“You Cannot Escape Responsibility Tomorrow by Evading it Today”

While the contents of this blog are certainly not what Abraham Lincoln had in mind when he made the statement that I’m using to title this blog post, it does speak volumes to the pertinence of what will be discussed today. “Design by others” or some variation of this appears in many parts of Simpson Strong-Tie details.

Simpson Strong-Tie receives technical calls from contractors and plans examiners inquiring about information that requires input from the Designer. When these calls occur during construction there can be confusion and frustration in the field because the Designer is needed to evaluate and resolve the issue. Designers and engineers that identify these conditions ahead of time will reduce confusion and delays on their projects.

Strong-Wall® Shearwalls

The Strong-Wall® shearwall product line uses several iterations of “design by others” within its installation details. It is important to note that many details within the installation drawings may require input from the Designer when certain conditions exist.

For example, Detail 7 on SSW2 shows an alternate first-story installation where the Steel Strong-Wall shearwall bypasses the floor framing and bears directly onto concrete. A ledger may be attached to the shearwall to support perpendicular floor framing. The specification of the hanger and attachment is project specific and would require evaluation by the Designer.
framing1

This condition is also on the Strong-Wall SB shearwall installation details as seen in the following detail. The detail is generic and requires attachment information from the Designer.  When these conditions do not exist on the project, it may be beneficial to cross out the detail or delete and state “not used” to lessen confusion during plan check and construction.

framing2

The note “Foundation Dimensions are for Anchorage only. Foundation design (size and reinforcement) by others” can be found in multiple locations on the Wood Strong-Wall,  Steel Strong-Wall and Strong-Wall Shear Brace detail sheets.

framing3

Soil type and loading conditions not related to the product design vary from project to project and cannot be designed into a one-size-fits-all foundation solution. Thus, the information provided by Simpson Strong-Tie in the installation drawings only addresses the concrete anchorage requirements of ACI 318 (Section D5.2.9/ACI 318-11; Section 17.4.2.9/ACI318-14). These details assume no reinforcement in the footing, resulting in rather large foundations. Since design requirements vary with every project, it’s important for Designers to evaluate and verify each condition.

The new Steel Strong-Wall shearwall grade beam solutions reduce the size of the footings required for anchorage. However, the Designer must specify the grade beam reinforcement for proper performance. The details for the grade beam solutions (see SSW1.1 sheet) are based on ACI 318 and testing that was conducted by Simpson Strong-Tie.

For the grade beam solutions, Designers have two options:

(1) Design grade beam to resist the moment induced from amplified forces to the anchor, or

(2) The lesser of the tabulated moment or the amplified LRFD design moment for seismic (ASD Shear / 0.7) x Ω0 x (SSW Height).

Furthermore, the Designer is responsible for specifying the size and number of shear and flexural reinforcement throughout the grade beam beyond the anchor reinforcement depicted in the details.

Delivery of forces to the Strong-Wall shearwall (to the top of wall), including properly sizing the structural members, should be based on project specific requirements.

Strong Frame® Moment Frames

Simpson Strong-Tie Strong Frame® ordinary moment frames and special moment frames contain similar requirements for the Designer. Moment frames have been discussed many times in this blog: Special Moment Frame Installation: What Structural Engineers Should Watch For, Steel Moment Frame Beam Bracing and Breaking News:  Simpson Strong-Tie Strong Frame Special Moment Frame Testing Today.

The notes on SMF2 state “Footing/Grade beam size and reinforcing shall be specified by the Designer as required to resist the imposed loads, such as foundation shear and bending, soil bearing pressure, shear transfer, and frame stability/overturning.”

Moment frame foundation solutions are based on satisfying the minimum concrete anchorage requirements. Detailing can be a crucial area for this product line as it is common to find deeper footings at these locations, which should be reflected on your construction documents.

Like Strong Wall shearwalls, Designers must evaluate the project conditions and detail the load path of the forces to the resisting element (in this case the Strong Frame moment frame). Ensuring proper transfer of forces that are detailed within your construction documents will reduce headaches down the road.

Strong-RodTM Systems

Strong-Rod™ systems are continuous rod tiedown solutions for multi-story, light-frame wood construction. These systems include Anchor Tiedown Systems (ATS) for shearwall overturning restraint and Uplift Restraint Systems (URS) for roofs. Both systems utilize the same components (i.e. bearing plates, rods, couplers and shrinkage compensation devices), however the detailing, design and locations of these two systems differs.

For ATS, the Designer is responsible for the following:

  • Developing the cumulative tension/compression loads,
  • Determining the system displacement requirements as defined in ICC-ES Acceptance Criteria AC316 to satisfy code required drift equations (this specifically addresses the holdown part of the Equation 4.3-1 of the 2015 Special Design Provisions for Wind and Seismic)
  • The location of each holdown/shearwall.

A more elaborate description of the Designer’s responsibilities for ATS can be found on page 23 of our  Strong-Rod Systems design guide (F-L-SRS15). Simpson Strong-Tie incorporates this information into our design of the system and provides calculations and installation drawings to the Designer for review (a sample two-story run is shown below).

framing4

For URS, there are two different approaches to design that have different level of responsibilities. The Designer has fewer responsibilities when specifying a rod with a “system (CRTS)” evaluation report per AC391, but they are still responsible for developing the project’s wind uplift loads, specifying URS details and designing systems for shearwall overturning.

More requirements must be taken into account when designing and specifying a rod system using steel components with a “rod-run only (CRTR)” AC391 evaluation report, or no report at all. (A more elaborate description of the Designer’s responsibilities for URS can be found on page 43 of the  Strong-Rod Systems design guide).

Based on the project information provided, the rod manufacturer will design and detail the system and submit calculations and installation drawings to the Designer for review.

Podium Deck Anchorage

The newest details published by Simpson Strong-Tie on podium deck anchorage solutions were developed to reduce the impact of an industry-wide challenge; resolving large tension forces (upwards of 50 kips) from four- to five-story narrow shearwalls into thin (often 10-14 inch thick) concrete podium decks. These solutions (e.g., design tables installation drawings and sample calculations), which are on our website, rely on special anchor reinforcement details using standard construction rebar.

framing5

More information about these solutions can be found this blog post and in the shallow anchor page on our website. It is important to note that the Designer is responsible for selecting the best anchorage detail to satisfy the demand loads based on his/her concrete specification and specific project conditions. The Designer is also responsible for designing and detailing the flexural reinforcement within the slab to achieve the amplified forces.

Summary

Adding standard installation details to your construction documents saves significant design time. However, the responsibility does not end with the copy-and-paste. The installation details by Simpson Strong-Tie contain details of many common applications. Some may not apply to your project, while others may require additional input from you. Of course, the Designer is permitted to use alternate details and is not limited to what is shown on the installation drawings. Providing complete information will save time and frustration during plan check and construction. Simpson Strong-Tie is here to answer questions and help with your next project. Please reach out to us by calling 800-999-5099 or by clicking here.

 

Steel Strong-Wall® Footings Just got a Little Slimmer!

While 54 inches is a good height and will get you on most amusement park rides, what about this dimension for the width of a footing? We did some tests recently — actually a lot of tests — that answered that question.

Steel Strong-Wall® narrow panels are great for resisting high seismic or wind loads, but due to their narrow widths, their resulting anchor uplift forces can be rather hefty, requiring very large pad footings. How large? For Seismic Design Categories C-F, the largest cracked concrete solution per ACI318-11 Appendix D has a width of 54 inches and an effective embedment depth of 18 inches in order to ensure the anchor remains ductile. The overall length of this footing, as seen in Figure 1, can be up to 132 inches. While purely code driven, these solutions have historically presented challenges in the field. Most concrete contractors have to dig footings this size by hand. This often leads to discussions with their engineers about finding a better solution.

Figure 1:  Slab-on-Grade Installation (Traditional Solution)

Figure 1: Slab-on-Grade Installation (Traditional Solution)

Simpson Strong-Tie has been studying cast-in-place anchorage extensively in recent years. Our research has been featured in a couple of blog posts: The Anchorage to Concrete Challenge – How Do You Meet It? and Podium Anchorage – Structure Magazine. Concrete podium slab anchorage was a multi-year test program that started with grant funding from the Structural Engineers Associations of Northern California for initial concept testing at Scientific Construction Laboratories Inc. and wrapped up with full-scale detailed testing completed at the Simpson Strong-Tie Tye Gilb Laboratory in Stockton, California. This joint venture studied the performance of anchorage reinforcement into thin podium deck slabs (10-14 inch) to resist the high overturning forces of continuous rod systems on 4-5-story mid-rise construction. The testing confirmed the need to comply with Appendix D requirements to prevent plastic hinging at anchor locations. Be on the lookout for an SE Blog post on that topic in the near future. Armed with what we learned, we decided to develop tested anchor reinforcing solutions for the Steel Strong-Wall.

The newly developed anchor reinforcement solutions for grade beams are calculated in accordance with ACI318 Appendix D and tested to validate performance. Anchor reinforcement isn’t a new concept, as it’s been in ACI318 for some time. Essentially, anchor reinforcements transfer load from the anchor bolt to the reinforcing, which restrains the breakout cone from occurring. For the new grade beam details, the additional ties near the anchor are designed to resist the load from the anchor and are developed into the grade beam. The new details offer solutions with widths as narrow as 18 inches when anchor reinforcement is used.

Two details have been developed: one for the larger panels (SSW18, SSW21, SSW24) as shown in Detail 1/SSW1.1, and one for the smaller panels (SSW12, SSW15) as shown in Detail 2/SSW1.1. The difference between the two is the number of anchor reinforcement ties specified in Detail 3/SSW1.1. For SSW18, SSW21 and SSW24 panels (Detail 1/SSW1.1), the total number of reinforcement per anchor is specified. Due to their smaller sizes, the anchor reinforcement ties specified in Detail 2/SSW1.1 for the SSW12 and SSW15 panels are the total required per panel.

Detail 1/SSW1.1

Detail 1/SSW1.1

Detail 2/SSW1.1

Detail 2/SSW1.1

Detail 3/SSW1.1

Detail 3/SSW1.1

Validation Testing

From the concrete podium deck anchorage test program, we discovered that the flexural and shear capacity of the slab is critical to anchor performance and must be designed to exceed the demands created by the attached structure. For grade beams, this also holds true. In wind-load applications, this demand includes the factored demand from the Steel Strong-Wall. In seismic applications, our testing and analysis showed that achieving the anchor performance expected by Appendix D design methodologies requires the concrete member design strength to resist the amplified anchor design demand from Appendix D Section D.3.3.4.3.

Validation testing was conducted to evaluate this concept. The test program consisted of a number of specimens with different configurations, including:

  • Closed tie anchor reinforcement
  • Non-closed tie u-stirrup anchor reinforcement
  • Control specimen without anchor reinforcement

Flexural and shear reinforcement were designed to resist Appendix D amplified anchorage forces and were compared to test beams designed for non-amplified strength level forces. The results of the testing are shown in Figure 2. In the higher Seismic Design Categories (C-F), the anchor assembly must be designed to satisfy Section D.3.3.4.3 in ACI318-11 Appendix D. In accordance with D.3.3.4.3 (a), the concrete breakout strength needs to be greater than 1.2 times the nominal steel strength of the anchor, 1.2NSA. This requires a concrete breakout strength of 87 kips for a Steel Strong-Wall that uses a 1-inch high-strength anchor.

Figure 2: Steel Strong-Wall Grade Beam Testing

Figure 2: Steel Strong-Wall Grade Beam Testing

Grade beams without the anchor reinforcement detail and with flexural and shear reinforcement designed to the Appendix D amplified anchorage forces performed similar to those with closed-tie anchor reinforcement and flexural and shear reinforcements designed to the non-amplified strength level forces. Both, however, came up short of the necessary forces required by Section D.3.3.4.3 (a). From Test V852, we discovered that even though the flexural and shear reinforcement were designed with the amplified forces, the non-closed tie u-stirrups did not ensure the intended performance. From observation, the u-stirrups do not provide adequate confinement of the concrete and tend to open up under loading conditions, resulting in splitting of the beam at the top as can be seen in the photo.

Test V852: Non-Closed U-Stirrups

Test V852: Non-Closed U-Stirrups

Tests W785 and W841 resulted in the best performance. Both test specimens contained flexural and shear reinforcement designed for the amplified forces, as well as closed-ties. Two configurations were tested to study their performance — two piece closed-tie anchor reinforcement in W785 and a single piece closed-tie anchor reinforcement in Test W841. As seen in Figure 2, their performance was very similar, and met the requirements of Section D.3.3.4.3 (a). The closed-ties helped confine the concrete near the top of the beam, allowing the assembly to reach the expected performance load (See the photo below). It’s important to indicate the following specifics in the New Grade Beam Anchor Reinforcement Details:

  • Anchor Reinforcement is #4 closed-ties
  • SSWAB embedment depth is 16″ +/- ½” (as shown in Detail 3/SSW1.1). This is to ensure there is enough development length of the anchor reinforcement on both sides of the theoretical breakout surface as required by ACI318-11 D.5.2.9.
  • The minimum distances from the anchor bolt plate washer to top and bottom of closed tie reinforcement are 13 inches and 5 inches respectively to ensure proper development above and below the concrete breakout cone (refer to Detail 3/SSW1.1).
  • The spacing between the two vertical legs of the anchor reinforcement tie must be 10 inches apart. While this may differ from your shear reinforcement elsewhere in the grade beam, it ensures the reinforcement is located close enough to the anchor and adequate development length is provided.
  • Flexural reinforcement (top and bottom) and shear reinforcement (ties throughout the grade beam length) are per the designer. Simpson Strong-Tie has provided information in Detail 3/SSW1.1 for the applicable minimum LRFD Applied Design Seismic Moment (See Figure 3) to make sure the grade beam design will at least resist the applied anchor forces. Project design loads not related to the Strong-Wall panel also should be considered and could control the grade beam design.
Closed-Tie Anchor Reinforcement

Closed-Tie Anchor Reinforcement

Figure 3: LRFD Applied Design Seismic Moment

Figure 3: LRFD Applied Design Seismic Moment

Simpson Strong-Tie is interested in hearing your thoughts on the new details. What is your opinion? How have the new details been received on your job sites?