4 Common FRP Myths and Misconceptions: The Stuff Not Everyone Talks About

This week’s post is written by Kevin Davenport, who is the Southeast US Field Engineering Manager for Simpson Strong-Tie. Kevin is also responsible for providing technical support on Simpson Strong-Tie products for Infrastructure, Commercial and Industrial market segments within his own Southeastern territory. He is a registered professional engineer in Georgia and received his B.S. (’97) and M.S. (’98) from Clemson University. Kevin is a member of ICRI, ACI and various local chapters of SEA. 

The primary benefit of fiber-reinforced polymer (FRP) systems as compared with traditional retrofit methods is that significant flexural, axial or shear strength gains can be realized using an easy-to-apply composite that does not add significant weight or section to the structure. Many times it is the most economical choice given the reduced preparation and labor costs and may be installed without taking the structure out of service. However, it is important to make sure the composite is properly designed following industry standards in order to ensure that it is the right product for the application.

To provide you with a better understanding of the topic, it’s important to dispel some common myths and misconceptions that you might hear about FRP:

1. “FRP can solve all my retrofit and strengthening problems”
Composite strengthening systems are another tool for your toolbox, providing a possible solution to certain specific retrofit problems. However, they can’t do everything, and there are times when they may not be able to meet the project requirements. Simpson Strong-Tie’s design team will work with you to prepare a feasibility study to ensure suitable solutions for your application. One very important check when strengthening a structure is to verify that the existing, unstrengthened capacity is capable of resisting a certain percent of the newly applied loads. The following equations are strengthening limit checks that should be considered. These checks will sometimes determine how much additional strength the FRP composites are capable of providing to the existing structure.

  • ACI 440.2R-08

(φRn)existing ≥ (1.1SDL + 0.75SLL)new            (9-1)

  • ACI 562-13

Uex ≥ 1.2D + 0.5L + Ak + 0.2S                     (5.5.1)

2. “FRP is 10 times stronger than steel”
Although the ultimate tensile strength of some FRP dry fibers can exceed the yield strength of mild reinforcing bars (60 ksi) by up to 10 times, there are two main reasons an engineer should not assume that using FRP will provide 10 times the capacity of steel. First, the cured composite properties, not the dry fiber properties, are more relevant when designing with FRP composites. The ultimate tensile strength of cured composites will be more on the magnitude of two to three times stronger than 60 ksi (not 10 times stronger). Second, the ultimate tensile strengths of FRP systems occur at ultimate strain. When full design calculations are performed, the FRP design strain and resulting FRP strength will often be much lower after accounting for all possible failure modes and all recommended reductions based on durability testing and/or environmental reduction factors. Code limits often govern design over published ultimate strength properties.

For this reason, it is not good practice to size the required area of FRP using:

AFRP = (Arebar x fy rebar) / ffu FRP

 

Material Properties of Cured Composite 3. “FRP can triple the flexural capacity of the member or replace all the corroded steel”

It may be possible to achieve higher increases depending on member properties, but the following are some good rules of thumb when estimating the amount of strengthening that can be provided by FRP: flexural = 40%, shear = 20%, axial = 20%. Design is usually governed by the existing strength check, the FRP debonding strain (can’t develop infinite tension capacity through the bond line), or a ductility check (flexural φ factor based on strain in rebar at section failure).

4. “Stamped calculations and drawings were submitted, so it must have been designed properly”
Often, the FRP design engineer may make various assumptions in the design calculations, and the EOR (reviewer) should ensure that the FRP is designed “correctly” and verify that any assumptions made by the FRP engineer are accurate. Note that Simpson Strong-Tie calculations have an “Assumptions” section to make it is very easy for the EOR to identify where we took educated guesses.

Blueprints

 


Simpson Strong-Tie Can Help

We recognize that specifying Simpson Strong-Tie® Composite Strengthening Systems™ is unlike choosing any other product we offer. Leverage our expertise to help with your FRP strengthening designs. Our experienced technical representatives and licensed professional engineers provide complimentary design services and support – serving as your partner throughout the entire project cycle. Since no two buildings are alike, each project is optimally designed to the Designer’s individual specifications. Our pledge is to address your specific condition with a complete strengthening plan tailored to your needs, while minimizing downtime or loss of use, at the lowest possible installed cost.

Concrete Structures

Your Partner During the Project Design Phase 

During the Designer’s initial evaluation or preparation of the construction documents, Simpson Strong-Tie can be contacted to help create the most cost-effective customized solution. Simpson Strong-Tie Engineering Services will work closely with the Design Engineer to provide all the necessary information required to design the system. The solution we deliver will include detailed design calculations for each strengthening requirement and design drawings with all the necessary details to install the CSS.

Why Use Simpson Strong-Tie Design Services?

  • To assess feasibility studies that will help ensure suitable solutions to your application
  • To receive customized FRP strengthening solutions
  • To work with our trained contractor partners to provide rough-order-of-magnitude (ROM) budget estimates
  • To collaborate during the project design phase
  • To receive a full set of drawings and calculations to add to your submittal
  • To maintain the flexibility to provide the most cost-effective solution for your project
  • To gain trusted technical expertise in critical FRP design considerations

 

The Design Fundamentals of FRP Strengthening
If you would like to view the recorded webinar, you can find it at our website at https://www.strongtie.com/css.

The Design Fundamentals of FRP Strengthening

WATCH THE RECORDED WEBINAR NOW!

Learn the best practices for FRP strengthening design during this innovative webinar presented by Simpson Strong-Tie.


If you’d like more information about FRP design, you can learn the best practices for fiber-reinforced polymer (FRP) strengthening design from a recorded webinar offered by Simpson Strong-Tie professional engineers. We look at FRP components, applications and installation. We also take you behind the scenes to share the evaluation process informing a flexural beam-strengthening design example and talk about the assistance and support Simpson Strong-Tie Engineering Services offers from initial project assessment through installation.

For complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/css or call your local Simpson Strong-Tie RPS specialist at (800) 999-5099.

An Introduction to Helical Wall and Stitching Ties

This week’s post is written by Kevin Davenport, who works as a Field Engineer with Simpson Strong-Tie. Kevin is responsible for providing technical support on Simpson Strong-Tie products for Infrastructure, Commercial and Industrial market segments within his Southeastern territory. He is a registered professional engineer in Georgia and received his B.S. (’97) and M.S. (’98) from Clemson University. Kevin is a member of ICRI, ACI and various local chapters of SEA. 

What do you do when brickwork is in bad condition? Depending on what state the brickwork is in, a tear-down may be called for. However, often brickwork can be restored and strengthened using helical ties such as Simpson Strong-Tie® Heli-Tie™ wall ties and stitching ties. This post introduces these two types of helical ties, which might be just what you need for your next brick restoration project.

What is a helical tie? 

A helical tie is made by twisting a metal profile into the shape of a helix. The design of the Simpson Strong-Tie Heli-Tie wall tie also  incorporates a large core diameter in order to provide higher torsional capacity. The benefit of this feature is less axial deflection due to a propensity for normal helix shape to “uncoil” under tension load. Since helical ties are typically used in building façades, they are generally made from stainless steel in order provide the necessary corrosion resistance.  Helical ties can be used to retrofit and stabilize brickwork in two common applications: 1) Wall anchor applications, and 2) Stitching tie applications.

heli tie

What are common helical wall tie applications?

Application #1: Anchoring building façades to structural members

In a wall anchor application, the helical tie is used to stabilize the façade by transferring out of plane façade forces through the anchor into the backup material. The need for this type of reinforcement arises when pre-existing wall anchors were never installed, were inadequately spaced or have corroded away over time. Helical ties are an economical solution that can be installed directly through a brick façade into various backup materials such as solid concrete, CMU block, and even wood or metal studs.

Image 1

Image 1: Installing a helical tie.

A pilot hole is drilled through the existing brick wall and any air gap into the backup material. Then the helical tie is placed in an installation tool and driven into the pre-drilled hole. As it is driven, the fins of the helical tie tap into both the masonry and backup material and provide an expansion-free connection that will withstand tension and compression loads. Some helical wall ties, like the Heli-Tie, use an installation tool that countersinks the tie below the surface of the brickwork. This allows the hole to be patched and concealed with a color matching material.  Thereby, helical anchors allow the repair to be both efficient and inconspicuous when completed.

Image 2Image 3Image 4

There are presently no specific U.S. design standards for the use and qualification of helical wall ties. However, a rational calculation of required spacing given the demand load can be easily calculated using ACI 530 (Building Code Requirement and Specifications for Masonry Structures), Section 6.2, and test data with an appropriate factor of safety. In addition, many Designers also follow the detailing practice for prescriptive anchored veneer in Section 6.2.2.5.6 that prescribes the following:

  1. At least one anchor for each 3.5 ft2 of wall area, and
  2. A maximum anchor spacing of 32″ horizontal and 25″ vertical, and
  3. Around openings larger than 16″ in either dimension: Additional perimeter anchors at maximum 36″ spacing within 12″ of the opening.

Prescriptive anchors also require bed joints to be at least twice the thickness of the embedded anchor. However, this provision is not relevant for helical anchors since they are installed into a drilled hole, rather than embedded into a wet mortar joint.

Application #2: Stabilizing multiple-wythe brick walls

In this application, the wall tie is used to attach wythes of brick to one another in an effort to stabilize the wall. By intermittently alternating installation angles (0 o, 45 o, 0o, -45 o, etc.) the tie promotes more monolithic behavior of the wall.

Image 5

What are common helical stitching tie applications?

Unlike wall anchor applications, in a stitching tie application the helical tie is used to stabilize brickwork by transferring in- and out-of-plane shear and bending forces across an existing crack. Stitching ties are placed in the plane of the wall within the horizontal bed joint.

The existing bed joint is routed out deep enough to recess the helical tie and cleaned out. Then, the recess is filled about 2/3 deep with a repair mortar (such as Simpson Strong-Tie® FX-263 Rapid Hardening Vertical/Overhead Repair Mortar). The helical stitching tie is then pressed into the mortar, followed by a trowelling with encapsulating grout. The installation provides an inconspicuous repair and preserves the appearance of the structure.

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For red brick, we recommend placing stitching ties  at a minimum vertical spacing of 12″ and  extending the ties at least 20″ on either side of the crack.

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Helical ties are not something that you see on every jobsite, however, they can provide a fast and cost-effective solution for brickwork rehabilitation. Hopefully this post provided you some background about them and an insight into our Heli-Tie product offering. The recent launch of our Repair Protection Strengthening Systems product line complements the Heli-Tie with a wide array of other repair products.

Do you have any past experience with helical wall ties, or questions? Please share in the comments below.