Multi-Storey Timber Frame: When Does Wind-Governed Uplift Change the Hold-Down Design?
A four-storey residential building uses a light timber frame with 38 x 140 mm studs at 400 mm centres, sheathed with 9 mm OSB on one side. The site is exposed coastal (basic wind velocity 27 m/s, terrain category 2). The designer has sized the shear walls using a simple racking check and has specified M12 bolts at 600 mm centres into a 100 mm concrete slab for the bottom storey. During a site visit the engineer observes that the hold-down brackets at the corners of the ground-floor shear walls are bolted through the bottom plate into the slab with two M12 bolts at 300 mm centres, and that the OSB sheathing stops 50 mm above the bottom plate on some panels. The building is 12 m tall, 20 m long and 10 m wide.
1. Assess whether the observed hold-down detail is adequate and identify the governing failure mode.
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First establish the overturning demand. For a 12 m tall, 10 m wide building in exposed terrain, the base overturning moment from wind is substantial; the total horizontal force acts at roughly mid-height, so the uplift at the leeward corner is approximately the overturning moment divided by the building width, minus the stabilizing dead load of the frame above. With a light timber frame the dead load is small (perhaps 2–3 kN/m² including finishes), so the net uplift at the corner can easily exceed the capacity of two M12 bolts.
The governing failure mode is rarely the bolt in shear; it is the bolt group in tension combined with the concrete breakout cone and the bottom-plate bending. Two M12 bolts at 300 mm centres into a 100 mm slab give a small breakout cone and little edge distance; the tension capacity per bolt is likely 15–25 kN, so the pair gives 30–50 kN, which may be less than the calculated uplift. Check also the bottom plate for bending between bolts and the stud-to-plate connection for the uplift being transferred. If the demand exceeds capacity, the fix is a engineered hold-down with a longer anchor or a cast-in strap, not more bolts at the same spacing.
2. The sheathing stops 50 mm above the bottom plate on some panels. What is the structural consequence and how should it be resolved?
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The racking resistance of a timber shear wall depends on the sheathing being continuous and fastened to the bottom plate so that the shear is transferred into the foundation. A 50 mm gap at the base means the lowest row of fasteners is missing or ineffective, so the panel acts as a shorter shear wall with a free bottom edge. The consequence is a reduction in racking capacity and stiffness, an increase in drift, and a concentration of shear at the remaining fasteners.
Quantify it: the missing fasteners along the base reduce the panel's unit racking resistance, and the effective height of the wall increases, which increases the overturning moment at the hold-down. The resolution is to require the sheathing to extend to the bottom plate and be fastened at the specified edge spacing, or to add a continuous blocking and a positive connection. Do not accept a cosmetic filler; the load path must be continuous from sheathing to plate to anchor. This is a classic detailing failure that a calculation alone will not catch.