Resolving Field Problems

Because the controlling component in shear wall design is the nailing, it is no surprise that the correction for field problems generally revolves around the installation of additional fasteners. In some instances we can continue to use nails, but where nails are spaced closely or where the materials have low moisture content due to aging, wood screws are the right choice. Here's our hit list:

Improper Nailing

Too Close To Edge Of Panel
Subdrill and add wood screws as shown.

Too Close To Edge Of Framing Member
Supplement with another 2x stud, internailing the new and existing stud together to transfer the whole shear load.

Into Too Narrow A Stud - The 1997 Uniform Building Code requires 3x studs at panel edges where the design shear exceeds 350 pounds per foot. This is a change from previous codes in that 2x framing was commonly used at 4""o.c. nailing. Word will be slow to get around and sure as not you'll be looking at a wall with 4" o.c. nailing and 2x studs.

• Solution One: The wall may be nailed at 4" o.c. because you just exceeded the allowable shear for 6" o.c. nailing. If you can show that the design shear on a wall is less than 350 pounds per foot, your assembly is acceptable.
• Solution Two: Add studs each side of the plywood joint and provide edge nailing to them. Internail the 2x's to transfer forces one to the next. The loss of strength in the wall due to inboard nailing is (3/48) times the Code allowable load…for a net reduction in strength of 6.25%.
• Solution Three: Add sheathing to the inside face of studs under the gypboard. Edge nail to different members at 6" o.c. The combination of interior and exterior sheathing will make up for the loss of strength caused by being limited to 350 plf.

Overdriven Nails - Treat as if the nail was misplaced and follow the recommendations for adding screws.

Box Versus Common Nails - The Uniform Building Code accepts galvanized box nails as equivalent alternates to common nails for shear wall nailing. The galvanization increases the diameter of the thinner box nail, making the plywood think it is bearing on a common nail. If you are using untreated box nails, the shear wall has lost significant strength.  This formula will permit you to calculate the reduced allowable load:

[(Allowable lateral load for box nail) / (Allowable lateral load for common nail )] * (Shear nailing value per Code)

Partial Head Nails - As was noted previously, buckling of the plywood is one of the mechanisms by which shear walls tend to fail. The plywood is allowed to buckle because it is insufficiently anchored to the framing lumber. It tends to fail by slipping over the head of the nail. As might be expected, partial head nails lack a full circular nail head. Thus their ability to resist buckling is less than a full head nail.

Nailing To The Wrong Member
The limits of the shear wall are indicate by the arrows. But when the framer installs sheathing over the entire face of wall, all indications of the thicker member attached to the hold-down are obscured.  Chalk lines snapped on the exterior of the plywood would minimize the problem.

Improper Materials

Stud Grade - The grade of a stud (No.1, No. 2, Construction, Utility) has some bearing on the strength of a shear wall. Material properties that vary according to this classification alter a stud's allowable strength in compression. Overturning forces can significantly increase the compressive load on studs close to the ends of walls. The solution is to add sister studs to those already present.

Stud Species - The specific gravity (G) of wood varies from species to species. Fastener lateral strength is decreased with decreasing specific gravity. According to the current Code…

Nail Design Values for Single Shear Connections - Combined Table
(Both members of identical species)
Side Member Thickness (inches)	Nail Length (Inches)	Nail Diameter (Inches) D		Penny Weight	G=0.55 Southern Pine (lbs.)		G=0.50 Douglas-Fir Larch (lbs.)		G=0.42 Spruce-Pine-Fir (lbs.)
ts	L	Common	Box		Common	Box	Common	Box	Common	Box
0.5	2	0.113	0.099	6d	67	55	59	48	47	38
	2.5	0.131	0.113	8d	85	67	76	59	61	47
	3	0.148	0.128	10d	101	82	90	73	73	59
	3.25	0.148	0.128	12d	101	82	90	73	73	59
	3.5	0.162	0.135	16d	117	89	105	79	87	65
	4	0.192	0.148	20d	137	101	124	90	103	73
	4.5	0.207	0.148	30d	148	101	134	90	112	73
	5	0.225	0.162	40d	162	117	147	105	123	87
	5.5	0.244	-----	50d	166		151		127
	6	0.263	-----	60d	188		171		144
0.75	2	-----	0.099	6d		61		55		47
	2.5	0.131	0.113	8d	104	79	90	72	70	57
	3	0.148	0.128	10d	121	101	105	87	83	68
	3.25	0.148	0.128	12d	121	101	105	87	83	68
	3.5	0.162	0.135	16d	138	108	121	94	96	74
	4	0.192	0.148	20d	157	121	138	105	111	83
	4.5	0.207	0.148	30d	166	121	147	105	119	83
	5	0.225	0.162	40d	178	138	158	121	129	96
	5.5	0.244	-----	50d	182		162		132
	6	0.263	-----	60d	203		181		149
1	2.5	-----	0.113	8d		79		72		61
	3	0.148	0.128	10d	128	101	118	93	96	79
	3.25	0.148	0.128	12d	128	101	118	93	96	79
	3.5	0.162	0.135	16d	154	113	141	103	109	86
	4	0.192	0.148	20d	183	128	159	118	124	96
	4.5	0.207	0.148	30d	192	128	167	118	131	96
	5	0.225	0.162	40d	202	154	177	141	140	109
	5.5	0.244	-----	50d	207		181		143
	6	0.263	-----	60d	227		199		159
1.5	3.25	-----	0.128	12d		101		93		79
	3.5	0.162	0.135	16d	154	113	141	103	120	88
	4	0.192	0.148	20d	185	128	170	118	144	100
	4.5	0.207	0.148	30d	203	128	186	118	158	100
	5	0.225	0.162	40d	224	154	205	141	172	120
	5.5	0.244		50d	230		211		175
	6	0.263		60d	262		240		191

If you reference the plywood shear wall tables in the building code, you'll see a message similar to the following:

Allowable shear values for nails in framing members of other species shall be calculated for all other grades by multiplying the allowable shear for nails in Structural I by 0.82 for species with a specific gravity greater than 0.42 but less than 0.49, and 0.65 for species with a specific gravity less than 0.42.

Sheathing Type - As we've seen in the section on sheathing selection, the difference between Structural I and Structural II sheathing can have an effect on the strength of a shear wall. But for all the hooplah made about Structural I panels, the real difference in strength is less than 15 percent. The allowable shear on Structural II panels can be found in the UBC Tables.