Structural Design Considerations for Bearing Wall Top Plates that Support Concentrated Loads
Design and detail issues where trusses/joists don’t align with bearing wall studs
For light-frame wood structures, cost-effective material use often means changing the stud spacing of bearing walls from one level to another, resulting in spacing that differs between levels or from the trusses/joists being supported.
In many cases, bearing wall stud design is controlled by the bearing area of the stud on the wall’s bottom and top plates. For large bearing wall loads resulting from longer-span floor joist/trusses or accumulated multiple floor loads, this can drive stud spacing to 16-in. o.c., with the studs supporting truss/joist reactions every 24-in. This creates a scenario where the wall double top plate needs to act as a structurally-spanning member, resisting a concentrated load (truss or joist reaction) at the middle of a stud bay, near the end of a stud bay, or anywhere in between.
When considering how to address this issue, there are a number of questions at play, such as:
- If using typical double plate construction with staggered joints, do both plates act together as a composite member and, if so, does adequate nailing between the two need to be provided for shear flow?
- What is the load distribution between the top plates if they are assumed to act independently of each other? Does the top plate take as much load as it can and then the bottom plate take the rest? Is the load evenly split between the two plates?
- What justification is there for this load transfer between plates (regardless of what magnitude is transferred)? Is equal deflection/stiffness of the two plates assumed? Is direct bearing of the top plate on the bottom plate considered adequate for load transfer? Is load transfer between the two plates different when looking at moment vs. shear?
- Should the analysis assume both plates are simple span for one stud bay, should one of the plates be assumed continuous for several adjacent bays, or should both plates be continuous for several bays?
- Will the wall gypsum or wood structural panel (WSP) sheathing contribute to resisting the vertical forces acting on the top plate?
This condition is not specifically addressed in the International Building Code (IBC), National Design Specification (NDS®) for Wood Construction®, or other codes or referenced standards. There are also multiple methods of conducting this analysis, which can produce significantly different results.
The least conservative analysis is to assume that the two plates are acting together as one composite, 3-in.-deep member for both shear and bending. Although some engineers use this method, it can be unconservative, since nailed connections in wood members are relatively flexible and the typical nailing pattern between the two plates is not even close to being adequate to make them act compositely due to nail slip. The textbook shear flow equations don’t work well for light-frame wood construction since they are based on rigid connections (such as built-up steel I-shapes with welded connections). This makes achieving adequate shear flow transfer between the two wood plates unreasonable.
The general consensus among many engineers is to assume that the two plates act independently with the load distributed to each plate based on deflection/stiffness. In other words, for common top plate construction, half the load goes to each plate.
As far as splice locations and looking at continuous vs. simple span members, there should be some offset in top plate splices to provide continuity in at least one top plate member. The worst-case approach of both analyses should be used. A simple span will produce the worst bending effects and a continuous span will produce the worst shear effects.
The downside to this approach is that the capacity of the plates is low. A (2)-2×6 spruce-pine-fir top plate with studs at 16-in. o.c. has a truss or joist reaction capacity of approximately 1,000 to 1,400 lbs, depending on load location. The American Wood Council’s Wood Frame Construction Manual (WFCM) prescriptively limits floor joist spans to 26 ft to address this issue. The WFCM Commentary to Section 18.104.22.168a states:
C22.214.171.124a Framing Member Spans. Framing member spans are limited to 26 feet for floors based on the bending capacity of the double top plate supporting floor framing members. The worst-case assumption is that a floor framing member bears directly between two studs creating a concentrated load at mid-span of the top plates. Section 126.96.36.199g required band joists, blocking, or other methods to transfer roof, wall, and/or floor loads from upper stories to alleviate the concern of additional loads being transferred through the floor framing members into the top plate.
If the method mentioned above is used, any moderate-to-long span truss will have reactions requiring top plate reinforcing or adjustments in stud/truss/joist spacings. This is not to say that other analysis methods that produce larger plate capacities can’t be used, but justification for those methods may be more difficult to achieve.
Options for achieving larger plate capacities include using a solid 3x or 4x plate, using a triple 2x top plate, adjusting stud or truss/joist spacings to make them align, adding studs under trusses/joists that don’t align with typical stud layout, or using a higher strength/grade or engineered wood top plate. If a triple top plate is used, a repetitive member factor of 1.15 could be used. Recall that the repetitive member factor only applies to the bending capacity and the plate capacity is frequently governed by shear capacity.
In summary, it is not uncommon in wood-frame structures to have truss and/or joist spacings that differ from the spacings of the studs in the walls supporting them, resulting in the top plate acting as a spanning member. The capacity of a conventional (2)-2x top plate is often overlooked when designing multi-story light wood-frame projects. The design process for bearing walls, as well as the floor-to-wall intersection details, should consider whether alignment of studs from one level to the next, and alignment of studs with trusses/joists, is required.