The DETAL Simpson Strong-Tie Embedded Truss Anchor

In recent years the Insulated Concrete Form (ICF) system has, at least around here (Inland Northwest), brought reinforced concrete into the world of above ground residential construction. Concrete is used all the time at or below grade (around here), but for above, light frame wood construction has dominated. It took some time for some contractors to `break form' and try it. (After all, no matter how good your project looks on paper, you still need someone willing to build it.) But now it is catching on.

What is sweet is that the building code (for example, the International Residential Code, IRC) has been accommodating this trend, and provides prescriptive design paths for residential concrete construction, like it has for light frame wood construction. That means, if you follow the prescriptive provisions, you can design a concrete residence, without necessarily having to hire a structural engineer.

So, as a part of all this we see the new DETAL Embedded Truss Anchor by Simpson Strong-Tie (2009-2010 Wood Construction Connectors Catalog). Currently the prescriptive provisions of the IRC are limited to concrete wall construction, with light frame floor(s) and roof (except for the basement floor, which may be concrete slab on grade). This particular connector is for attaching light frame wood trusses to concrete walls.

The New DETAL Attaching a Truss to an IRC (ICF) Concrete Wall

Let's consider attaching a truss to the top of a 6-in. thick (core) ICF upper story wall. The calculated uplift force per Table R802.11 for a structure with Exposure Category C, Basic Wind Speed 90 mph, spanning 32 ft, with 18 in. eaves is ...

... 242 lb, plus 43.22 lb/ft x 1.5 ft eave gives ... 242 lb + 64.8 lb = 307 lb.

But, wait, Table R802.11 is for Exposure B. Increasing it for Exposure C, at a roof height of 25 ft, per Table R301.2(3) gives a multiplying factor of 1.35 ...

... 307 lb times (1.35 / 1.00) = Uplift = 414 lb.

Now let's go to page 145 of the Catalog.

DETAL20 in Concrete ... Uplift Capacity ... 2480 lb.

The allowable capacities on pages 144 and 145 are for `SP' (Southern Pine). For DF (Douglas Fir) commonly used around here we must multiply the capacity by 0.95 (See adjustment by Simpson, ... here.)

Uplift Capacity = 2480 x 0.95 = 2356 lb.

Since uplift load = 414 lb ≤ 2356 lb = capacity ... Done!

Wait; in fact, there are a lot of anchors on pages 144 and 145 that would work. Perhaps the DETAL is overkill in this situation.

Note that tables on pages 144 and 145 also provide lateral load capacities, both parallel and perp to the wall (F₁ parallel to the wall and F₂ perpendicular). Since the truss anchor connects directly to the concrete wall, the top-of-wall sill plate `goes away', and along with it the presumed anchorage provided by the (prescriptive) attachment of plate to wall (e.g., ½ in. anchor bolts @ 6 ft o.c., per R611.9, for Seismic Design Categories A and B).

Backward calculating the presumed capacities of the sill plate anchorage from Table 11E of the National Design Specification (NDS), we get (using DF with Specific Gravity of 0.50), ...

... ½ in. anchor bolt resisting loads parallel to wall (and wood grain), Z_para' = 650 lb times 1.6 for short term loading = 1040 lb every 6 ft ... divided into trusses @ 2 ft o.c. gives 1040 lb per 6 ft divided by 1 truss per 2 ft = 347 lb per truss.

Page 145 gives F₁ = 2000 lb for the DETAL20 in concrete. Adjusting for DF, F₁= 2000 x 0.95 = 1900 lb.

... ½ in. anchor bolt resisting loads perpendicular to wall (and wood grain), Z_perp' = 380 lb (again DF) times 1.6 for short term loading = 608 lb every 6 ft ... divided into trusses @ 2 ft o.c. gives 608 lb per 6 ft divided by 1 truss per 2 ft is ... 203 lb per truss.

Page 145 gives F₂ = 1505 lb for the DETAL20 in concrete. Adjusting for DF, F₂= 1505 x .95 = 1430 lb.

Considering, for example, the uplift and load resisted by F₂ acting simultaneously (say a strong stagnation pressure on the wall and underside of the eave, and suction as the wind goes up and over), we turn to page 14 of the Catalog for examining the capacity of the connector with simultaneous loads, ...

Design Uplift / Allowable Uplift + Design Lateral Perp / Allowable Lateral Perp < 1.0? ...

So, is ... (414 / 2356) + (203 / 1430) = 0.18 + 0.14 = 0.32 < 1.0? Yes!

So, we would argue that the DETAL20 accommodates the uplift attachment of truss to sill plate and sill plate to wall, in one piece of hardware.

And so would a lot of other of the truss anchors under these conditions.

Conclusion

We have illustrated how the new DETAL20 embedded truss anchor can be used to attach wood trusses to the top of a concrete wall (presumably ICF construction). With this connector the top of wall sill plate `goes away'. But, we have shown that the capacity of the DETAL20 also accommodates the presumptive capacity of the sill plate and its anchorage to the wall, thus providing resistance to uplift and anchorage for the roof (at least for Seismic Design Categories A and B). Looking at the pages 144 and 145 of the Catalog we see that a lot of other anchors would also work in this example (META, etc.). So, specifically, the new DETAL connector could be used for more robust wind (and seismic) conditions.

References

2006 International Residential Code, International Code Council, 4051 West Flossmoor Road, Country Club Hills, IL 60478.

2009-2010 Wood Construction Connectors Catalog, C-2009, Simpson Strong-Tie, 5956 W. Las Positas Blvd., Pleasanton, CA 94588, www.strongtie.com.

"from-sst-regarding-embedded-truss," reinforced-concrete.blogspot.com.

National Design Specification for Wood Construction, American Forest & Paper Association / American Wood Council, 1111 Nineteenth St., NW, Suite 800, Washington, D.C., 20036, www.awc.org.

ICF Connectors, F-ICFL08, Simpson Strong-Tie, 5956 W. Las Positas Blvd., Pleasanton, CA 94588, www.strongtie.com.