Holding the roof down: bolts, steel plates and the uplift load

Hibbeler Chapter 3 found 870 lb of wind uplift on the 10×10 ft foundry roof against 700 lb of dead weight, and concluded that the beam-to-post connections need real hardware, not gravity or friction. It left two things open: how much the connection must carry, and what a bolt or steel plate can hold. This chapter does both.

What the connection must carry

Chapter 3 netted the uplift against the full dead weight (870 − 700 = 170 lb). The allowable-stress rule for uplift is 0.6 × dead load + 0.6 × wind (ASCE 7-16, 2.4.1): only 60% of the dead weight counts as resisting, because a roof can be lighter than assumed. The chapter's wind speed is already at the allowable-stress level, so the net to design for is 870 − 0.6 × 700 = 870 − 420 = about 450 lb across the whole roof. Chapter 3 now says so. How that splits between the posts and the rear wall depends on the real post count, which is still an open item; the calculator starts with all 450 lb through a single connection, which is conservative.

How a bolt differs from a lag screw

• A through-bolt is full body: the equations use its full diameter (Breyer Chapter 2 used the smaller thread root diameter for lag screws). It bears on both members and is clamped by a washer and nut.
• A bolt can go through three members, which doubles the number of shear planes. The NDS yield equations for double shear carry the same terms, doubled where both side members work.
• A steel side plate changes the picture: steel bears at 87,000 psi (A36), so the wood in the main member and the bolt's own bending set the capacity, not the plate. The plate itself still has to be designed as steel (NDS 10.2.3).

The calculator reproduces the NDS's own tables for these cases: all 72 blocks of Tables 12A, 12B, 12F and 12G (wood to wood and wood to a 1/4 in steel plate, in single and double shear, across ten wood groups), 5,510 published values in all, to within 10 lb of rounding, and the group action factor against Table 11.3.6C for steel plates (396 entries, none off by more than 0.01).

One bolt at the post, Southern Pine

A 2× beam (1.5 in) bolted through a 4×4 post (3.5 in). The uplift pulls the beam straight up: that is across the beam's grain and along the post's grain.

• 1/2 in bolt, single shear, wood to wood: Z = 400 lb. With the wind load duration factor of 1.6 that is about 640 lb, more than the 450 lb net uplift. It would drop to about 450 lb if the wood stayed wet (×0.7).
• 5/8 in bolt: about 565 lb (900 lb with wind). 3/4 in bolt: about 665 lb.
• The same bolt with the post between two beam plies (double shear): about 800 lb (1,280 lb with wind).
• A 1/4 in steel plate on each side of the post, load along the post grain: about 1,720 lb for a 1/2 in bolt (2,750 lb with wind). One plate on one side: about 860 lb (1,370 lb with wind).
• Each bolt past the first adds a little less than its own value because of group action: three 1/2 in bolts in a row in steel plates carry about 99% of three times one.

The distance that catches people: the post's end

Uplift makes the bolt push up against the wood above it, toward the top of the post. That is a bolt bearing toward the end of the member, and the NDS asks for 7 diameters of end distance for the full value (3.5 in for a 1/2 in bolt), with 3.5 diameters as the absolute minimum at half value. A bolt only an inch below the top of a 4×4 is below that minimum and tears out the top of the post. In Appendix E terms the post's row tear-out is about 1,960 lb with the bolt 3.5 in down and only 980 lb with it 1 in down, but the bolt's own value is also cut by the geometry factor well before that. Put the bolt at least 3.5 in from the post top, and 1.5 diameters (0.75 in) from the edges.

Answer

One 1/2 in through-bolt at each beam-to-post joint, 3.5 in down from the post top, carries about 640 lb of wind-duration uplift, more than the whole roof's net uplift of about 450 lb, so even with only two or three posts sharing it there is a wide margin. That is the wood. The bolt does nothing unless the hardware is real: a cut washer under the head and the nut, a snug accurate hole, and a post bolted to a footing with a connection that holds the same load from below. Steel plates or double shear roughly double or triple the bolt's value if a joint ever needs it.

Where to be cautious

• Drill the hole 1/32 to 1/16 in larger than the bolt, accurately aligned through all members, and never drive the bolt with a hammer (NDS 12.1.3.2). Put a standard cut washer, or a metal plate or strap of at least equal size, under both the bolt head and the nut (12.1.3.3).
• The wet service factor of 0.7 applies if the wood stays above 19% moisture; an outdoor roof post can be wet more than you think.
• The calculator does not apply the geometry factor if a distance is short, so keep the end distance of 7D and the spacing of 4D.
• The steel plate, its own connection to anything else, bolt tension and the strap or hanger hardware are separate checks, designed by the steel rules. The numbers here are for the wood-and-bolt side only.
• The post-to-footing connection is not covered: it must hold the same uplift from below.
• Everything here is a worked example. A roof over people or fire is for a structural engineer to design and stamp.

Open items for a later pass

The real post count and how the uplift splits across the posts and rear wall (Chapter 3's open item). The post-to-footing connection and the strap or hanger hardware. Steel plate thicknesses other than 1/4 in (the NDS tables assume 1/4 in), glued laminated timber, loads at an angle, and the bolt's own tension strength (the wood under the washer is checked in Breyer Chapter 7: a standard 1/2 in washer limits a hold-down to about 700 to 890 lb of bolt pull). The Ogopogo joints (Hibbeler Chapter 8) with bolts and plates in place of lag screws.

Sources

• American Wood Council, NDS 2018 with Commentary (free at awc.org): Chapter 12, Table 12.3.1A (yield equations, single and double shear), Table 12.3.1B, Table 12.3.3 (bearing strengths), 12.1.3 (bolt installation), 12.5.1 (distances), Tables 12A, 12B, 12F and 12G (bolt values); Chapter 11, Equation 11.3-1 and Tables 11.3.6A and 11.3.6C (group action), Table 11.3.3 (wet service); Appendix E (tear-out). AWC Technical Report 12, Appendix A (steel bearing strength 87,000 psi, bolt bending yield 45,000 psi).
• ASCE 7-16 Section 2.4.1 (the 0.6D + 0.6W uplift combination), read in secondary summaries rather than the standard itself, including a SEAOC Wind Committee article that argues against using full dead load against allowable-stress wind.
• Written from these public documents, not from the Breyer textbook itself. If the book disagrees, the book and the current NDS win.