The optimization design of Piled raft foundation for substation building in US code

: In order to get a more safety and economic foundation, finite element method was performed for a piled mat cap and piled strip cap by the software of STAAD. Through the finite element analysis, plate distribution region of bending moment and shear stress were researched; meanwhile, considering the compatibility of deformation of element plate, how to chose the design value of plate stress was researched. The result showed that: single plate value is not representative , average of value of 9 plates (3x3 array) is close to the fact. Strip cap is better than mat cap both from the moment value and shear value of plate element in this project; strip cap was recommended due to its remarkable economic benefit.


Introduction
Substation building is an essential facility in the substation, which ensures the reliability of urban power supply. Figure 1 is a building of 230kV substation in the Philippines. The substation building is a fifth-layer concrete frame structure, which is 26.8 meters high. 230kV gantry on the top of substation building , which is 21 meters high. Pile foundation was used in substation building. Pile arrangement and pile cap types were the major work for the engineer designer [1] .

Seismic loads
Seismic zone factor Z=0.4,Soil type S D , Seismic source type B.
R(response modification factor) for substation building the structure system should choose Special moment-resisting frame, the value of R is 8.5; refer to steel supports (non-buildings structures not similar to buildings), proposed R will be 3 for gantries [2] .

Load combinations
This project was suggested to design with US code, both Ultimate Limit State and Serviceability Limit State should be considered in piled raft foundation design [3] . The ULS and SLS load general combinations according to contractual specifications are based on ASCE7-10.

Pile parameter
This project uses pile foundation. Type of the pile and single pile capacity are shown in the following: Type of pile : cast-in-situ bored pile.

Calculation principle [4-6]
Staad Pro V8i(SS6) software shall be adopted for substation building structure analysis and design. The strength design and allowable stress design were adopted for the analysis. The STAAD plate finite element is based on hybrid finite element formulations. A complete quadratic stress distribution is assumed. For plane stress action, the assumed stress distribution is as follows.

Project Design
The customer suggest us use mat pile cap. We wanted to optimize the pile cap.So we suggest two schemes to piled raft cap design of substation building. One is mat cap ,another is strip cap. The analysis model is shown in the following figure. The parameter of pile and pile cap is shown in the following table. In order to get accurate result, cap plates were meshed to square plate element, which is 0.5m x 0.5m.

Bending moment
The result of bending moment for piled raft cap is shown in the following figure. Myd-bot -710 -285 The color nephogram of bending moment showed that only a few moment value of plates reached the maximum value. Consider the compatibility of deformation of element plate, the average of value of nearest 9 plates (3x3 array) was performed as the design value conservatively, which made the calculate results close to the fact.

Shear stress
The result of shear stress for piled raft cap is shown in the following figure. The color nephogram of shear stress showed that only a few stress value of plates reached the maximum value. Consider the compatibility of deformation of element plate, the average of value of nearest 9 plates (3x3 array) was performed as the design value conservatively, which made the calculate results close to the fact.
From the 3)The shear stress on the top plate of Strip cap reduce 18.9% than the shear stress of mat cap. The shear stress on the bottom plate of Strip cap reduce 11.0% than the shear stress of mat cap. Strip cap is better than mat cap from the shear stress value of plate element. 4) Strip cap reduce 20.8% reinforcement concrete than mat cap. We recommend strip cap in the project due to its remarkable economic benefit.