the in-place conditions with the design loading, the connections were found to be acceptable. Figure 5 : Stress distribution for the pile cap plate Conclusion Single pile directly under the structural columns is a very advance foundation concept and has many fold advantages on the projects & industry. It provides a clean construction method eliminating concrete and excavation for foundations. Construction period is fairly short compare to the traditional concrete foundation system and provides schedule advantages in critical projects. In many countries it is a cheaper option compared to the traditional foundation systems. The facility after construction is far more reliable and safe.

7 Design of Single Steel Pile under Steel Column Foundation System 217 Elevating modules above grade would result in additional cost benefits for the foundations due to the elimination of the excavation, mud-mat, concrete infill, and backfill. Site work hours can be reduced and more work can be done at shop floor. Hence the concept is becoming more popular in the advanced countries. Fig. 6 & fig 7 show a couple of site photographs from a recent project where we used this foundation system with great success and brought down the concrete quantities greatly. Figure 6: Single piles without pile caps for supporting structures Figure 7: A compete structure being supported on single piles on a North American project Acknowledgement Author would like to acknowledge the help, support and encouragement provided by the P-4 (Professional publication and presentation program) committee of the Fluor Corporation, New Delhi, India. References [1] Timoshenko s Theory of Plates. [2] Fluor 3rd Generation Design Guidelines. [3] RISA 3D, structural Analysis Software Suite.