Cold-Formed Steel Curtain-Wall Systems

In August 2012, Simpson Strong-Tie launched a comprehensive, innovative solution for curtain-wall framing. Our lead engineer for developing our line of connectors for curtain-wall construction explains the purpose of the curtain wall with the illustrations below.

steel-stud-framingFirst, curtain walls are not what you put up if you shared a room with your brother and sister when you were growing up. When I first learned about the use of cold-formed steel curtain walls, I laughed and thought: “Gosh, how useful this would be for someone growing up with 5 siblings in one bedroom!” I have always enjoyed the sense of humor that our engineers use to help explain technical topics.

Curtain walls can be described as exterior building walls with the primary purpose of protecting the interior building against the exterior weather and natural phenomena such as sun exposure, temperature changes, earthquakes, rain and wind.

To put it in structural terms, a curtain-wall system consists of non-load-bearing exterior walls that must still carry their own weight. Curtain walls are not part of the primary structural framing for the building, but they typically rely on the primary structural framing for support. Additionally, curtain walls receive wind and seismic loads and transfer these forces to the primary building structure.

Types of Curtain Walls

Glass and cladding curtain walls make up two basic types of curtain-wall systems. Glass curtain-wall systems are usually designed using aluminum-framed walls with in-fills of glass. The cladding curtain wall is a system with back-up framing that is covered in some type of cladding material. The cladding curtain-wall system is the type in which Simpson Strong-Tie products can be used.

mid-rise-buildings-1The back-up framing is the structural element of the curtain-wall system. It is typically constructed with cold-formed steel studs ranging from 31/2″ to 8″ deep, in 33 mil (20 ga.) to 97 mil (12 ga.) steel thicknesses. The framing studs are typically spaced at 16″or 24″ on center. There are many different types of cladding materials. They include, but are not limited to, exterior insulation finish systems (EIFS), glass-fiber-reinforced concrete (GFRC), bricks, metal panels and stone panels.

building-material-examplesDeflection

One essential function of the curtain wall is to allow for relative movement between the curtain-wall system and the main building structure. At first, it was not obvious to me why making this allowance was necessary, but our product development team creatively explained some of the reasons why this is an important must-have feature for curtain walls.

deflection-examplesFirst, the primary building will move up and down as it is loaded and unloaded by the live-load occupancy, similar to beam live-load deflections.

Second, the structure sways and has torsional displacement due to movement from lateral wind or seismic loads.

Third, concrete structures typically encounter creep and shrinkage, and there may be foundation differential settlement or soil compression from high-gravity loads.

Lastly, the temperature differential may cause the building elements to expand and contract, which, again, can result in relative movement between structural elements. This is similar to a bridge’s steel plate expansion joint system.

And if you are a curious designer like me, you probably wonder why the relative vertical moment is so significant in engineering design.

One key reason is to ensure that the curtain walls do not collect gravity loads from the building, so as to prevent overloading and possible failure of the stud framing. In addition, a well-designed curtain-wall system needs to retain the primary structural load path as intended by the building designer.

The other reason is to protect the cladding of the building. If you remember earlier, the cladding material may be marble, granite or natural stones that are often very expensive and heavy. In some cases, the cladding can be one of the most expensive systems in a building. And there are times when it’s much more cost-effective to design for relative movement than it is to over-design structural framing to address the stringent deflection requirements.

Construction Type

Bypass framing is a term that is often used in curtain-wall construction. In this system, the metal studs bypass the floor and hang off the outside edges of the building. You can see from the illustration how the studs run past, or bypass, the edge of the slab. In this case, the studs are supported vertically on the foundation at the bottom, and then run continuously past multiple floor levels.

Picture by Don Allen of Super Stud Building Products.

Picture by Don Allen of Super Stud Building Products.

In steel construction, concrete fill over metal deck is typically constructed with a heavy-gauge bent plate or structural angle. Connectors can attach directly to the steel angle or the web of an edge beam.

Simpson Strong-Tie SCB Bypass slide clip connections.

Simpson Strong-Tie SCB Bypass slide clip connections.

SSB Bypass Framing Slide-Clip Strut connections.

SSB Bypass Framing Slide-Clip Strut connections.

 

 

 

 

 

 

 

 

It may seem that this type of construction is too complex and requires great efforts to detail the many connections needed to hang the curtain wall off the outside of the building. So what are the compelling reasons to choose bypass framing construction?

Bypass framing can accommodate flexibility for the architect. In another words, the bypass configuration easily allows architects to create reveals, set-backs and other architectural features.  Plus, there are fewer joints to detail for movement when stud length can run continuously for several floors.  Another benefit is that the exterior finish can also be installed on a curtain-wall system with a tighter tolerance than the edge of the structure.

One other special bypass framing type is known as ribbon window or spandrel framing. Ribbon windows are a series of windows set side by side to form a continuous band horizontally across a façade. The vertical deflection for this type of bypass framing is typically accommodated at the window head. This type of bypass usually works well for panelized construction.

Another common curtain-wall system is infill framing, where the studs run from the top of one floor to the underside of the floor above. Sometimes it’s a challenge to attach bypass framing to the edge of thin concrete slabs. In the following illustration, deflection is designed at the top track of wall panels.

bypass-framing-in-actionIn Part 2 of this blog post series, I will provide more details about how we have innovated products to be used for this application, plus a more comprehensive post about the products we offer and how they are typically used.

In the meantime, you can check out our product offering. Our recent SC slide-clip and FC fixed-clip connectors are designed for high-seismic areas.

I would like to invite you to comment and provide feedback on this topic and tell us whether you’ve had any experience working with a Designer on a CFS curtain-wall project. If you are a Designer who specializes in this discipline, how are you designing curtain-wall systems for seismic forces?

 

Concrete Anchorage for ASD Designs

One of the first things I learned in school about using load combinations was that you had to pick either Load and Resistance Factor Design (LRFD)/Strength Design (SD) or Allowable Stress Design (ASD) for a building and stick with it, no mixing allowed! This worked for the most part since many material design standards were available in a dual format. So even though I may prefer to use LRFD for steel and ASD for wood, when a steel beam was needed at the bottom of a wood-framed building that was designed using ASD load combinations, the steel beam could easily be designed using the ASD loads that were already calculated for the wood framing above since AISC 360 is a dual- format material standard. And when the wood-framed building had to anchor to concrete, ASD anchor values were available in the IBC for cast-in-place anchors and from manufacturers for post-installed anchors in easy-to-use tables, even though ACI 318 was not a dual-format material standard. (Those were good times!)

Then along came ACI 318-02 and its introduction of Appendix D – Anchoring to Concrete, which requires the use of Strength Design. The 2003 IBC referenced Appendix D for Strength Design anchorage, but it also provided a table of ASD values for some cast-in-place headed anchors that did not resist earthquake loads or effects. This option to use ASD anchors for limited cases remained in the 2006, 2009 and 2012 codes. In the 2015 IBC, all references to the ASD anchor values have been removed, closing the book on the old way of designing anchors.

ICC-ES-equation-tensionSo what do you do now? Well, there is some guidance provided by ICC-ES for manufacturers to convert calculated SD capacities to ASD allowable load values. Since there is no conversion procedure stated in the IBC or referenced standards, designers may want to use this generally accepted method for converting anchor capacities designed using ACI 318. ICC-ES acceptance criteria for post-installed mechanical and adhesive anchors (AC193 and AC308) and cast-in-place steel connectors and proprietary bolts (AC398 and AC399) outline a procedure to convert LRFD capacities to ASD using a weighted average for the governing LRFD/SD load combination. So if the governing load combination for this anchor was 1.2D + 1.6L and the dead load was 1,000 pounds and the live load was 4,000, then the conversion factor would be (1.2)(0.2) + (1.6)(0.8) = 1.52 (keep in mind that the LRFD/SD capacity is divided by the conversion factor in the ICC-ES equation shown here for tension).

Right away, there are a few things that you may be thinking:

  1. What about load factors that may exist in ASD load combinations?
  2. It may just be easier to just recalculate my design loads using LRFD/SD combinations!
  3. The resulting allowable loads will vary based on the load type, or combination thereof.
  4. If the ACI 318 design strength is limited by the steel anchor, then the conversion will result in an allowable load that is different from the allowable load listed for the steel element in AISC 360.

Let’s take a look at these objections one by one.

Item 1: Since unfactored earthquake loads are determined at the ultimate level in the IBC, they have an LRFD/SD load factor of 1.0 and an ASD load factor less than 1.0, which is also true for wind loads in the 2012 and 2015 IBC (see graphic below). Using the LRFD/SD load factor of 1.0 obviously does not convert the capacity from LRFD to ASD so you must also account for ASD load factors when calculating the conversion factor. To do so, instead of just using the LRFD load factor, use the ratio of LRFD Factor over ASD Factor. So if the governing load combination for an anchor was 0.9D + 1.0E and the dead load was 1,000 pounds and the seismic load was 4,000, then the conversion factor would be (0.9)(0.2) + (1.0/0.7)(0.8) = 1.32.

ICC-ES-equations

Item 2: Even though the weighted average conversion requires you to go back and dissect the demand load into its various load types, often this can be simplified. ICC-ES acceptance criteria permit you to conservatively use the largest load factor. The most common application I run into is working with ASD-level tension loads for wood shearwall overturning that must be evaluated using SD-level capacities for the concrete anchorage. Since these loads almost always consist of wind or seismic loads, using the largest factor is not overly conservative. Depending on the direction in which you are converting the demand loads or resistance capacities, the adjustment factors are as shown in the figure below. Affected Simpson Strong-Tie products now have different allowable load tables for each load type. (For examples, see pp. 33-36 of our Wood Construction Connectors catalog for wind/seismic tables and pp. 28-30 of our Anchoring and Fastening Systems catalog for static/wind/seismic tables.)

IBC-ealier-later

Item 3: I am unsure whether there is any sound rationale for having allowable loads for an anchor resisting 10% dead load and 90% live load differ from those of an anchor that resists 20% dead load and 80% live load. Perhaps a reader could share some insight, but I just accept it as an expedience for constructing an ASD conversion method for a material design standard that was developed for SD methodology only.

Item 4: We have differing opinions within our engineering department on how to handle the steel strength component of the various SD failure modes listed in ACI 318. Some believe all SD failure modes in ACI 318 should be converted using the load factor conversion method. I side with others who believe that the ASD capacity of a steel element should be determined using AISC 360. So when converting SD anchor tension values for a headed anchor, I would apply the conversion factor to the concrete breakout and pullout failure modes from ACI 318, but use the ASD steel strength from AISC 360.

Finally, I wanted to point out that the seismic provisions in ACI 318, such as ductility and stretch length, must be considered when designing anchors and are not always apparent when simply converting to ASD. For this reason, I usually suggest converting ASD demand loads to SD levels so you can use our Anchor Designer™ software to check all of the ACI 318 provisions. But for some quick references, we now publish tabulated ASD values for our code-listed mechanical and adhesive anchors in our C-A-2016 catalog —  just be sure to read all of the footnotes!

Get There Quicker! How CFS Designer Can Help Speed Up Your Design Process

Did you know that Simpson Strong-Tie is celebrating its 60th birthday this year? We started out with one punch press and the ability to bend light-gauge steel. Then, one Sunday evening in the summer of 1956, Barclay Simpson’s doorbell rang and a request for our first joist hanger led us into the wood connector business. Since then, we’ve continued to grow that business by focusing on our engineering, research and development efforts. Some might say that nowadays we’re an engineering company that also happens to manufacture products, as evidenced by our focus on developing technology tools over the past few years such as web calculators, an updated website and design software. Our focus on technology, however, is really another aspect of our continued commitment to excellence in manufacturing and our application of the tenets of lean manufacturing.

Many of you may already be familiar with the idea of lean manufacturing made famous by Toyota in the early 2000s, along with the principles of continual improvement and respect for people. The concept of continual improvement is based on the idea that you can always make small changes to improve your processes and products. Although they were established in a manufacturing setting, these ideals ring very true for engineering as well; eliminate steps in your design process that don’t add any value to the final project and always be on the lookout for tools or techniques that can speed up your process. Thinking lean isn’t about cutting corners to get your result faster, it’s about mindfully getting rid of the steps that aren’t helping you and finding better ways of doing everyday tasks.

As structural engineers, we can find ourselves working on a variety of projects that lead us to perform repetitive calculations to check different conditions, such as varying parapet heights on the exterior of a building, or we may find ourselves working with an unfamiliar material, such as light-gauge or cold-formed steel (CFS), where we have to take some time away from design to review reference materials such as AISI S200-12 North American Standard for Cold-Formed Steel Framing. Wouldn’t it be great if there were a design tool that could help you complete your light-gauge projects more quickly, in complete compliance with current building codes?

It turns out that Simpson Strong-Tie offers a design tool called CFS Designer™ to help structural engineers improve their project design flow. This program gives engineers the ability to design light-gauge stud and track members with complex beam loading and span conditions according to building code specifications. What does that actually mean, though? Allow me to illustrate with an example of a design project.

Let’s say you’re designing a building and part of your scope is the exterior wall framing, or “skin” of the building. You probably get sent some architectural plans that look something like this:

Figure 1. Sample building elevation with section marks.

Figure 1. Sample building elevation with section marks.

The architectural elevations will have wall section marks indicated for different framing situations. Two sample wall sections are shown in Figure 2.

Figure 2. Sample building wall sections.

Figure 2. Sample building wall sections.

This building has several different wall section types that include door and window locations, varying parapet heights, diverse finish materials that need to meet different deflection criteria, and different connection points back to the base building. The traditional design calculation that you would need to run for one wall section might begin with a loading diagram similar to Figure 3 below.

Figure 3. Sample calculation of wall stud loading diagram.

Figure 3. Sample calculation of wall stud loading diagram.

Once you have your loading diagram generated, you would need to use reference load tables or a computer analysis program to solve for the axial and moment demands, the reactions at the pinned supports, and the member deflections. 

After you determine the demand loads, you would then need to select a CFS member with sufficient properties, and you may need to iterate a few times to find a solution that meets the load and deflection parameters. After you’ve selected a member with the right width, gauge and steel strength, you’ll need to select an angle clip that can handle the demand loads, as well as fasteners to connect the clip to the CFS stud and to the base building. You would also need to also check the member design to ensure that it complies with bridging or bracing requirements per AISI. Then, after all that, you’d have to repeat the process again for all of the wall section types for your project.

Figure 4. Hmm, CFS design would sure be a lot easier if buildings were just huge windowless boxes…

Figure 4. Hmm, CFS design would sure be a lot easier if buildings were just huge windowless boxes…

Just writing out that whole process took some time, and you can imagine that actually running the calculations takes quite a bit longer. I think we can all agree that the design process we’ve outlined is time-consuming, and here’s where using CFS Designer™ to streamline your design process can really help.

CFS Designer is a structural engineering design program that can automate many of the manual steps that are required in the design process. It has an easy-to-understand graphical user interface that allows you to input your project parameters within a variety of design modules from walls and beams, jambs and headers, X-brace walls, shearwalls, floor joists, and roof rafters. The program also enables the design of single stud or track members, built-up box-sections, back-to-back sections, and nested stud or track sections. Figure 5 shows an example of how you would input the same stud we looked at before into the program.

Figure 5. CFS Designer™ user interface for wall stud design.

Figure 5. CFS Designer™ user interface for wall stud design.

The program will generate the loading diagrams and complete calculation package for all of these different situations. And along with checking the member properties and deflection limits, CFS Designer will also check bridging and bracing requirements and provide connector solutions for the studs using tested and code-listed Simpson Strong-Tie products. Figure 6 shows an example of the summary output you would receive.

Figure 6. The comprehensive summary output page that covers the complete member design down to the bracing and connection solutions.

Figure 6. The comprehensive summary output page that covers the complete member design down to the bracing and connection solutions.

One unique part of the output is toward the center of the second page, under the heading “Simpson Strong-Tie Connectors.” This section summarizes the tension and compression loads at each reaction point and then shows a connector solution (such as the SCB45.5) along with the number of screws to the stud and the number of #12 sheet-metal screws to anchor back to the base building. Simpson Strong-Tie has developed and tested a full array of connectors specifically for CFS curtain-wall construction as well as for interior tenant improvement framing, which allows designers to select a connection clip straight out of a catalog without needing to calculate their own designs per the code. It’s just another way we’re helping you to get a little leaner!

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Figure 7. A typical SCB/MSCB bypass framing slide-clip connector showing directional loading along with the table of allowable connector loads.

Figure 7. A typical SCB/MSCB bypass framing slide-clip connector showing directional loading along with the table of allowable connector loads.

The last part of the output shown in Figure 6 is titled “Simpson Strong-Tie Wall Stud Bridging Connectors.” It checks the bridging and bracing requirements per AISI S100 and selects a SUBH bridging connector, an innovative bridging solution developed by Simpson Strong-Tie that snaps into place and achieves design loads while only requiring one #10 screw to connect for 75% of applications.

Figure 8. A close-up of the SUBH installed (left) and a wall of studs with bridging installed using the LSUBH clips (right).

Figure 8. A close-up of the SUBH installed (left) and a wall of studs with bridging installed using the LSUBH clips (right).

You can download a free trial of CFS Designer™ and give it a test drive to see how much time it can save you on a design project. The trial version has almost full functionality, with the exception of not being able to print the output sheets. You can see purchasing information online, and you should always feel free to contact your local Simpson Strong-Tie engineering department with any questions you may have. I hope you are able to take advantage of this great tool to further improve your everyday design processes. We will be sure to keep you updated on our latest technology tools that help speed up the design process.  If you’re using CFS Designer, we’d like to hear your thoughts about the program. Please share them in the comments below.

 

Installation Errors – They Happen

A few years ago, we did a post on creative uses of our products. Most of the uses shown were artistic, or functional do-it-yourself projects, with one odd car spoiler modification. This week, I was reviewing some slides in a presentation that I give a few times a year regarding product installation errors. I call them misinstallations, but I’m not sure that’s a word. I thought I’d share a few of the more instructional ones. Most of the photos were curated by our northwestern region training manager, Olga Psomostithis – thanks Olga!

Double Shear Hangers

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Double shear hangers require joist fasteners that are long enough to penetrate through the hanger, through the joist and into the header. The joist nails help transfer load from the joist into the header, resulting in higher allowable loads.

install2.1

The installation shown has had the double shear tabs bent back, and nails installed straight into the joist. Since the joist nails do not penetrate the header, this would result in a reduced capacity.

Holdowns

I’m including the trailer hitch installation because it makes me laugh no matter how many times I see it.

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A very common question we get about holdowns is related to posts being offset too far from the anchor bolt (or is the anchor too far from the post?). In the installation shown below, the holdown is not flush with the post as the anchor bolt is offset about 1 inch. For small offsets up to about 1½”, a common solution is to raise the holdown off the sill plate and extend the anchor bolt with a coupler and bend it so there is a small (1:12) slope to it.

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The holdown test standard, ICC-ES AC155, which is discussed in this post, requires that holdowns are tested raised off the test bed, which you can see in the photo below. Holdowns may be raised up to 18” above the top of concrete without a reduction in load provided that the additional elongation of the anchor rod is accounted for.

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I like this photo because the installer put on the nail stops to protect the pipes. It is good to remember that plumbing happens when laying out a structural system.

Oh boy, does it happen.

Oh boy, does it happen.

install9STHD Holdowns

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The photo above is not a misinstallation, but something that can happen. Embedded strap-style holdowns are cost-effective solutions for shearwall overturning or wind uplift. It is permitted to bend the straps to horizontal and back to vertical one cycle. If spalls form, they should be evaluated for reduced loads. Any portion of the strap left exposed should be protected against corrosion.

Hanger Gaps

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Gaps can occur between trusses and supporting girders for a variety of reasons. For standard hanger tests, a 1/8″ gap is required between the joist and header per ASTM D7147. A resource for evaluating conditions with larger gaps is our technical bulletin Allowable Loads for Joist Hangers with Gaps. The technical bulletin has load data for a variety of hangers with gaps up to 3/8″, as well as recommended repairs for larger gaps. Our HTU product series comprises truss hangers specifically engineered to allow gaps up to ½”.

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After going through a design project and carefully selecting the members and details of construction, it can be frustrating as an engineer to get that phone call from the general contractor or building inspector informing you that something is not right with the construction. Understanding some of the resources available to address installation errors can help solve these problems more quickly, and get you back to designing the next project.

California Has Funding for $3,000 Grants for Home Retrofits

Are you an engineer working with California clients whose homes were built before 1979 on a raised foundation?

Evident earthquake damage

Earthquake damage sustained by a two-story building over a cripple wall system after the Mexicali Earthquake (M7.2).

If you are, these clients may be among the 1.2 million California homeowners eligible for a seismic home retrofit. The state of California has approved the continuation of an initiative known as Earthquake Bolt + Brace (EBB). In its second year, this program plans to make as many as 1,600 grants to selected homeowners, nearly three times the number given the previous year. The EBB grant program provides up to $3,000 to homeowners residing in more than 150 California zip codes. Check to see whether your clients live within one of these communities here.

Simpson Strong-Tie has several different resources to assist you in helping your clients understand how to mitigate seismic risks to houses with raised foundations. The Seismic Retrofit Details sheet provides various ways to retrofit the cripple wall system using prescriptive methodologies, which can be adapted for engineered solutions. The picture below highlights the use of the Simpson Strong-Tie universal foundation plate (UFP) to attach the boltless sill plate of the cripple wall to the concrete stemwall. This simple step can help prevent the house from sliding off its foundation. The picture also reveals plywood sheathing used to reinforce the weak cripple wall system. Additional resources for retrofit can be found here.

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Retrofit with UFP foundation plate in Napa, California

To help your clients better understand the impact these simple steps can have in preventing structural damage in an earthquake, click here to watch the story of a Napa business women who had purchased a structure with a raised foundation for her business and retrofitted it just prior to the 2014 M6.0 Napa earthquake, which caused considerable damage to many similar structures.

Let your clients know that the time to apply is very limited if they think they qualify for a retrofit grant. Registration for the 2016 EBB program ends on February 20. To register or learn more about the program, visit www.earthquakebracebolt.com.

When you finish a retrofit for one of your clients, we want to hear how it went. Let us know in the comments below.

Fiber Reinforced Polymer (FRP) Design Example

The following FRP Design example walks the reader through the typical process for designing an FRP strengthening solution for a concrete T-beam per ACI 440.2R Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures.

One of the most important initial checks for an Engineer of Record is to confirm that the unstrengthened structure can support the load combination shown in Equation 5.5.1 in ACI 562 Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings:

Eq. 5.5.1: (φRn)existing ≥ (1.2SDL + 0.5SLL)new

This check is to prevent a structural failure in case that the strengthening is damaged in an extraordinary event. If the structural element cannot pass this check, then external reinforcement is not recommended.

We have a Design Questionnaire where we ask Engineers of Record for more specific information related to the element to be strengthened:

doc1clone1

For this particular example, the following information was provided for the concrete T-beam.

1.  Structure Type (e.g., building, bridge, pier, garage):

  • 5-story commercial concrete building

2. Element(s) to be Strengthened/Repaired (e.g., beam, column, slab, wall):

  • Reinforced concrete beams

3. Type of Deficiency (e.g., shear, flexural, axial):

  • Flexural

4. Existing Factored Capacity of Section (e.g., kips, kip-ft):

  • 265 kip-ft

5. Ultimate Demand to be Supported (e.g., kips, kip-ft):

  • 320 kip-ft

6. Existing Concrete Compressive Strength:

  • 4,000 psi

7. Existing Rebar Yield Strength:

  • 60 ksi

8. Existing Reinforcement Layout:

  • 3 #7s 2.6875 inches from bottom of web to centroid of steel

9. Existing Dimensions:

  • 36 inches total beam height, 8 inches slab, 24 inches web width, 120 inches effective slab width

10. Relevant Existing Drawing Sheets and/or Pictures:

  • See attached

11. Finish Coating Requirements/Preferences:

  • None

12. For Flexural Strengthening:

  1. Dead Load Moment Applied at Time of Installation
    1. 60 kip-ft
  2. Service Dead Load Moment After Installation
    1. 80 kip-ft
  3. Service Live Load Moment After Installation
    1. 140 kip-ft

We then plug this information into our design program to come up with an FRP solution that meets the needs of the member:

masterdoc

For a beam that was at 83% of the capacity required for the new loading, we specified a simple, low-impact FRP solution to maintain clearances under the beams. If a traditional fix of adding cross-section to the beam had been specified instead, then additional concrete and rebar would need to be added to the beam, which would impact clearances under the beam and also increase the seismic weight of the building. The additional weight could also translate all the way through the building and even impact footing designs.

FRP can be used to increase the flexural strength up to 40% per ACI 440.

For your next retrofit project, please contact Simpson Strong-Tie to see if FRP would be an economical choice for strengthening your concrete or masonry element.

Concrete Anchor Design for the International Building Code: Part 3

Specification of Concrete Anchors
The 2012 IBC and its Referenced Standard, ACI 318-11, is the first to mandate that contract documents specifically address installation, inspections and design parameters of concrete anchorage. For this reason, the specification of anchors in drawing details alone is impractical. To fully and effectively address these code mandates, concrete anchorage is more practically specified in both drawing detail(s) and the General
Structural Notes or specifications of the contract documents. The drawing detail(s) would typically call out the anchor type, material specification, diameter, and embedment depth. The General Structural Notes or specifications would include the name of the qualified anchor(s) and address the installation, inspections and design parameter requirements of ACI 318-11.


The following sections of ACI 318-11 discuss the contract document requirements for concrete anchorage:

cadp3.1

The commentary in ACI 318-11, RD.9.1 discusses the sensitivity of anchor performance to proper installation. It emphasizes the importance of qualified installers for all anchors, and compliance with the Manufacturer’s Printed Installation Instructions (MPII) for post-installed anchors. Training is required for adhesive anchor installers per ACI 318-11 D.9.1. Simpson Strong-Tie Co. Inc. provides free installer training by experienced Technical Sales Representatives for our adhesive, mechanical and specialty anchors. Contact us at 1-800-999-5099. Special inspection and proof loading are addressed in ACI 318-11 D.9.2
and D.9.2.1.

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Per the section above, anchor installation requires inspection per Section D.9.2. In addition, the design parameters for adhesive anchors are required to be specified in the contract documents. An explanation of the design parameters listed in ACI 318-11 D.9.2.1 is provided below:

  1. Proof loading where required in accordance with ACI 355.4. Proof loading is only required for adhesive anchors loaded in tension in which the inspection level chosen for the adhesive anchor design is “Continuous” (Ref. ACI 355.4 Section 10.4.6). Selecting “Continuous Inspection” can result in a higher “Anchor Category,” which in turn results in a higher strength reduction factor, φ. Reference Section 13.3.4 of ACI 355.4 for the minimum requirements of the proof loading program, where required. The Design Professional is responsible for performing the quantity, the duration of
    the applied load, and the proof load to which the anchors will be tested. These parameters will be specific to the anchor design conditions.
  2. Minimum age of concrete at time of anchor installation. Per ACI 318 D.2.2, adhesive anchors must be installed in concrete having a minimum age of 21 days at time of anchor installation. Simpson Strong-Tie® has performed in-house testing of SET-XP®, AT-XP®, and ET-HP® adhesive anchors installed in 7-day- and 14-day-old concrete. The results of testing are published in an engineering letter (L-A-ADHGRNCON15.pdf), which can be viewed and downloaded at www.strongtie.com.
  3. Concrete temperature range. This is the in-service temperature of the concrete into which the adhesive anchor is installed. Temperature Ranges are categorized as 1, 2 or 3. Some manufacturers use A, B, or C as the category designations. Each Temperature Range category has a maximum short-term concrete temperature and a maximum long-term concrete temperature. Short-term concrete temperatures are those that occur over short intervals (diurnal cycling). Long-term concrete temperatures are constant temperatures over a significant time period.
  4. Moisture condition of concrete at time of installation. Moisture conditions, as designated by ACI 355.4, are “dry,” or “water-saturated.” Moisture condition impacts the characteristic bond stress of an adhesive.
  5. Type of lightweight concrete, if applicable.
  6. Requirements for hole drilling and preparation. These requirements are specific to the adhesive, and are described in the Manufacturer’s Printed Installation Instructions (MPII). Reference to the MPII in the contract documents is sufficient.

 

Adhesive anchors installed in a horizontal or upwardly inclined orientation that resist sustained tension loads require a “certified” installer.

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A certification program has been established by ACI/CRSI. Installers can obtain certification by successful completion of this program. Contact your local ACI or CRSI chapter for more information. Other means of certification are permitted, and are the responsibility of the licensed design professional.

The installation of adhesive anchors in a horizontal or upwardly inclined orientation presents unique challenges to the installer. Simply put, the effects of gravity for these applications make it difficult to prevent air bubbles and voids, which can limit full adhesive coverage of the insert (threaded rod or reinforcing bar). Due to the increased installation difficulty of these anchors, they are required to be continuously inspected by a certified special inspector.

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Suggested General Structural Notes or specifications for post-installed anchors can be viewed and downloaded at here, or contact a Simpson Strong-Tie® representative for help with your post-installed General Structural Notes or specifications.

Simpson Strong-Tie Suggested General Note for Anchor Products

Post-Installed Anchors into Concrete, Masonry and
Steel and Cast-in-Place Anchors into Concrete

The below products are the design basis for this project. Substitution requests for products other than those listed below may be submitted by the contractor to the Engineer-of-Record (EOR) for review. Substitutions will only be considered for products having a code Report recognizing the product for the appropriate application and project building code. Substitution requests shall include calculations that demonstrate the substituted product is capable of achieving the equivalent performance values of the
design basis product. Contractor shall contact manufacturer’s representative (800-999-5099) for product installation training and a letter shall be submitted to the EOR indicating training has taken place. Refer to the building code and/or evaluation report for special inspections and proof load requirements.

  1. For anchoring into cracked and uncracked concrete

a) Mechanical anchors shall have been tested in accordance with ACI 355.2 and/or ICC-ES AC193 for cracked concrete and seismic applications. Pre-approved products include:
i. Simpson Strong-Tie® Strong-Bolt® 2 (ICC-ES ESR-3037)
ii. Simpson Strong-Tie® Titen HD® (ICC-ES ESR-2713)
iii. Simpson Strong-Tie® Torq-Cut® (ICC-ES ESR-2705)
iv. Simpson Strong-Tie® Titen HD® Rod Hanger (ICC-ES ESR-2713)
v. Simpson Strong-Tie® Blue Banger Hanger® (ICC-ES ESR-3707, except roof deck insert)

b) Adhesive anchors shall have been tested in accordance with ACI 355.4 and/or ICC-ES
AC308 for cracked concrete and seismic applications. Adhesive anchors shall be installed
by a certified adhesive anchor installer where designated on the contract documents.
Pre-approved products include:
i. Simpson Strong-Tie® AT-XP® (IAPMO-UES ER-263)
ii. Simpson Strong-Tie® SET-XP® (ICC-ES ESR-2508)
III. Simpson Strong-Tie® ET-HP® (ICC-ES ESR-3372)

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Concrete Anchor Design for the International Building Code: Part 2

Designing “Alternative Materials”
Concrete anchor types whose designs are not addressed in the IBC or its Referenced Standards, or are specifically excluded from the scope of the Referenced Standard (ACI 318-11), may be recognized as Alternative Materials. Section 1909 of the 2012 IBC requires that “The strength design of anchors that are not within the scope of Appendix D of ACI 318, shall be in accordance with an approved procedure.” Section D.2.2 of ACI 318-11 lists some concrete anchor types that are considered “Alternative Materials” and specifically excludes these anchors from its scope. The list of “Alternative Material” anchors provided in this section is not, however, a comprehensive list.

Section 104.11 of the 2012 IBC describes how the design professional must approach the design of Alternative Materials.

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Section 104.11 provides the design professional with two options for the substantiation of the acceptable performance of an Alternative Material:

a.
Research Reports. As described in the previous section (Design of Code Anchors), Research Reports are referenced as the primary source for the design and qualification of Alternative Materials. Research Reports for anchors are published by IAPMO UES or ICC-ES, both ANSI ISO 17065 accredited agencies. Publicly developed, majority-approved acceptance criteria are used to establish the test program and minimum performance requirements for an anchor type. Some Alternative Material anchor types have established acceptance criteria to which a product can be evaluated:

  • Screw Anchors in Concrete (such as Simpson Strong-Tie® Titen HD®): ICC-ES AC193
  • Headed Cast-in Specialty Inserts (such as Simpson Strong-Tie® Blue Banger Hanger®): ICC-ES AC446
  • Powder- or Gas-Actuated Fasteners (such as Simpson Strong-Tie® PDPA and GDP): ICC-ES AC70

If Research Reports are used to substantiate an anchor’s performance, the design professional is bound by the design methodology and product limitations described in the Research Report.

b.
Tests. If a Research Report is not available, and no acceptance criteria exists for a given anchor type, IBC Section 104.11 permits the use of tests performed in accordance with “recognized and accepted test methods” by an “approved agency” to substantiate performance. One example of an anchor type for which no acceptance criteria exists is:

  • Helical Wall Ties (such as Simpson Strong-Tie® Heli-Tie™)

Cracked Concrete Determination
One of the many design considerations that the design professional must determine when designing either “Code Anchors” or anchors qualified as “Alternative Materials” is whether to consider the state of the concrete “cracked” or “uncracked.” The concrete state can significantly influence the anchor’s capacity. Neither the IBC nor ACI 318, Appendix D explicitly defines which applications should be categorized as “cracked” or “uncracked” concrete. The design professional must determine by analysis whether cracking will occur in the region of the concrete member where the anchors are installed. Absent an analysis to determine whether cracking will occur, the design professional may conservatively assume that the concrete state is “cracked.” With that said, there are two circumstances that require the design professional to design for “cracked” concrete:

a) Anchors in structures assigned to Seismic Design Categories C, D, E, or F (per 2012 IBC, Chapter 16) are required to be designed for “cracked” concrete unless the design professional can demonstrate that cracking does not occur at the anchor locations. The prequalification requirements of ACI 355.2 for mechanical anchors and ACI 355.4 for adhesive anchors include a test program that evaluates the performance of anchors in cracked concrete. Only anchors that have been tested and have passed the cracked concrete test program qualify for use in “cracked” concrete. The Research Report for a post-installed anchor (mechanical or adhesive) will clearly indicate whether it qualifies for use in “cracked concrete.”
b) Anchors located in a region of the concrete element where analysis indicates cracking at service level loading must be designed for “cracked” concrete (e.g. fr ≥ 7.5λ√f’c, ACI 318-11 eq. 9-10).

The design professional must consider additional factors that have the potential to result in concrete cracking in the region of anchorage. These factors include restrained shrinkage, temperature changes, soil pressure, and differential settlement. If no cracking is assumed in the region of the anchorage, the design professional should be able to justify that assumption.

Design Calculations

The design methodology in ACI 318 Appendix D is cumbersome. Calculations can be performed by hand using the design equations in Appendix D, inserting the substantiated data from an anchor manufacturer’s data tables or Research Reports to design with post-installed anchors. Designing with cast-in-place “Code Anchors” does not require additional data beyond what is included in ACI 318, Appendix D since these are “standard” anchors with standard design characteristics.

Performing hand calculations can be time-consuming, and for most design professionals is impractical due to the complexity of the design equations associated with multiple failure modes required to be considered. Design software, such as Simpson Strong-Tie® Anchor Designer™ Software for ACI 318, ETAG and CSA provides a fast, reliable method of calculating anchor performance for both cast-in-place and post-installed anchors. This software designs both “Code Anchors” and “Alternative Materials” for which an acceptance criteria exists.

Simpson Strong-Tie® Anchor Designer™ Software for ACI 318, ETAG and CSA is free and can be downloaded here.

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Concrete Anchor Design for the International Building Code: Part 1

Purpose
The intent of this technical bulletin is to clarify code language and outline the correct path for the design of concrete anchors under the International Building Code (IBC). The reader will be able to clearly distinguish between “code anchors” and anchors that are considered “alternative materials,” as well as understand the logical sequence of code language for designing each type. The distinction between “cracked” concrete and “uncracked” concrete anchor design will be made. This technical bulletin will lend clarity to the qualification of post-installed anchors for use in concrete. Excerpts from the IBC and its Referenced Standards will be provided to facilitate the description of the design requirements.

Background
More than a decade after the introduction of the American Concrete Institute’s ACI 318, Appendix D design methodology for anchor design in 2002, many design professionals either do not fully understand or are unaware of the code requirements for the design of concrete anchors. Several factors contribute to the challenges associated with understanding the code mandates:
1. The incorrect notion that ACI 318, Appendix D is exclusively for anchors designed for “cracked concrete,” leading to regionally varying degrees of enforcement and implementation of the design requirements
2. Multiple Reference Standards for the design and qualification of different anchor types
3. The evolving scope of Reference Standards, which have reclassified some anchors as “Code Anchors” that were previously considered “Alternative Materials”
4. Confusing language in IBC sections that address concrete anchorage
5. Complexity of the anchor design methodology itself
6. Varying levels of special inspections enforcement

It is nevertheless incumbent upon the licensed design professional to design anchors in accordance with the minimum provisions of the code in order to protect public safety, reduce liability risk and fulfill professional responsibilities.

The International Building Code, beginning with the 2000 edition, describes the design methodology of concrete anchors by virtue of the language within the IBC itself, or through language in the Referenced Standard (ACI 318). In this technical bulletin, specific reference to the 2012 IBC and ACI 318-11 will be made, since this is currently the most widely adopted edition of the IBC.

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“Code Anchors” and “Alternative Materials”
Anchors can be divided into two major categories: 1) “Code Anchors”, which are those that are specifically addressed in the IBC or its Referenced Standards, and 2) “Alternative Materials”, the design and qualification of which are not addressed in the IBC or its Referenced Standards.
The following “Code Anchors” recognized by the 2012 IBC:

  • Headed studs
  • Headed bolts
  • Hooked (J- or L-) bolts
  • Expansion anchors(such as Simpson Strong-Tie® Strong-Bolt® 2)
  • Undercut anchors (such as Simpson Strong-Tie® Torq-Cut™)
  • Adhesive anchors (such as Simpson Strong-Tie® SET-XP®, AT-XP®, and ET-HP®)

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Anchor types not listed above are considered “Alternative Materials.”
The following are anchors qualified as such:

  • Screw anchors (such as Simpson Strong-Tie® Titen HD®)

Alternative materials also apply to anchor types specifically excluded from ACI 318-11 calculation and analysis requirements.

  • Specialty inserts (such as Simpson Strong-Tie® Blue Banger Hanger®)
  • Through-bolts
  • Multiple anchors connected to a single steel plate at the embedded end
  • Grouted anchors
  • Powder- or gas-actuated fasteners (such as Simpson Strong-Tie® PDPA)

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Designing “Code Anchors”
The starting point for the design of all anchors is Section 1908 of the 2012 IBC.

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Section 1908.1 states that only cast-in-place headed bolts and headed studs are permitted to be designed using “Allowable Stress Design,” provided that they are not used to resist earthquake loads or effects. For these anchors, Section 1908.2 references Table 1908.2 for the determination of the allowable service load. Section 1908.1 makes explicit reference to post-installed anchors (anchors installed into hardened concrete), stating that the provisions of “Allowable Stress Design” is not permitted. For the design professional, this means that determining anchor by means of “Allowable Load Tables” based on previous test criteria that used a safety factor of 4.0 to determine allowable loads, as in the example below, is not permitted under the IBC.

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Section 1909 of the 2012 IBC, “Anchorage to Concrete – Strength Design” makes explicit reference to Appendix D of ACI 318 as the required design standard for the anchors listed in this section.

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Cast-in-place headed bolts and headed studs used to resist earthquake loads or effects must be designed using “Strength Design” in accordance with ACI 318 Appendix D. Additionally, Section 1909 does not make reference to adhesive anchors, despite their status as “code anchors.” ACI 318-11 was the first edition to include adhesive anchors in its scope; however, the 2012 IBC was approved prior to the approval of ACI 318-11. This resulted in the omission of adhesive anchors from the language in Section 1909 of the 2012 IBC. Section 1901.3 of the 2015 IBC, entitled “Anchoring to Concrete” includes language for adhesive anchors and their applicability to the ACI 318-14 design and qualification requirements. The omission of adhesive anchors from Section 1909 of the 2012 IBC, however, does not exclude them from the design and qualification requirements of ACI 318-11 by virtue of their inclusion in ACI 318-11 Section D.2.2. The design professional must then reference Section D.2 of ACI 318-11, Appendix D to confirm that the anchors being designed fall within its scope.

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Note that anchors used for temporary construction means, such as tilt wall panel bracing, are not addressed in the IBC. As a result, they are not required to be designed in accordance with the provisions of ACI 318, Appendix D. Section D.2.2 lists anchor types that fall within its scope, and those that are excluded (considered “Alternative Materials”).

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Code Anchors are required to meet the ACI 318-11 Section D.2.3 qualification requirements described below.

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ACI 355.2 (Qualification standard for expansion and undercut anchors) and ACI 355.4 (Qualification standard for adhesive anchors) are referenced here as the qualification criteria for specific types of postinstalled anchors. For the design professional it can be difficult to determine, without fully investigating these Referenced Standards, whether a specific proprietary anchor has been tested and is qualified for use in concrete. A simpler means by which to identify whether a proprietary anchor has been qualified to the Referenced Standard is a current Research Report (e.g., Evaluation or Code Report) which provides third-party review and verification that the product has been tested to and meets the qualification standard. There are two primary Research Report providers: IAPMO UES (International Association of Plumbing & Mechanical Officials Uniform Evaluation Service) and ICC-ES (International Code Council Evaluation Service).
These agencies are ANSI ISO 17065 accredited. They review independent laboratory test data, witnessed or conducted by an accredited third party, for a product and verify its conformance to publicly developed and majority-approved qualification criteria (or acceptance criteria) established for a given anchor type. Research Reports are an invaluable tool to the design professional and building official as evidence of conformance with the IBC.

There are two acceptance criteria that apply to post-installed “Code Anchors”:

  • ICC-ES AC193 – Acceptance Criteria for Post-Installed Mechanical Anchors in Concrete Elements
  • ICC-ES AC308 – Acceptance Criteria for Post-Installed Adhesive Anchors in Concrete Elements

These acceptance criteria reference ACI 355.2 and ACI 355.4, respectively, as the foundation for the test program by which the anchor is evaluated, and establish minimum performance standards for qualification. A Research Report is issued for an anchor that meets these minimum standards.

Masonry Reinforcement and Concrete Strengthening with Composites

Guest blogger Brad Erickson, Engineering Manager: Composite Strengthening Systems™

Guest blogger Brad Erickson, Engineering Manager

This week’s post comes from Brad Erickson, who is the Engineering Manager for the Composite Strengthening Systems™ product line at our home office. Brad is a licensed civil and structural engineer in the State of California and has worked in the engineering field for more than 17 years.  After graduating from Cal Poly, San Luis Obispo with a B.S. in Architectural Engineering, he worked for Watry Design, Inc. as an Associate Principal before coming to Simpson Strong-Tie.  Brad is the Engineering Manager for Composite Strengthening Systems and his experience includes FRP design, masonry and both post-tensioned and conventional concrete design.  While not at work, Brad enjoys spending time carting his three kids around to their competitive soccer games and practices.

Have you ever had a concrete or masonry design project where rebar was left out of a pour? Chances are, the answer is yes. Did you wish you could solve this problem by putting rebar on the outside of that element? That’s exactly what Simpson Strong-Tie Composite Strengthening Systems™ (CSS) can do for you and your project. In effect, composites act like external rebar for your concrete or masonry element. Composites can be used in similar configurations to rebar but are applied on the exterior surface of the element being strengthened.

The initial offering in our CSS line is our fiber-reinforced polymer (FRP) product group. An FRP composite is created by taking carbon or glass fabric and saturating it with a two-part epoxy which, when cured, creates the composite. Together, the weight of the fabric and the number of layers in the composite determine how much strength it will add to your concrete or masonry element.

reinforce1 Another form of FRP composite is a precured carbon laminate. The carbon fibers are saturated in the manufacturing facility and are attached to the structure using CSS-EP epoxy paste and filler, an epoxy with a peanut butter–like consistency. We also carry paste profilers (pictured below) that help contractors apply the proper amount of paste to a piece of precured laminate.

reinforce2Of course, before any concrete or masonry reinforcement project can succeed, proper surface preparation is of the utmost importance. Without a good bond with the substrate, a composite will not be able to achieve the intended performance. Concrete voids must be repaired, cracks must be injected and sealed, and any deteriorated rebar must be cleaned and coated. Prior to composite placement, the surface of the substrate must be prepared to CSP-3 (concrete surface profile) in accordance with ICRI Guideline No. 310.2. Grinding and blasting are the most common surface-preparation techniques.

reinforce3The following are just a few applications where composites can be used for concrete and/or masonry retrofits. The orange arrows show the direction of the fibers in the fabric – in other words, the direction in which the composite provides tension reinforcement.

FRP Confinement

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Flexural Strengthening

Shear Strengthening

Shear Strengthening

Wall Flexural Strengthening

Wall Flexural Strengthening

This is a summary of the basics of composites and their installation on strengthening projects. As composites are not yet in the design codes in the United States, the American Concrete Institute has produced 440.2R-08: Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. This guide has numerous recommendations for using fiber-reinforced polymer systems to strengthen your concrete or masonry construction.

If you would like more information about FRP design, you can learn the best practices for fiber-reinforced polymer (FRP) strengthening design during 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 to installation.

screen-shot-2017-01-06-at-11-39-27-amFor complete information regarding specific products suitable to your unique situation or condition, please visit strongtie.com/rps or call your local Simpson Strong-Tie RPS specialist.