In this post, we follow up on our August 28 webinar, Making the Right Anchor Choice: Best Practices in Anchor Design, by answering some of the interesting questions raised by attendees.
During the webinar where we discussed the critical performance factors and code requirements you need to consider when designing with or installing adhesive or mechanical anchors. In case you weren’t able to join our discussion, you can watch the on-demand webinar and earn PDH and CEU credits here. Continue Reading
Not all anchoring adhesives are created equal. There are important differences between acrylic-based and epoxy-based adhesive systems — differences that affect installation, gel and cure times, and anchoring performance. In the following post, Marlou Rodriguez, S.E., of Simpson Strong-Tie, lays out some of the comparative installation advantages of each system.
There are two common types of adhesives for anchoring threaded rod or rebar into concrete — epoxy-based systems and acrylic-based systems. What’s the difference? When should you specify one rather than the other? This blog post will help you understand the differences and guide you in choosing the best adhesive for your anchoring solution. Continue Reading
I was driving under a concrete bridge one nice clear day in Chicago, and I happened to look up to see rusted rebar exposed below a concrete bridge. My beautiful wife, who is not a structural engineer, turned to me and asked, “What happened to that bridge?” I explained that there are many reasons why spalling occurs below a bridge. One common reason is the expansion of steel when it rusts or corrodes.
Over the weekend, I had the pleasure of watching my daughter in her cheer competition. I was amazed at all the intricate detail they had to remember and practice. The entire team had to move in sync to create a routine filed with jumps, tumbles, flyers and kicks. This attention to detail reminded me of the new ratcheting take-up device (RTUD) that Simpson Strong-Tie has just developed to accommodate 5/8″ and ¾” diameter rods. The synchronized movement of the internal inserts allows the rod to move smoothly through the device as it ratchets. The new RTUDs are cost effective and allow unlimited movement to mitigate wood shrinkage in a multi-story wood- framed building. When designing such a building, the Designer needs to consider the effect of shrinkage and how to properly mitigate it.
Shrinkage is natural in a wood member. As moisture reaches its equilibrium in a built environment, the volume of a wood member decreases. The decrease in moisture causes a wood-framed building to shrink.
The IBC allows construction of light-framed buildings up to 5 and 6 stories in the United States and Canada respectively. Based on the type of floor framing system, the incremental shrinkage can be up to ¼” or more per floor. In a 5-story building, that can add up to 1-¼” or more and possibly double that when construction settlement is included.
The Simpson Strong-Tie Wood Shrinkage Calculator is a perfect tool to determine the total shrinkage your building can experience.
In order to accommodate the shrinkage that occurs in a multi-story wood-framed building, Simpson Strong-Tie offers several shrinkage compensating devices. These devices have been tested per ICC-ES Acceptance Criteria 316 (AC316) and are listed under ICC-ES ESR-2320 (currently being updated for the new RTUD5, RTUD6, and ATUD9-3).
AC316 limits the rod elongation and device displacement to 0.2 inches between restraints in shearwalls. This deflection limit is to be used in calculating the total lateral drift of a light-framed wood shearwall.
The 0.2-inch allowable limit prescribed in AC316 is important to a shearwall’s structural ability to transfer the necessary lateral loads through the structure below to the foundation level. This limit assures that the structural integrity of the nails and sill plates used to transfer the lateral loads through the shearwalls is not compromised during a seismic or wind event. Testing has shown that sill plates can crack when excessive deformation is observed in a shearwalls. Nails have also been observed to pull out during testing. Additional information on this can be found here.
In AC316, 3 types of devices are listed.
Compression-Controlled Shrinkage Compensating Device (CCSCD): This type of device is controlled by compression loading, where the rod passes uninterrupted through the device. Simpson Strong-Tie has several screw-type take-up devices, such as the Aluminum Take-Up Device (ATUD) and the Steel Take-Up Device (TUD), of this type.
Tension-Controlled Shrinkage Compensating Device (TCSCD): This type of device is controlled by tension loading, where the rod is attached or engaged by the device and allows the rod to ratchet through as the wood shrinks. The Simpson Strong-Tie Ratcheting Take-Up Device (RTUD) is of this type.
Tension-controlled Shrinkage Compensating Coupling Device (TCSCCD): This type of device is controlled by tension loading that connects rods or anchors together. The Simpson Strong-Tie Coupling Take-Up Device (CTUD) is of this type.
Each device type has unique features that are important in achieving the best performance for different conditions and loads. The following table is a summary of each device.
The most cost-effective Simpson Strong-Tie shrinkage compensation device is the RTUD. This device has the smallest number of components and allows the rod unlimited travel through the device. It is ideal at the top level of a rod system run or where small rod diameters are used. Simpson Strong-Tie RTUDs can now accommodate 5/8″ (RTUD5) and ¾” (RTUD6) diameter rods.
How do you choose the best device for your projects? A Designer will have to consider the following during their design.
Rod Tension (Overturning) Check:
Rods at each level designed to meet the cumulative overturning tension force per level
Standard and high-strength steel rods designed not to exceed tensile capacity as defined in AISC specification
Standard threaded rod based on 36 / 58 ksi (Fy/Fu)
High-strength Strong-Rod based on 92 / 120 ksi (Fy/Fu
H150 Strong-Rod based on 130 / 150 ksi (Fy/Fu)
Rod elongation (see below)
Bearing Plate Check
Bearing plates designed to transfer incremental overturning force per level into the rod
Bearing stress on wood member limited in accordance with the NDS to provide proper bearing capacity and limit wood crushing
Bearing plate thickness has been sized to limit plate bending in order to provide full bearing on wood member
Shrinkage Take-Up Device Check
Shrinkage take-up device is selected to accommodate estimated wood shrinkage to eliminate gaps in the system load path
Load capacity of the take-up device compared with incremental overturning force to ensure that load is transferred into rod
Shrinkage compensation device deflection is included in system displacement
System deformation is an integral design component impacting the selection of rods, bearing plates and shrinkage take-up devices
Rod elongation plus take-up device displacement is limited to a maximum of 0.2″ per level or as further limited by the requirements of the engineer or jurisdiction
Total system deformation reported for use in Δa term (total vertical elongation of wall anchorage system per NDS equation) when calculating shearwall deflection
Both seating increment (ΔR) and deflection at allowable load (ΔA) are included in the overall system movement. These are listed in the evaluation report ICC-ES ESR-2320 for take-up devices
Optional Compression Post Design
Compression post design can be performed upon request along with the Strong-Rod System
Compression post design limited to buckling or bearing perpendicular to grain on wood plate
Anchorage design tools are available
Anchorage design information conforms to AC 318 anchorage provisions and Simpson Strong-Tie testing
In order to properly design a continuous rod tie-down system for your shearwall overturning restraint, all of the factors listed above will need to be taken into consideration.
A Designer can also contact Simpson Strong-Tie by going to www.strongtie.com/srs and filling out the online “Contact Us” page to have Simpson Strong-Tie design the continuous rod tie-down system for you. This design service does not cost you a dime. A few items will be required from the Designer in order for Simpson Strong-Tie to create a cost-effective rod run (it is recommended that on the Designer specify these in the construction documents):
There is a maximum system displacement of 0.2″ per level, which includes rod elongation and shrinkage compensation device deflection. Some jurisdictions may impose a smaller deflection limit.
Bearing plates and shrinkage compensation devices are required at every level.
Cumulative and incremental forces must be listed at each level in Allowable Stress Design (ASD) force levels.
Construction documents must include drawings and calculations proving that design requirements have been met. These drawings and calculations should be submitted to the Designer for review and the Authority Having Jurisdiction for approval.
More information can be obtained from our website at www.strongtie.com/srs, where a new design guide for the U.S., F-L-SRS15, and a new catalog for Canada, C-L-SRSCAN16, are available for download.
Have you ever been at home during an earthquake and the lights turned off due to a loss of power? Imagine what it would be like to be in a hospital on an operating table during an earthquake or for a ceiling to fall on you while you are lying on your hospital bed.
This week’s post comes from Marlou Rodriguez who is an R&D Engineer at our home office. Prior to joining Simpson Strong-Tie, Marlou worked as a consulting engineer. His experience includes commercial, multi-family residential, curtain wall systems and the design of seismic bracing for non-structural components. Marlou is a licensed professional Civil and Structural Engineer in California, and too many other states to list. He received his bachelor’s degree in Architectural Engineering from Cal Poly San Luis Obispo. Here is Marlou’s post.
I recently had the amazing opportunity to volunteer as a judge at The Tech Museum of Innovation’s The Tech Challenge 2015 in San Jose, Calif. My role involved evaluating projects designed by teams of students in grades 4-12 whose challenge was to build an earthquake-safe structure.
The museum’s annual Tech Challenge is a great event that excites young minds by introducing kids ages 8-18 to the science and engineering design process with a hands-on project based on solving a real-world problem. This year’s challenge was to build a scaled structure that supports live load and is earthquake safe. Simpson Strong-Tie was a sponsor of this year’s event and I was excited to be invited as a judge.
While The Tech Challenge is always focused on solving a real-world problem, no one could have anticipated how real this one would become. On the first day, April 25, I woke up to the horrifying news that a 7.8 magnitude earthquake had devastated Nepal. Thousands of lives were affected by this devastating earthquake. On that morning, this terrible tragedy thousands of miles away really highlighted how important this design challenge really is.
The Tech Challenge spans two days and is divided into three categories: elementary, middle and high school. The judging process consists of two phases. The first phase – a Pre-Performance Interview – gives students a chance to be creative in presenting their teams and designs. They discuss their roles within their team, describe how they chose their design and materials, and explain their method for solving problems and challenges. The second phase places their structures on a test rig and simulates three earthquake movements to test stability. Each structure was judged on its ability to:
Stay standing during all three seismic events
Return back to its original position
Perform with the least amount of drift or the horizontal movement at the top most part of the structure.
As an engineer, I have spent 20 years designing structures to withstand earthquakes. But when I was in elementary school years ago, my thoughts were focused on which parking lot with new curbs, banks and rails or empty pools I could skateboard in. These kids are spending their weekends thinking of how to come up with a system to vertically support a high-rise building and ways to laterally support the building while dissipating the seismic energy induced by the testing rig.
It was amazing to see the ideas the children had in their designs. There were structures with fixed bases, some with innovative base isolation systems and even a few with mass dampers attached to the top of the structure. The lateral systems chosen by the children consisted of moment frames, braced frames and solid core systems – closely resembling the systems used in most buildings today.
The design rules included:
Plan dimension of the building was limited to 16” square, while the base of the building could not exceed 20” square
Structure height could not exceed six feet
Floor-to-ceiling height had to be a minimum of 5 inches
Gravity weight of the structure could not exceed 7 lbs.
In addition, there were some size and length limitations for the supporting materials, based on grade levels, and an additional live load was added to the structure by using bolts that were inserted into drilled holes. Not only did the teams have to adhere to the rules, but they also had to calculate the area of living space within their structure. All of these rules, calculations and how they overcame the challenges had to be documented in a detailed journal.
One design that stuck out to me was developed by a team of elementary school children. I had the pleasure of conducting their pre-performance interview. They had a typical rectangular building with an interior compression member made of stacked plastic PVC pipes. The lateral system comprised of some tension wires that were attached to the top of the building at the interior PVC column. The tension wires were angled as they went through the floors and finally attached to the four corners at the base. The central PVC column bear on the base but was not directly attached to the base. This was a form of base isolation. The four tension wires attached to the four corners of the building were turned toward the central PVC column and attached via a spring. The spring acted as a way for the central column to return to its original position. The design was very interesting and had some innovative features built into it. I could only imagine how it would have performed in the device performance phase of the event, since I wasn’t able to observe that part.
Earthquakes occur all over the world. These natural occurring events profoundly affect and change people’s lives. Although there are a lot of buildings that withstand earthquakes, there are still a lot of failures of existing buildings. Structural engineers learn from these failures and develop building codes and innovative products to resist future earthquakes. These future scientists, engineers and innovators that I had a pleasure of meeting are truly amazing kids. What struck me was how well these children were able to document their thought process and how they developed their final design. It makes me believe the future of this world is in great hands. I can’t wait to come back next year to judge another challenge.
Thanks for reading our blog – have a nice holiday weekend!
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