The great room's exterior bearing walls are framed with continuous 12-foot studs and the floor framing above them is 14 inches deep, and the approved plans carry no engineering for these walls. The prescriptive limit here is the laterally unsupported bearing wall stud height the stud table permits, which is 10 feet, plus a floor framing depth of not more than 16 inches. What is the finding?
- The wall exceeds the prescriptive story height limit, so it needs an engineered design rather than prescriptive approval
- BIt complies, because the story height comes to 13 feet 2 inches, under the 13 feet 7 inches the code permits for a story
Thirteen feet seven inches is a story height figure the code states for masonry walls. Applying a masonry allowance to a wood-framed wall borrows a real number from the wrong material.
- CIt complies, because a 12-foot stud is permitted wherever the wall is sheathed with wood structural panel on its outside face
Sheathing braces a wall against racking; it does not extend the permitted unsupported height of a stud. The stud table governs the stud, sheathing or no sheathing.
- DIt fails, and the remedy is to double the studs at 16 inches on center, which restores the prescriptive limit without engineering
Doubling studs changes the compression capacity and the spacing but not the unsupported height, which is what the limit is about. It also is not a substitution the prescriptive provisions offer.
Why A is correct
The prescriptive limit is stated as the permitted stud height plus a limited floor framing depth. A 12-foot continuous stud exceeds the 10-foot stud height the table allows, so the prescriptive provisions no longer cover the wall and an engineered design is required.
What this question is testing
Whether the candidate knows the prescriptive provisions are bounded by story height, can assemble that limit from stud height and floor depth, and knows the remedy is design rather than a field substitution.
On the job
Tall great room and stairwell walls are the most common place a house steps outside the prescriptive provisions without anyone saying so. What the inspector looks for is a continuous stud from plate to plate with no floor or blocking line breaking it, and then for engineering in the approved set. Balloon-framed gable ends behind a two-story window wall are the same problem, and they usually come with a strap and blocking detail somebody has to have drawn.
Memory technique
Ten feet of stud plus sixteen inches of floor is the prescriptive ceiling.
Exam tip
Prescriptive rules have an envelope. A 12-foot stud is usually outside it.
Where to look it up
IRC Chapter 3, building planning, story height, read with the stud table in Chapter 6.