Questions and Answers corresponding to AWC Webinar: STD 312 – 2012 Wood Frame Construction Manual – Part 3: Connections

Q: How critical is attachment of rafter tie & collar tie to rafter?

A: If there are no rafter ties or ceiling joists (or if the rafter ties or ceiling joists aren’t adequately attached to the rafters), the roof framing must be constructed using a ridge beam so the rafters do not impose an outward thrust at the top of the wall. If there are no collar ties (or if they aren’t adequately attached to the rafters), then the tension load due to uplift has to be designed to transfer via ridge straps per WFCM Table A-3.6.

Q: In your discussion, is the "rafter tie" assumed to be 16"o.c., or can they be at 48" or 72" o.c., as a collar beam situation?

A: Tabulated rafter tie connection values assume a rafter tie is attached to each set of opposing rafters. See WFCM Table 3.9 and 3.9A.

Q: Can a rafter tie be eliminated by use of a mechanical connection between the top plates and the rafter. And at what width do you attain when a rafter tie is required.

A: No. If there is no ridge beam, the outward thrust (perpendicular to the plane of the wall) has to be resisted by either a ceiling joist or rafter tie. See Footnote 1 to Table 3.26A-B.

Q: Can you eliminate the collar tie with the use of a mechanical connection over or at the ridge beam.

A: Yes, see WFCM 3.2.6.1 for ridge connection requirements.

Q: Do the collar ties also provide some "shared stiffness" with unbalanced roof loading (drifts)?

A: It is not considered in the analysis. The rafters are designed for the maximum unbalanced loading required by ASCE7-10.

Q: Is there a way to recognize the rafter tie as it is and not mistake it as a bottom chord for light storage in an attic?

A: If a ceiling joist is also used as a rafter tie, the ceiling joist would have to be designed for gravity loads per WFCM Table 3.25B for attics with storage. The fastener requirements for a rafter tie are shown in WFCM Table 3.9 for rafter/ceiling joist heel joint connection requirements.

Q: Figure 3.8a, you implied that the hold-down takes compression loads down into the foundation. I thought it only worked in tension, and the compression just transferred on its own.

A: There was confusion during the presentation. The compression is taken through the endpost studs.

Q: I don't believe the bolt in the shear wall anchor is intended to take compression due to the "slenderness" of the bolt as it goes through the floor framing.

A: There was confusion during the presentation. The compression is taken through the endpost studs.

Q: I assume that uplift (tension) connectors are needed at both ends of the wall for wind direction reversals?

A: Yes, hold-downs are required at each end to provide a continuous load path. See WFCM Section 3.4.4.2.3.

Q: If the corner walls from both directions are used as shear walls, can we use one hold-down for one direction only?

A: Yes. WFCM Section 3.4.4.2.3 allows a single hold-down to be used to resist overturning forces in both directions where full height shear wall segments meet at a corner, provided the corner framing in the adjoining walls is adequately fastened to transfer the load.

Q: In connection 4 can we substitute hold-down with the tension straps.

A: Yes however, the strapping needs to be sized for the overturning forces and extend over the floor beam.

Q: What would you recommend for the tension hold-down force on a perforated wall? The standard calculation is force times height divided by width; however, the perforated wall cannot account for that entire width.

A: WFCM Table 3.17F provides minimum requirements for segmented and perforated shear wall hold-down capacities and are based on vtabulated (plf) * Hwall (ft) (e.g. Overturning force can be calculated as unit shear strength, plf, times height, ft.) This approach also works well for standard full height shear wall segments.

Q: Table 3.17A it was not made clear that you need to use the adjacent wall length to determine the amount of shear wall length you need. It sounded like you need more shear wall on the longer wall.

A: The header in the Table 3.17A just below the wind speed states that the shear wall length is the “Minimum Length of Full Height Sheathing on Exterior Shear Walls Perpendicular to Building Dimension, L or W (ft)”

Q: Is there a hierarchy, that is to say, one shear wall type that is better suited than others in 100 mph zones?

A: One shear wall type or method is not preferred over another.

Q: What is the limit on height of studs in design of perforated or segmented shear wall design? A: WFCM 2.1.3.3a allows loadbearing walls to be 20’ in height, but requires engineering analysis. WFCM 3.1.3.3a limits prescriptively designed loadbearing wall heights to 10’ and non- loadbearing wall heights to 20’.

Q: What if wall height is 11'?

A: WFCM 2.1.3.3a allows loadbearing walls to be 20’ in height, but requires engineering analysis. WFCM 3.1.3.3a limits prescriptively designed loadbearing wall heights to 10’ and non- loadbearing wall heights to 20’.

Q: What level of detail is appropriate for connection information on contract documents? Design Reactions only? Reference to a nailing standard? Details depicting mechanical connections/ hardware? All of the above?

A: We are unsure to which “contract documents” you are referring, but it will depend upon the local jurisdiction and what they require.

Q: If multiple connectors are used, are their capacities additive?

A: If you are asking about mechanical connectors, that is a question for the connector manufacturer. For multiple fasteners and/or fastener types, there are specific design requirements (see NDS 10.1.4 and 10.2.2). In general, capacities of different fastener types cannot be directly summed.

Q: Can you use toe nails to account for lateral & shear and allow the connector to only handle the uplift?

A: Yes, this is allowed as a prescriptive alternative in accordance with WFCM Section 3.2.1.2 and Table 3.4A.

Q: Does a unity check need to be made for a nail fastener subject to both uplift and lateral load in the diaphragm? E.G. Design uplift/Allowable uplift + Design Lateral/Allowable Lateral <1

A: Yes. See WFCM Commentary to Table 3.10.

Q: However, if the load tributary area is small it probably should still be C&C rather than MWFS loads as the local gust and pressure effects may be greater than the Main Force System average level for the entire structure

A: Correct. In the WFCM, local uplift of the sheathing is calculated with C&C loads and is based on the tributary area of the nail. The lateral loads on the roof diaphragm and the required diaphragm nailing is based on MWFRS loads.

Q: Isn't that really unlikely though? Wind really only blows one direction at a time. You will have F1 and F2 typically non-concurrent A: If you are referring to slide 13, you are correct. The WFCM considers the parallel-to-ridge and perpendicular-to-ridge load directions separately (in slide 13, these are analogous to the F1 and F2 load directions, respectively). However, roof-to-wall connectors such as the ones depicted in slide 13 may be subjected to uplift loads simultaneously with the horizontal (F1 and F2) load directions. Thus, these simultaneous loading conditions must be considered. These concurrent loading conditions can be resisted by a single connector if rated for concurrent loading for loads per WFCM Table 3.4, or can be resisted with two different fasteners specified to handle uplift and lateral or shear loads. See WFCM Table 3.4A for prescriptive lateral and shear connector requirements for rafter and/or ceiling joist to top plate. See WFCM Table A-3.4 for prescriptive uplift strap provisions for connecting the roof assembly to wall assembly.

Q: Does the 2012 WFCM have tables for smaller gun nails? Engineers always design with common or box nails and framers always use smaller gun nails. This causes weak structures.

A: The WFCM primarily specifies common and box nails for connections. WFCM Table 3.1 provides a prescriptive nailing schedule. For power driven nails, the International Staple and Nail Tool Association (ISANTA.org) develops information on power driven nails that are judged equivalent to specified common and box nails (see ISANTA’s ESR-1539).

Q: Do the prescriptive connections (such as for anchor bolts) in the International Residential Code meet any design criteria requirements as you've discussed? What issue year shifts from marginal to reasonable criteria?

A: The IRC fastener schedule is the minimum required for all structures within its scope. Use of the WFCM generally exceeds the wind speed thresholds and the IRC minimum fastening requirements.

Q: Fig 3.2k does putting 5 connector nails in the gusset plate affect the integrity of the truss connection?

A: Based on review of connector manufacturer’s literature and TPI 1-2002, nailing through the connector plate is permitted as long as nails are not located in critical locations. In this case, the “critical location” is generally defined as being within 1” of the heel joint line.

Q: Will spacing of nail in the field be the same as that of edge when you design the roof decking attachments to rafter or trusses?

A: WFCM Table 3.10 provides roof sheathing attachment requirements for wind (suction) loads based on the spacing of roof framing. Both edge and field nailing requirements are shown in the table and are sometimes the same depending on wind speed, panel location on the roof, and specific gravity of the framing members.

Q: Will spacing of nail in the field be the same as that of edge when you design the board decking attachments to rafter or trusses?

A: When using board decking, WFCM Table 3.10 provides nailing requirements for the board to each support. The terms edge and field nailing are not directly applicable.

Q: Plywood seams just need to be blocked

A: See WFCM Section 3.5.4.2 for roof sheathing edge support requirements. See WFCM Section 3.3.4.2 for floor sheathing edge support requirements. See WFCM Section 3.4.4.2 for wall sheathing edge support requirements.

Q: Again, I have the same comment as last week: If you don't have blocking at the edge of the roof diaphragm, how do you nail the diaphragm chord per the requirements?

A: See WFCM section 3.5.1.3 for depth-to-thickness ratios requiring blocking. Shear transfer through rafter/ceiling joist to top plate connections is assumed to happen without blocking if depth-to-thickness ratios are in accordance with WFCM 3.5.1.3. This assumption only applies for unblocked diaphragm capacities in accordance with WFCM Chapter 3 requirements.

Q: What is the maximum roof pitch angle, beyond which the sloped roof can no longer be considered an effective resistive diaphragm element for transfer of lateral loads?

A: WFCM 2.1.3.4d and 3.1.3.4d limits the maximum roof slope to 12:12.

Q: Your last diagram showed uplift looking like it was vertical when in fact it would be perp to slope...if vertical load then you would have to use hankinson formula

A: If you are referring to Slide 8, you are correct that it acts perpendicular to the slope. It is hard to show that direction in that figure.

Q: Can you talk about how wind forces are distributed along various wall lines?

A: The WFCM assumes flexible diaphragms and no internal shear walls, so diaphragm shear forces are distributed to vertical resisting elements (i.e., shear wall lines) based on tributary area. See ASCE 7 for more information.

Q: Are wind exposures to be considered separately in each 45 degree direction as in previous ASCE 7 issues?

A: See ASCE7-10 commentary C26.7.4.

Q: How to transfer a load in case of piling house if walls are not perfectly aligned with stringers?

A: WFCM Sections 2.1.3.2c and d and 3.1.3.2 c and d limit cantilevers and setbacks in floor systems to maintain load path. There may be a variety of options for an engineered load path.

Q: How is the 145# resisted at the level of the wall plate?

A: In slide 22, the 145# load is acting parallel to the length of the wall bottom plate. Typically nails from the bottom plate into the band joist (not shown) will be used to provide for shear transfer at that location. The shear load path into the pile also includes connection of the band joist to the support beam (shear connection not shown) and connection of the support beam to the pile (shown as consisting of 4 through bolts).

Q: When would you suggest the use of "Structural 1" rated panels in lieu of non-structural 1 rated panels for shear walls and diaphragms?

A: Tabulated values for both Structural I and Structural Sheathing are provided in the WFCM and SDPWS standards. Either option is permitted.

Q: Is this the publication that makes the 2001 WFCM manual obsolete?

A: The 2012 WFCM is referenced in the 2012 IRC and 2012 IBC. The 2001 WFCM is still referenced in previous building codes, so would still be applicable in jurisdictions enforcing these earlier codes.

Q: Does WFCM legally supersede IRC?

A: There are instances where the 2012 IRC recognizes the WFCM. In R301.1.1, the WFCM is recognized as an alternative to the code, but subject to the limitation of the code. So, the code still prevails in case of more conservative requirements. In R301.2.1.1 the code defers to the WFCM (among others) when the wind speed equals or exceeds110 mph for the design of the building, but other IRC content still applies.

Q: Can we interpolate tabular values from WFCM

A: WFCM 2.1.4 and 3.1.4 provides guidance on interpolation. It is permitted unless noted otherwise in table footnotes.

Q: Slides 30 & 32 are labeled "Lateral" should be "Shear". WFCM convention is lateral is out of plane loads, we're talking about in-plane loads

A: Correct.

Q: I missed last week. Was there any discussion of Sheathing - Stud composite behavior for normal to surface loading (wind)?

A: A recording of last week's webinar is available at http://www.awc.org/helpoutreach/ecourses/index.php#std311-2012. Also, you can download a PDF of the accompanying slide presentation here: http://www.awc.org/pdf/education/20130911_webinar_wfcm2_loadpaths.pdf. However, there was not discussion about sheathing – stud composite behavior.

Q: Why were wind load speeds changed in ASCE 7-10 to ultimate wind to get about same psf loads?

A: See Webinar 1 and the accompanying Q&A.