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Project 2013-02

Project Title:

Improved Procedures for the Design of Slender Structural Concrete Columns

Institution:

Purdue University
West Lafayette, Indiana

Researchers:

Dr. Robert J. Frosch, PE

Award Period:

2012-2013 school year

Status:

Ongoing

Description:

With the availability of higher strength concrete and higher strength reinforcement, the use of reduced column cross-sections is possible. However, as the cross-section is reduced, slenderness effects become more significant and can control design of the column. Slender columns are controlled by their geometry, such that their cross sectional area is small in proportion to their length. Slender columns are controlled by their geometry, and will buckle before the concrete crushes or steel reinforcement yields.

The current slenderness provisions in the ACI 318 Building Code are based on provisions developed for the 1971 Code and have essentially remained unchanged since that time. Recognizing the importance of slender columns on the future of structural concrete design, a 318 task group was formed during the 2008 Code cycle to evaluate the slenderness provisions which resulted in a reorganization of the design provisions, although the procedure remains essentially the same. Simplification was also achieved through the elimination of direct stability analysis by limiting the magnitude of second-order effects. Through this evaluation, several shortcomings were identified; however, there was inadequate background information to result in a Code change. In addition, several problems have surfaced since publication of 318-08. In particular, designers have indicated that the new limit of total moment to 1.4 times the first-order moment is unnecessarily restricting column design, particularly for non-sway structures. Columns that once could be constructed and were considered safe can no longer meet this requirement.

Based on critical review of these issues as a member of the 318 task group, there are several issues restricting slender column design. However, there is extremely limited test data to support changes to the Code. It is interesting to note that the current provisions accounting for sustained loading effects on slender columns are based on only five tests conducted between 1964 and 1966. In fact, the majority of test data available today for slender columns, in general, is from the 1960’s with materials quite different from those used in modern structures. New test data is needed to support Code changes as well as to fill in a significant gap of prestressed columns not being addressed in the current design provisions.

Proposed Research – The proposed research will focus on three interconnected issues related to slender column design restricting the sizing and applicability of slender columns. Experimental tests will be conducted on fullscale, non-prestressed and prestressed columns. These tests in combination with analytical research will be used to develop improved code provisions.

a. Short-Term Column Stiffness (ACI 10.10.6.1). The stiffness used in slenderness analysis is the primary variable influencing second-order effects. Current design expressions for short-term stiffness are either too conservative which can greatly overestimate design forces resulting in increased column size, or too complex such that they are not practical for use in design. The proposed research will focus on the development of a simple, yet improved design expression to estimate the stiffness of both reinforced and prestressed concrete columns. An analytical study utilizing existing experimental data to evaluate the primary parameters influencing column lateral stiffness was recently completed at Purdue. Additional testing is needed to provide data on columns utilizing higher strength materials as well as on precast, prestressed columns.

b. Long-Term Column Stiffness (ACI 10.10.6.1 and 10.10.6.2). Sustained load effects provided by creep are accounted by reducing the short-term column stiffness. However, the current design procedure is not based on the sustained stress in the column, but rather the maximum factored sustained load relative to the maximum factored load for the same load combination. For gravity load dominated columns, a 50 percent decrease of the shortterm stiffness is required regardless of stress level which may be excessive. The proposed research will evaluate the influence of sustained loads on column stiffness and develop an improved method for integration into the Code. As noted, existing data is scarce. Experimental tests providing long-term sustained loading are essential for this research phase as this behavior cannot be reliably modeled analytically.

c. Second-Order Limit (ACI 10.10.2.1). The rationality of the Code limiting the total moment including second order effects to 1.4 times the first-order moment will be evaluated. For this evaluation, columns will be tested at various total moment to first-order moment ratios to evaluate the applicability of this limit.