ODOT CROSSBEAM LOAD RATING SOFTWARE

OTIA III State Bridge Delivery Program Management & Design

FIBER REINFORCED POLYMER (FRP) FLEXURAL STRENGTHENING DESIGN PROGRAMUSER MANUAL

Version 1.2Prepared by

1165 Union St., Suite 200

Salem, Oregon 97301

Issue Date July 2006DisclaimerPortions of the contents of this design program were developed cooperatively by Oregon Department of Transportation (ODOT) and Oregon Bridge Delivery Partners (OBDP). ODOT and OBDP assume no liability or responsibility for and make no representations or warranties as to applicability or suitability of this program. Anyone making use thereof or relying thereon assumes all responsibility and liability arising from such use or reliance.

TABLE OF CONTENTS

Section 1Introduction

Section 2Software Details and DiscussionSection 3Software UseSECTION 1INTRODUCTION

Recent load ratings revealed that many of the vintage 1950s and 1960s Reinforced Concrete Deck Girder (RCDG) bridges in the State of Oregon have deficient flexural capacity. One of the acceptable methods to strengthen those girders, when the controlling flexural load ratings are no less than 0.85, is installing externally bonded fiber reinforced polymer (FRP) strips along the girder soffits. These strips act as additional longitudinal reinforcement, subject to the design limitations described herein. Fiber reinforced polymers are typically comprised of high strength fibers (e.g., carbon, glass) impregnated with an epoxy, polyester, or vinyl ester resin. Currently, only carbon fiber reinforced polymer (CFRP) is acceptable to ODOT. As more polymer types are approved, revisions to the ODOT QPL will be made. Several projects have been rehabilitated by ODOT using FRP to strengthen the existing reinforced concrete bridge girders, but these have typically been designed as temporary repairs. This design procedure is directed at rehabilitation with a design life of 25 years.

Based on the design criteria recommended by ACI 440.2R-02 and the technical memo, Externally Bonded FRP Strengthening For OTIA III Bridge Program (DRAFT), by Oregon Bridge Delivery Partners, OBDP developed a general FRP flexural strengthening design software using MathcadTM.

The purpose of this software is to help engineers convert girder moment demands from an ODOT Tier-2 Load Rating, Load Resistance Factor Rating (LRFR), into design loads per ODOT Bridge Design and Drafting Manual (BDDM). From this, the program assists in determining the required FRP strip size. It should be noted, however, that, in some cases, FRP rehab may not be suitable. To use this design software without modifications, the bridge should be successfully load rated based on ODOT LRFR Interim Scoping Load Rating Guidelines, hereafter referred to as LRFR Scoping Rating.The procedures stated in this manual are to explain the methodology used in this software, which is consistent with BDDM and ACI 440.2R-02.

A list of the files that are included in the design software package is given below:

FRP_Flexure_Design.MCDJuly 26, 2006.

The sample LRFR Scoping Rating files (EXTGIR.DAT and EXTGIR.OUT for BR. #00933) are also included in the design software package for the users reference.The manual is subdivided into sections to allow for easy documentation of future modifications.

SECTION 2SOFTWARE DETAILS AND DISCUSSION

MathcadTM was selected as the software to develop the FRP flexural strengthening design package. This decision was driven by the fact that the same software is used in LRFR Scoping Rating. Using MathcadTM also helps to standardize the FRP flexural strengthening design, and reduce any duplicating efforts by design engineers.A sample MathcadTM file was created for Bridge 00933. This bridge will serve as the basis for the design example. According to the LRFR Scoping Rating summary sheet, LS00933.XLS, the exterior girder of span 1 has a flexural deficient section at 0.4L with a RF=0.98 (this is used as an example, and may not be in accordance with current repair policy). Because its RF>0.80, an RFP repair was considered. The following procedure demonstrates how to use this software to design FRP flexural strengthening in order to increase the girder moment capacity to an acceptable level.At the beginning of the FRP_Flexure_Design.MCD file, the author has listed all the load modifiers and pertinent load and resistance factors per AASHTO LRFD Bridge Design Specifications. The load modifiers should be determined based on the existing bridge configuration and BDDM Section 1.1.7.4. These factors should not be changed, unless the Engineer of Record deems otherwise. Following that, the user inputs the load factors, material properties, sectional properties, dead and live loads as used in the LRFR Scoping Rating. All those values, except dead loads, can be found under POINT OF INTEREST SUMMARY in the BRASS output file, EXTGIR.OUT. Please note: 1)Similar to BRASS, the depth of girder is defined as the web depth only. The total girder depth should include the flange thickness. 2)Since load ratings only consider the existing wearing surface thickness, and designs should include a future overlay of 25 psf per BDDM Section 1.1.7.1, the additional dead load due to wearing surface should be adjusted to take into account the future overlay, as well as any future utility (if required).

3)The live load demands shown in BRASS output files include LL Factors and Dynamic Load Allowance. The LL Factors should be removed, but the Dynamic Load Allowance should remain. The dynamic load allowance, however, should be checked for consistency between the load rating and the design values.4)In calculating the ultimate moment demand, the user should choose either the max or min dead load factor based on its impact to the live load.Currently, ODOT lists only CFRP on the Qualified Products List (QPL). Therefore, this program was written solely for CFRP applications. Since CFRP products could vary significantly from one manufacturer to another, it is impossible for the author, nor is the intention of this program, to standardize the sizes and properties of FRP. Therefore, as with any FRP design, the Designer must assume certain key material properties of the FRP product before carrying out the design. The assumption should be made based on the approved manufacturers data. The final FRP design based on the assumed material properties should not be construed as a sole-source design.FRP systems can be categorized into the following four groups based on how they are delivered to the site and installed: Wet layup systems, Prepreg systems, Precured systems, and Other FRP forms. Precured systems consist of a wide variety of composite shapes manufactured off-site. Typically, an adhesive along with the primer and putty is used to bond the precured shapes to the concrete surface. Precured systems are relatively easy to construct and fast to install. Therefore, for FRP flexural strengthening applications installed on long, flat surfaces like girder soffits, Precured systems are recommended.

The flexural strength of a section depends on its controlling failure mode. Potential flexural failure modes include: (1) Crushing of the concrete in compression before yielding of the reinforcing steel; (2) Yielding of the steel in tension followed by rupture of the FRP laminate (FRP Rupture); (3) Yielding of the steel in tension followed by concrete crushing; (4) Shear/tension delamination of the concrete cover (cover delamination); and (5) Debonding of the FRP from the concrete substrate (FRP debonding). The program described herein and illustrated in the design example is to help the engineer investigate these failure modes.Based on Florida DOTs Temporary Design Bulletin C06-03, it is recommended not to use more than three (3) layers of FRP in wet layup systems.For simplicity within the program when estimating the ultimate flexural capacity for beams controlled by FRP rupture, the program assumes the concrete reaches its maximum usable strain of 0.003. This approach does not provide a more conservative result, but the error introduced by this approach is believed to be negligible. It is expressly stated that it is the engineers responsibility to determine if a refined model is required to more accurately determine the neutral axis location and the effective level of concrete strain.In calculating the design flexural strength of the retrofitted section, ACI 440.2R recommends adjusting the strength reduction factor, , when the reinforcing steel strain level is less than 0.005. Even though this adjustment is not required in the current AASHTO LRFD Specifications, it is included in this software to take into account the loss of ductillity due to the application of FRP on reinforced concrete beams. According to ACI 318-02, sy can be assumed as 0.002.

As a part of the FRP flexural strengthening design, but not included in this program, the Designer shall determine the termination point of FRP reinforcement. According to ACI 440.2R, FRP strips shall be terminated a minimum of d, the effective depth, beyond the point on the moment diagram that represents concrete cracking. In addition, the factored shear force at the FRP flexural termination location should also be checked against 2/3 of the concrete shear strength. If the shear force is greater than 2/3 of the concrete shear strength, FRP U-wraps are recommended to reinforce against cover delamination.SECTION 3SOFTWARE USESection 3 outlines the process to use the FRP Flexural Strengthening Design Software. A detailed description is provided in Section 2 and within the Design Example.

Step 1. Open the FRP_Flexure_Design.MCD file using MathcadTM version 11 or higher.

Step 2. Based on the BRASS Girder (LRFD) output file, enter the LRFR load factors, girder material properties, sectional properties, and Dead Load and Live Load demands at the section of interest.

Step 3. Select the controlling Permit Load Case, and calculate the Ultimate Demands.

Step 4. Calculate the moment capacity of the existing structure (without any contribution from the FRP strengthening).Step 5. Determine the FRP reinforcement dimensions, number of plies, and properties, according to the FRP Manufacturers product data. Approved FRP products and their Manufacturers are listed in the ODOT QPL.Step 6. Determine the cracked section neutral axis and moment of inertia, Icr, of the existing structure (without any contribution from the FRP strengthening) using Trial and Error method assuming a concrete compression strain, c, of 0.003.

Step 7. Calculate the substrate strain, bi, based on DL+ADL only, of the existing structure (without any contribution from the FRP strengthening).Step 8. Determine the bond-dependent coefficient, m.

Step 9. Estimate the neutral axis location, C0, for the rehabilitated structure (considering the contribution from the FRP strengthening).Step 10. Determine the effective level of strain in the FRP strip, and the corresponding strain in the existing tension reinforcing steel.Step 11. Calculate the stress level in the FRP strip and the existing tension reinforcing steel.

Step 12. Calculate the internal force resultants and equilibrium based on the assumed C0. If C differs from C0 by more than 1%, adjust the neutral axis location (see Step 10 above) until force equilibrium is satisfied.Step 13. Calculate the flexural capacity of the rehabilitated section.

Step 14. Check service stresses in the reinforcing steel and the FRP strip for the rehabilitated section.

Step 15. Print the FRP_Flexure_Design.mcd file and save the file.PAGE