Screw-type anchors for bottom plates

By - , Build 166

There are several ways to fix timber bottom plates to concrete floor slabs. One popular option for residential and commercial construction is using screw-type anchors inserted into predrilled holes.

Figure 2: Concrete masonry header block foundation.
Figure 1: Formed concrete foundation.

ANCHORS THAT FIX bottom plates to concrete slab-on-ground floors are an essential component of timber frame construction as they secure the upper building structure to the foundation.

They must be able to withstand forces in three directions – uplift (tension), along the wall (in-plane shear) and across the wall (out-of-plane shear).

Two options for fixing bottom plate

Section 7 of NZS 3604:2011 Timber-framed buildings provides options for fixing bottom plates to concrete floor slabs:

  • Cast-in anchors consisting of M12 bolts that are cast into the concrete and 50 × 50 × 3 mm square washers.
  • Tested proprietary anchors that are inserted in the concrete once it has sufficiently cured.

A range of proprietary anchors are available including anchors screwed into predrilled holes, anchors chemically grouted into predrilled holes and expanding or wedge anchors.

The focus here is on screw-type anchors inserted into predrilled holes.

Anchors screwed into predrilled holes

Screw-type anchors were initially designed as removable fasteners for fixing plant and machinery to concrete walls and floors. In residential and commercial construction, they have become a popular option when inserted into predrilled holes for fixing timber bottom plates to concrete floor slabs.

Screw-type anchors offer some advantages over other types of proprietary anchors as they are:

  • able to be accurately installed without conflicts with the framing – drill a hole in the correct location in the concrete floor, align the timber framing and install the screw anchor using either a power driver or manual socket or wrench
  • easily removed
  • less likely to cause damage to the concrete as they do not have an expanding wedge or sleeve that applies an expansion force.

A few things to consider

There are some considerations if you are going to install screw-type anchors into concrete foundations:

    • Have the anchors been tested to demonstrate they have the required capacity to meet the requirements of NZS 3604:2011? Anchor capacity must meet the performance criteria for external and internal walls described in NZS 3604:2011 paragraphs 7.5.12.3 and 7.5.12.4. These are shown in Table 1.

Table 1 Minimum holding capacities for proprietary anchors 

TYPE OF FORCEblankCAPACITY
External
walls
Horizontal in-plane (along wall) 2 kN
  Horizontal out-of-plane (across wall) 3 kN
  Vertical (uplift) 7 kN
Internal
walls
Horizontal in-plane (along wall) 2 kN
blank Horizontal out-of-plane (across wall) 2 kN
  • Can screw-type anchors be used to hold down walls containing bracing? Proprietary anchors may be used as hold-downs for walls containing bracing but require additional uplift capacity depending on the bracing unit (BU) ratings:
    • 150 BUs/m – 15 kN uplift fixings.
    • 120 BUs/m – 12 kN uplift fixings.

Proprietary anchors should also have test results to demonstrate that they have been tested and meet NZS 3604:2011 requirements for their intended use.

  • Can adequate cover and edge distance to the anchor be achieved, particularly in exposure zone D (as described in NZS 3604:2011)? Figures 1 and 2 provide two details of how the maximum cover can be achieved.
Figure 1: Formed concrete foundation.
Figure 2: Concrete masonry header block foundation.

Edge distances and spacings

NZS 3604:2011 does not give edge distance dimensions for proprietary anchors as these depend on the particular product and the slab edge detail, frame width, concrete strength and load. Anchor spacings for proprietary anchors are given in the standard as:

  • 900 mm centres maximum for in situ concrete floor slabs
  • 600 mm centres maximum where the slab edges are formed using concrete masonry header blocks
  • no more than 150 mm from each end of every plate.

Durability depends on use

The protection required for all structural fixings for all zones in closed environments may be mild steel (that is, uncoated and non-galvanised) in accordance with NZS 3604:2011 Table 4.1.

However, the standard states that, where fixings are into ACQ and copper azole-treated timber, the fixings in closed environments should be at least hot-dipped galvanised steel.

Quiz

1. Proprietary anchors fixing bottom plates can be:
a. Cast into concrete floor slabs.
b. Inserted into the concrete once it has cured.

2. Which of the following is true for screw-type anchors?
a. They need to be accurately located before installation.
b. They need to be installed before the slab has been cast.
c. They place no expansion stress on the concrete.

3. Proprietary anchors used to hold down brace walls with 150 BUs/m must have uplift resistance capacity of:
a. 9 kN.
b. 12 kN.
c. 15 kN.
d. 20 kN.

4. Edge distances for proprietary anchors are:
a. 50 mm minimum.
b. 65 mm minimum.
c. Specified by the manufacturer.

Answers: 1. b  2. a  3. c  4. c

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Articles are correct at the time of publication but may have since become outdated.

Figure 2: Concrete masonry header block foundation.
Figure 1: Formed concrete foundation.

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