Retrofit Straps to Concrete

Last updated December 3, 2024
By Ian Story

One of the easiest way to tie down shear walls to an existing concrete foundation is with the use of steel straps fastened to the face of a concrete stem wall with concrete screw anchors. Typically we use Simpson’s MST straps for this purpose. Other potential straps include the MSTCM and HST.

It can be challenging when using this detail to ensure sufficient ductility. ACI 318R-19 17.10.6.3 requires concrete anchor connections to be designed for overstrength forces, unless the designer can show that a ductile fuse in the system limits the maximum force that can be transferred to the concrete. For tensile anchors and threaded rods, this process is well defined by ACI 318R-19 17.10.5.3(a), but the procedure is less clear when working with steel straps. This article documents our process for using steel straps in ductile tie-down connections.

Summary Results

The following table summarizes the expected yield strengths of the Simpson straps most commonly used in retrofit applications to concrete, along with the corresponding minimum concrete breakout cone depths required to achieve a ductile connection.

Strap	Expected Yield Strength	Minimum Concrete Breakout Cone Depth*
MST27/37	10,460 lb	16 3/4 inches
MST48	11,060 lb	17 1/2 inches
MST60/72	14,460 lb	23 inches
HST2	21,440 lb	(not achievable)

* Concrete breakout strength calculation is based on 2,500 psi concrete, cracked, with a Ψ_c,V factor of 1.0 (no supplemental reinforcement). These assume a 6 inch concrete stem wall at a midwall condition (not near the end of the stem wall).

Background: Expected Yield Strengths

ACI 318R-19 17.10.6.3 subsection (a) allows you to design the concrete connection based on the maximum shear load that can be transmitted to the concrete based on a ductile yield mechanism, including the effects of strain-hardening and material overstrength in the ductile component. In other words, you can design the concrete connection based on the expected strength of the weakest upstream element. The concept of expected strength comes up frequently in seismic design for steel systems.

Structural steels typically specify a minimum yield stress (F_y), which is the lowest stress value at which a material batch might yield. Because of material and production variations, the stress at which the steel is actually expected to yield is higher than the minimum yield stress. The structural codes define the expected yield stress as R_yF_y, where R_y is the ratio between expected and minimum yield stress. The AISC Seismic Provisions (AISC 341) includes tables of R_y for various structural steel grades.

Most Simpson straps are made from ASTM A653 or ASTM A1011 steel. Unfortunately, these specifications do not include a well-defined expected yield stress or R_y factor. With a bit of digging, however, we found that these straps are commonly specified for Cold-Formed Steel Light Frame Strap Braced Wall Systems, where the primary system ductility is provided through tension yielding of the strap bracing. The North American Standard for Seismic Design of Cold-Formed Steel Structural Systems (AISI S400) provides a table documenting R_y values for the typical steel strap grades. This table is reproduced here:

Expected Strap Strength Calculations

Using the above table, we can calculate the expected yield strength for each of the available bolted straps in Simpson’s catalog. Using these expected yield strength, we can then calculate the minimum concrete-controlled strength required to ensure a ductile failure mechanism, which lets us ignore overstrength when designing the concrete connection. Note that the expected steel strength and minimum required concrete strength are compared on a nominal (unfactored) level. This follows the procedure outlined in ACI 318R-19 17.10.5.3(a). To provide an additional factor of safety, we have marked up the minimum required concrete strengths by a factor of 1.2 (on the conservative assumption that the R_y factors in the table above do not include their own safety factor).

The “minimum factored concrete breakout strength” calculated at the end of each strap assumes a shear failure (φ = 0.7) and includes a seismic reduction factor of 0.75 for SDC C, D, E, or F.

MST27 & MST37

Strap width, W = 2 1/16 in
Strap thickness, t = 0.0975 in (12 gage)
Cross-sectional area, A_s = (2 1/16 in)(0.0975 in) = 0.2011 in² (conservatively neglect holes)
Expected yield stress, R_yF_y = (1.3)(40 ksi) = 52 ksi
Expected strap yield strength, N_sy = A_sR_yF_y = (0.2011 in²)(52 ksi) = 10,460 lb (nominal)

Check required concrete strength to ensure ductile failure:
Minimum nominal concrete-controlled strength, N_c > (1.2)(10,460 lb) = 12,560 lb
Minimum factored concrete breakout strength, φN_cb > (0.7)(12,560 lb) = 8,792 lb

MST48

Strap width, W = 2 1/16 in
Strap thickness, t = 0.0975 in (12 gage)
Cross-sectional area, A_s = (2 1/16 in)(0.0975 in) = 0.2011 in² (conservatively neglect holes)
Expected yield stress, R_yF_y = (1.3)(42 ksi) = 54.6 ksi
Expected strap yield strength, N_sy = A_sR_yF_y = (0.2011 in²)(54.6 ksi) = 10,980 lb (nominal)

Check required concrete strength to ensure ductile failure:
Minimum nominal concrete-controlled strength, N_c > (1.2)(10,980 lb) = 13,180 lb
Minimum factored concrete breakout strength, φN_cb > (0.7)(13,180 lb) = 9,226 lb

MST60 & MST72

Strap width, W = 2 1/16 in
Strap thickness, t = 0.1275 in (10 gage)
Cross-sectional area, A_s = (2 1/16 in)(0.1275 in) = 0.2630 in² (conservatively neglect holes)
Expected yield stress, R_yF_y = (1.3)(42 ksi) = 54.6 ksi
Expected strap yield strength, N_sy = A_sR_yF_y = (0.2630 in²)(54.6 ksi) = 14,360 lb (nominal)

Check required concrete strength to ensure ductile failure:
Minimum nominal concrete-controlled strength, N_c > (1.2)(14,360 lb) = 17,240 lb
Minimum factored concrete breakout strength, φN_cb > (0.7)(17,240 lb) = 12,068 lb

HST2

Strap width, W = 2 1/2 in
Strap thickness, t = 0.1715 in (7 gage)
Cross-sectional area, A_s = (2 1/2 in)(0.1715 in) = 0.4288 in² (conservatively neglect holes)
Expected yield stress, R_yF_y = (1.5)(33 ksi) = 49.5 ksi
Expected strap yield strength, N_sy = A_sR_yF_y = (0.4288 in²)(49.5 ksi) = 21,230 lb (nominal)

Check required concrete strength to ensure ductile failure:
Minimum nominal concrete-controlled strength, N_c > (1.2)(21,230 lb) = 25,480 lb
Minimum factored concrete breakout strength, φN_cb > (0.7)(25,480 lb) = 17,836 lb

Larger HST straps have higher factored concrete breakout strength requirements. The breakout cone size required to achieve this strength exceeds the length of any HST strap.