Link to Latest Report: March 2023 Progress Report
Background:
The advancement of Accelerated Bridge Construction has primarily been through focused effort on deck and superstructure design and construction methods. Further acceleration can be achieved through focused effort on bridge substructures. Ongoing efforts in the State of Iowa may provide benefit to ABC methods by identifying code conservatism. Reducing the total foundation requirements and effort required for substructure construction will result in a reduction of overall construction costs and time.
A project recently completed for the Iowa Department of Transportation investigated the contributions of concrete encasements of steel H-piles used for bridges which have historically not been considered in the design process. The initial reason for the project was to determine the remaining capacity of a pile when subjected to scour, leaving bare the uncased portion of a pile. A tool was developed to calculate capacity and a subsequent laboratory investigation was completed to validate the pile capacity assessment tool. The study concluded the pile capacity is greater than what has been otherwise calculated. As a secondary result, consideration has been given to revising the capacity calculations of new piles especially in fully-encased pile bents. This project aims to identify the unbraced height limits of steel H-piles when fully-encased and thus expedite foundation construction by taking advantage of the increased capacities and reducing the total number of required piles. By addressing design code conservatism, construction time may be decreased.
Objective:
The objective of this project is to identify the maximum unbraced height of fully-encased piles and to further develop the capacity calculation tool to include monolithic encasement options. Though this information is largely based on the deep foundation design practices in the State of Iowa, the deliverables are applicable to other state practices.
Scope:
This project
Task 1 – Kickoff Meeting
The research team will meet with the project Technical Advisory Committee (TAC) to review the project scope and work plan, and to establish a schedule of subsequent quarterly meetings. An outline of the proposed work will be presented and discussed with the TAC members. The purpose of this meeting is to discuss and clarify the scope of work, scheduling, and expected deliverables throughout the project and incorporate TAC input for any possible changes. A project plan that includes a detailed schedule and expectation for deliverables will be prepared per the meeting discussion.
Task 2 – Literature Review
A literature review of previously completed research will be added to the resources previously found and summarized in the already-completed project report. As before, the main objective of this task is to obtain an exhaustive understanding of concrete encased steel H-piles used for bridge projects. As part of this task, the design documents for previously constructed bridges in Iowa which monolithically encased pile bents will be gathered to be included in subsequent tasks.
Task 3 – Expand Pile Capacity Calculation Tool
The tool was first developed to rapidly assess existing piles subject to scour and it proved to be effective in calculating the capacity when including the concrete encasement into the evaluation. Further study expanded the application of the tool beyond assessment of existing piles and into the design of new piles. The concrete encasement used in the tool development is based on the P10L standard, which is the current Iowa design standard for pile bents with steel H-piles. The P10L standard specifies standard dimensions for specific piles sizes, either square or round in cross-section. Accordingly, the capacity of piles in non-standard encasement sizes or in monolithic pile bents is not directly calculated but, rather, inferred from the other calculations. The existing tool has been proven through analysis and laboratory validation and can now be further developed to directly calculate pile capacities beyond those in standard encasements.
As with the development of the first tool, the research team will investigate inelastic buckling, elastic buckling, and plastic yielding of the piles. Numerous finite element models will be created to ensure multiple combinations of encasements and pile exposure are considered. Upon completion of the analysis, the results will be incorporated into the existing tool for an easy user interface.
Task 4 – Tool validation through laboratory investigation
The previously developed tool was validated through laboratory investigation. Similarly, the updated tool will also be validated in the laboratory.
In fully-encased pile bents, the cross-sectional dimensions of the concrete encasement is greater than the P10L standard dimensions and therefore the concrete contributes a greater stiffness to the encased portion. Additionally, adjacent piles are positively connected through the encasement.
The influence of additional encasement size and adjoining piles on the capacity of any single pile is unknown. Therefore, the laboratory testing will aim to directly compare the results of the previous study where the P10L standard was used in the test specimens.
The specimen details from the first test are shown in Table 1
Table 1 – Specimen Details from Previous Pile Capacity Investigation
| Specimen | Section | Pile length (ft) |
Encasement length (ft) |
Experimental (kips) |
| 1 | HP10×42 | 16 | N/A | 612 |
| 2 | HP10×42 | 16 | 10 | 715 |
| 3 | HP10×42 | 30 | 20 | 563 |
| 4 | HP10×42 | 38 | 30 | 606 |
The test will directly compare to the results of Specimen 3 which consisted of a HP 10×42 pile with a total length of 30 ft. and an uncased length of 20 ft. In lieu of the P10L encasement, the pile will be encased with two other HP 10×42 piles with a spacing and concrete cross-section consistent with a monolithic encasement. The two additional piles will only be encased and not loaded. The extending pile will be individually loaded in a manner similar to that of the first laboratory study. In this way, the capacity results can be directly compared to the previous specimen and the influence of the encasement and additional piles can be determined.
Further, an additional test will be completed which specifically investigates the system behavior rather than the behavior of an individual pile. The investigation will be completed by constructing a specimen of the same size and dimension of the previous specimen, but instead of holding the adjacent piles short of the point of bearing, the three piles will be of equal length and will fully bear against a reaction block. The point of loading will be on the encasement instead of the pile bottom. This will allow the investigators to determine how loads are distributed between adjacent piles.
Task 5 – Final Report
The project findings from the previously described tasks will be prepared by means of a final report. A summary of activities and results of the finite element model analysis and corresponding laboratory study will be included. A discussion of the modified tool, its capabilities, and its limitations will be provided.
Research Team:
Principal Investigator: Justin Dahlberg
Research Engineer: Dr. Zhengyu Liu
Previous Progress Reports: