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Connection Options for Wood-Frame and Heavy Timber Buildings

Connection Options for Wood-Frame and Heavy Timber Buildings

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Earn 0.1 ICC Continuing Education Credit AWC ICC practical and technical design CONTINUING EDUCATION CONTINUING     Describe the basic categories of mechanical and joinery connections and their for modern structures, application in both light wood-frame and heavy timber . Examine the wide range of connection traditional from options available today, joinery methods to widely used and connectors to the innovative connection systems that are proprietary enabling longer wood spans, higher and taller . Apply performance effective guidance to ensure of connections in wood structures. Discuss key considerations for selection and installation of connections, for best performance and to avoid potential problems. able to: To receive credit, you are required the to entire read article and pass www.awc.org the for quiz. complete text Go and to to take free. for quiz the Learning ObjectivesLearning this article, you should be After reading 1. 2. 3. 4. This continuing education course course education This continuing WOOD CONNECTION BASICS the most appropriate determine To application, a given for connection it is necessary certain understand to starting withfundamentals, the terminology. course, education this continuing In provides an overview of provides options connection both light structures, modern wood for timber (see glossary). wood-frame and heavy connections—including mechanical covers It with plates shear and connector , as integral joinery teeth—and connections, in well as proprietary that are connections supporting in surge the current ways many selection Proper design. wood innovative potential avoid to and ways and installation, also discussed. are problems, extreme events such as hurricanes such and events extreme -disaster to According earthquakes. structural during these surveys, failures faults in connection to due often are events design assembly. or

he use of as a structural wood far grow to continues material such traditionalbeyond applications,

In wood buildings, effective connections connections effective buildings, wood In Portland Jetport, Portland, Maine Jetport, Portland, Portland : Gensler chosen was glulam Southern for this 40,000-sf because of its ability to span long distances intermediate for need minimal with supports.

Sponsored by reThink Wood reThink Wood by Sponsored T Effective solutions provide strength, stiffness, strength, Effective provide solutions stability and ductility Photo: Robert Benson Photography as single-family homes and buildings up as single-family and buildings up homes attention Although recent stories. four to the possibilityhas focused on of wood cost-effectiveness, wood’s high-rises, and light versatility footprint are of making it a material for already choice range ofan expanding types— and educational public, innovative from mass structures that utilize corporate main architectural as their timber products (and higher) five-story to expression, with built projects dimension multi-family techniques. using long-established all of to common these essential element An connections. buildings is effective stability and stiffness, strength, provide a ductility providing in addition to the . load path to continuous critical performance— They building are to and especially during loads, regular under Timber Buildings Wood-Frame and Heavy Wood-Frame Connection Options for CONNECTION OPTIONS FOR WOOD-FRAME AND HEAVY TIMBER BUILDINGS

Table 1: Example of Comparative Design Values WOOD BUILDING No. 2 Douglas- 2x or 4x Parallel to Grain Perpendicular to Grain SYSTEMS: A GLOSSARY The terms for different types of wood Compression Design Value (psi*) 775 625 building systems can be confusing, and the way they are used also varies Tension Design Value (psi) 575 No published values slightly in different parts of the world. Following is a brief glossary of terms Dowel Strength for 5,600 4,450 as they are used in this course: ¼-in.-dia (psi)

*Pounds per inch LIGHT-FRAME WOOD CONSTRUCTION is characterized by vertical and horizontal structural elements formed a connection encompasses all of the or compression perpendicular to grain by a system of repetitive wood components that attach one part of a building values. However, dowel bearing strengths members. Framing is made from element to another, be it wood to wood, wood perpendicular to grain are lower relative to standard sizes of 2x and 3x dimension to or wood to another material. Within dowel bearing strengths parallel to grain for lumber, and vertical supports are the connection are connectors and/or fasteners. larger diameter (less than ¼ in.) fasteners more numerous than with other wood Metal hangers, bases and many other (see 1). building systems. In addition to low-rise products fall within the category of connectors. buildings, four-story Type V buildings Fasteners include (among others) nails, , u Wood is weakest when tension is and five-story Type III buildings timber and bolts. These products do the applied perpendicular to grain. This are commonly light-frame wood ultimate work of fastening the elements of a should be avoided—which is why there construction. connection, either with additional connectors are no published design values for tension (such as hangers) or on their own. perpendicular to grain. MASS TIMBER CONSTRUCTION uses Several characteristics of wood are also large prefabricated wood members relevant to overall connection behavior. Second, wood connections are stronger made from cross laminated timber (CLT) First, wood is anisotropic, meaning it when the load is spread over a number of or structural (e.g., has different strength properties in different fasteners. Large concentrated loads should , laminated directions: longitudinal (strong), tangential be avoided unless designed not to exceed the strand lumber, parallel strand lumber) (weaker) and radial (weakest). wood’s strength capabilities (e.g., net tension for , and roof construction. and shear). Spreading the load also builds in laminated timber, another form of mass u Wood is stronger in compression and a degree of redundancy, which is useful in timber construction, has been utilized tension parallel to grain versus perpendicular high-wind or seismic events. To accomplish for over 20 years. It includes vertical 2x to grain; the former is the preferable way of this, designers are advised to: sawn lumber, nail laminated together to designing wood connections. However, this is create a wood floor system.Glued- not always practical or possible. u Use small fasteners (less than ¼ in. dia). laminated timber (glulam) can also be u Use multiple fasteners when possible. used in and column applications. u While wood is weaker in compression u Keep the scale of fasteners relative to the perpendicular to grain, this is the most size of wood members being connected. TIMBER-FRAME describes the structural common way to transfer loads in construction. configuration of a broad category of The most common loads are gravity loads, and Third, as with other building materials, systems, including post-frame, post-and- the typical way to transfer them is in bearing— wood moves in response to environmental beam, and heavy timber braced frame. It thus reliance on the compression perpendicular conditions. The main driver for this is characterized by the use of members to grain properties of wood. Fasteners and movement in wood is moisture. Allowances (typically larger than 4×6) arranged in connectors are required to transfer other must be made to accommodate potential two-dimensional frames at a consistent loads, such as wind uplift and the lateral loads shrinkage and swelling, particularly in interval throughout a building. caused by occasional wind and seismic forces connections. (See Moisture Effects on page 6.) to provide a continuous load path to the HEAVY TIMBER relates to foundation. CONNECTION TYPES resistance and is specifically defined as a There are two major types of connections construction type—Type IV—within the While capacities are more limited when used in wood buildings: mechanical and 2015 International (IBC). wood is loaded in compression perpendicular joinery (sometimes referred to as ). Buildings with mass timber and timber- to grain (versus parallel to grain), the limits Within these categories are many variations, frame systems can be used within the for bearing conditions on the surface of and thus a wide range of options to heavy timber category if the minimum wood members are deformation-based, accommodate virtually any building design. sizes specified for Type IV construction not strength-based, and published design are met. values can be increased for smaller bearing Mechanical Connections areas. Accordingly, dowel bearing strengths Mechanical connections used in wood are higher relative to compression parallel buildings can be classified into three major

2 rmed rmed rmed rmed rmed fo fo fo fo fo rker Nail rker Nail rker Nail rker Nail rker Nail 3 x 0.148 Common Nail 3 x 0.148 Common 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.120 2-7/8 x 0.135 Co 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.120 2-7/8 x 0.135 Co 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.120 2-7/8 x 0.135 Co 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.120 2-7/8 x 0.135 Co 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common CONTINUING Sinker Nail 2-7/8 x 0.120 2-7/8 x 0.135 Co EDUCATION 3

32 ⁄ 1 rmed rmed rmed rmed rmed fo fo fo fo fo rker Nail rker Nail rker Nail rker Nail rker Nail in. larger than the bolt diameter. A A than the bolt diameter. larger in. 16 ⁄ 1 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 2-7/8 x 0.120 Sinker Nail Nail 3 x 0.148 Common 2-7/8 x 0.135 Co 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common 2-7/8 x 0.135 Co 2-7/8 x 0.120 Sinker Nail 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.135 Co 3 x 0.148 Common Nail 3 x 0.148 Common 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.135 Co 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.113 Finish Nail 3 x 0.113 Finish 3 x 0.148 Common Nail 3 x 0.148 Common 2-7/8 x 0.120 Sinker Nail 2-7/8 x 0.135 Co 3 x 0.135 Flooring Nail, De 3 x 0.135 Flooring 2-7/8 x 0.113 Nail 2-7/8 x 0.113 Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.265 Boat Nail 3 x 0.265 Boat Nail 3 x 0.128 Bright Box 3 x 0.128 Casing Nail 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail 3 x 0.265 Boat 3 x 0.128 Bright Box Nail 3 x 0.128 Bright Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail 3 x 0.265 Boat 3 x 0.128 Bright Box Nail 3 x 0.128 Bright Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail 3 x 0.265 Boat 3 x 0.128 Bright Box Nail 3 x 0.128 Bright Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail 3 x 0.265 Boat Nail 3 x 0.128 Bright Box Lag screws are typically used when it isn’t typically are Lag screws used when it isn’t 10d Nails 10d 10d Nails 10d 10d Nails 10d 10d Nails 10d 10d Nails 10d

common mistake is not leaving enough room enough is not leaving room mistake common installation offor tightening or with nuts a device. torque other or wrench Bolts are inserted in pre-drilledBolts holes are u This chart shows nails, 10 all referred to as and10d, each with potentially different performance characteristics. Thus, specifying nail a 10d is not clear. in. to to in. possible to use a nut as required for bolts bolts for as required use a nut possible to

8 ⁄ 7 in., ¾ in., ¾ in., in., 8 ⁄ 5 CONNECTION OPTIONS FOR WOOD-FRAME FOR BUILDINGS OPTIONS HEAVY AND TIMBER CONNECTION 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 3 x 0.265 Boat Nail Nail 3 x 0.128 Bright Box Nail 2-7/8 x 0.113 Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail Nail 3 x 0.128 Bright Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail 3 x 0.128 Bright Box Nail 3 x 0.128 Bright Box 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.265 Boat Nail 3 x 0.128 Bright Box Nail 3 x 0.128 Bright Box 2-7/8 x 0.120 Cooler Nail 2-7/8 x 0.120 Cooler 3 x 0.128 Casing Nail 2-7/8 x 0.113 Box Nail 2-7/8 x 0.113 Box 3 x 0.265 Boat Nail Nail 3 x 0.128 Bright Box 10d Nails 10d Nails 10d Nails 10d Nails 10d Nails Bolt design values are tabulated in the in the tabulated Bolt design are values

u For flooring, APA – The Engineered – The Engineered APA flooring, For Nails come in many types (e.g., box types box in many come (e.g., Nails

NDS for five diameters (½ in., (½ in., diameters five NDS for u in. and 1 in.). Diameters greater than 1 in. are are than 1 in. greater Diameters and 1 in.). in. not permitted as they localized can initiate can which member, in the wood stresses brittlecause splitting other failures. or wood Wood Association recommends using a using a recommends Association Wood as described in the glued-nailed system, this In Construction. Guide Wood Engineered sheathing to glue attach is used to system, whether or lumber the substrate member, and increase squeaks minimize to an I-, All nailing action. T-beam stiffness to due Gluing glue sets. before be completed must bonding wall or for is not recommended roof framing because sheathing to it leads to may Although ductility. reduced B A, be used in Seismic Design Categories use is their R=1.5, with a reduced and C, in Seismic Design D, Categories prohibited E and F. nail, common nail, rink shank, sinker, sinker, rink shank, nail, common nail, pennyweights as well as multiple cooler), are There 60d). 6d to (the NDS tabulates also proprietary specific to nails unique the variety Due to of manufacturers. nails type, and diameter pennyweight, available, should belength specified. Designed by Path Architecture, Radiator is one of the first five- Designed by Path Architecture, story timber-frame office buildings to be built in Portland, Ore., Ore., buildings to be built in Portland, office story timber-frame since the early 1900s. Fabricated steel bucket-style connectors withsince the early 1900s. Fabricated steel bucket-style bolts were utilized for glulam beam-to-beam connections. bolts were Dowelled connections transfer the force transfer the force connections Dowelled Wood members members Wood Dowel-type fasteners. with are dowel-typeconnected fasteners mechanical the most common probably typeconnection at because effective they are transferring loads while also being relatively in They come install. to simple and efficient properties strength forms and their many Design using the National can be calculated Construction. Wood Specification® (NDS) for generally used when loads are are Nails and light multi-family as in light, relatively Staples can be used buildings. commercial capacities equivalent but ofin place nails, as the NDS doesn’t be determined need to may Screws staples. designpublish for values satisfactorybe more certain than nails under moisture) to as exposure (such conditions loose work to tendency less they have since high wind withdrawal and generally have Timber wind events. severe under resistance a high-strength dowel-type are fastener rivets withused in conjunction specially fabricated metal plates. a combination through members between of bearing of and bending dowel the dowel fastener. categories: dowel, metal connector plates plates metal connector dowel, categories: also are There and shear. with integral teeth, ofa number proprietary that connections characteristics ofcombine of each these types. Photos: Josh Partee Photography CONNECTION OPTIONS FOR WOOD-FRAME AND HEAVY TIMBER BUILDINGS

Modern Computer Numerically Controlled (CNC) connections provide the ability to fabricate joints with precision. This is especially useful when fabricating joints on large members with complex shapes. or when withdrawal capacity is required. plates. They are usually made from cast Proprietary connections. Some of the They are inserted into pre-drilled holes. The or light , and are capable of most innovative connection systems are clearance hole for the shank (unthreaded transmitting the load relying solely on proprietary—that is, produced exclusively portion of the lag ) should be the same bearing and shear resistance of the wood in by one manufacturer or designed specifically diameter as the shank, while the lead hole for the direction parallel or perpendicular to the for one building. This area has seen the threaded portion is a variable percentage grain. They can be used to connect wood to considerable growth in recent years, with of the shank diameter based on the wood’s wood members or wood to steel, and may be connection systems and products developed specific gravity. Because lag screws can concealed or visible. for use in larger and more complex buildings be fully threaded, the NDS assumes fairly Profiled using special , being designed to take advantage of wood’s conservative design values which are based split rings are usually fitted in a circular economic, aesthetic and environmental on the smaller “” diameter of the benefits. fastener. Higher lateral capacities for lag Custom-fabricated structural framing screws can be calculated if the designer can ONLINE CALCULATOR connections, for instance, provide a ensure the threads are far enough from the single-piece connection between two shear . For more information, see the The American Wood Council offers framing members. They generally consist American Wood Council (AWC) publication, a free online connection calculator of bent or welded steel, carrying load from Application of Technical Report 12 for Lag (www.awc.org), which allows users to the supported member (through direct Screw Connections, Design Aid No. 1. calculate capacities for single bolts, bearing) into the supporting member (by nails, lag screws and wood screws per hanger flange bearing, fastener shear or a u Timber rivets are high-strength steel the 2005 NDS. Both lateral (single combination of the two). nails typically hot-dipped galvanized, with a and double shear) and withdrawal ca- Pre-engineered metal connectors are flattened oval shank section and a - pacities can be determined. Wood-to- commonly used in all types of wood shaped head. When driven through the wood, wood-to- and wood- construction. Traditional fasteners like nails, pre-drilled steel plate and into the timber, to-steel connections are possible. bolts and rivets, as well as generic types the tapered heads wedge tightly in the holes. of pre-engineered metal connectors such Rivets must be driven with the long axis as metal hangers, are reliable and widely parallel to the . An excellent way used, but have some limitations in large, to spread the load with multiple fasteners, on the meeting side of the timber multi-story buildings that newer proprietary they can be used as an alternate to a large members. The split in the steel rings systems are designed to overcome. Newer number of bolts, resulting in a connection allows the gap in the to close or open systems, for example, are often concealed, with greater ductility and resilience. if the wood members shrink or swell to which makes them fire-resistant if they are ensure that the wood members and split sufficiently below the wood surface. They Metal connector plates with integral ring remain in contact. A bolt is installed can be designed for use in multiples without multiple rows of teeth partially penetrate through the center to hold the joint reducing the strength of the wood, and they the wood members and are used primarily assembly together. In connections with can be designed to be either stiffer or more with fabricated lightweight wood . shear plates, grooves must be precision flexible, depending on the application. With these connections, loads are transferred machined with special tools, which recess One example of a proprietary product near the surface of the wood member. the wood so the shear plates sit flush with that’s becoming widely used—especially in the surface. Typically, shear plates and split cross laminated timber (CLT) and glulam Shear connectors, sometimes referred to ring connections have brittle wood failure structures (see sidebar on page 8)—is the as bearing connectors, are typically used to modes and should be avoided in structures self-tapping or self- screw. A number accommodate heavier loads. They include located in high seismic regions unless the of manufacturers in North America, Europe split rings, shear plates and toothed shear designer employs elastic seismic design. and produce them with different

4 CONTINUING EDUCATION 5

the depth of the depth 10 ⁄ 1

One of of the benefits joinery-type connections dominate primarily because dominate connections they can be withoutmade expensive ease with and assembled equipment relative site. on made often is that they’re connections If of is the connection entirely wood. and drying, moisture change the to exposed ofin dimensions parts the wood will be subject connection a wood/steel In similar. be may the same moisture/drying, there to and the wood between change a differential steel. AVOIDING POTENTIAL PROBLEMS POTENTIAL AVOIDING Notching is an essential technique Although notching be required and may in traditional joinery, field any of a number in the field for reasons, made, not properly holes that are or notches , approved that deviateor from the capacity of significantly reduce may The designed member. a properly even and notching, on guidance NDS provides extensive the wood has produced with notes recommendations technical for of the size appropriate to notches related example, for glulam, With materials. different to is limited notching end the member or 3 in., whichever is less. When When is less. whichever 3 in., or the member designers should consider is used, a notch in screw driving self-tapping or a lag screw the bearing up past the going from 6–8 in. the develop, If to a crackneutral were axis. the down progressing will it from screw keep oflength the beam and acts as a transverse reinforcement. Effects Moisture and contracts as a result expands Wood of in its equilibrium moisture changes in the direction Expansion (EMC). content parallel grain to is member in a wood the direction in change however, minimal; perpendicular grain to can be considerable in connection for be accounted and must rule-of- The common design and detailing. will is that wood thumb a 1% experience in the radial direction tangential change or content. everyfor in moisture 4% change can decrease example, for 2x12, A nominal under just shrinkage through in depth by 10% EMC. 18% to from as it changes ¼ in. it is important to designing connections, In that such design and detail the connection shrinkage is not restrained. the member’s Otherwise, shrinkage of member the wood perpendicular- tension can cause excessive occur. to-grain and splitting may stresses CONNECTION OPTIONS FOR WOOD-FRAME FOR BUILDINGS OPTIONS HEAVY AND TIMBER CONNECTION Although joinery parts can be produced While fairly common in single-story fairly common While with great speed and precision using with using speed great and precision (CNC) Controlled Numeric Computer is not as widely this approach , America as it is in Japan used in North mechanical Here, Europe. Western and homes, commercial and recreational and recreational commercial homes, joinery seldom are connections structures, timber multi-story heavy modern, used for is the fact that One reason buildings. highly demand systems these connection with only associated usually developed connections Such carpenters. experienced them makes which labor intensive also are industrial for production. uneconomical in connected members so they members interlock— in connected as with and scarfsuch mortice/tenon joints. are these typesIn forces of connections, in principle transferred in compression/ in tension connections Interlocked bearing. to metal keys or pins or wood require connections Tension separation. prevent and in some a minimum, to kept often are cases metal straps provided bolts and are for mainly connections, better achieve to rooflong-span trusses structures. in historic designed joinery a properly In connection, tightening of interlocking, is sufficient there and friction the self- connections to due weight and supplementary support from buildings to and adjacent buttresses walls, design both gravity for forces and lateral withassociated wind events. seismic or Pre-engineered connectors are are connectors Pre-engineered mechanical design for values Reference At theAt Jackson Hole Airport in Jackson, Wyo., embedded steel plates with structural transfer -to-beam load at provide screws connections. Joinery joinery (also known connections Traditional as carpentry typically are connections) holes and tongues cutting notches, by created features and in many shapes and sizes. They shapes and sizes. and in many features higher load for lateral enhanced have in variouscapacity capacities and come for The main advantage applications. different without wood into is that they can be driven handheld power using a regular pilot holes, the risk This lessens ofdrill. in the mistakes and reliability. efficiency field and improves manufacturers, component by manufactured catalogues of products exhaustive have which in Categories, variousthat suit applications. previously the ones mentioned, addition to hold-down framing anchors, include components Many straps and ties. devices, fabricated using formed sheet metal, are as loads in thickness increase. increasing requirements Specific nailing and fastening and must the manufacturers by provided are develop to followed be rigorously capacity. full connection in various sources. provided are connections design for values reference The NDS contains bolts, as nails, dowel-type such connections shear split rings, screws, wood lag screws, drift pins and timber rivets. drift bolts, plates, are Design proprietary for values connectors evaluation reports. in code provided Photo:Gensler, WoodWorks 2015 Wood Design Award, Matthew Millman Photography CONNECTION OPTIONS FOR WOOD-FRAME AND HEAVY TIMBER BUILDINGS

Photo: Crescent Communities

Crescent Terminus Atlanta, Ga. Architect: Lord Aeck Sargent

Specific issues related to shrinkage include: Another issue related to moisture capacity of the connection. The following is exposed end grain. This can result are important considerations regarding the Beam-to-column connections: in checking and possible decay, even in wood members themselves: Continuous full-depth side plates can low-rainfall climates. Solutions: Redirect cause problems because they restrain flow around the connection. Use u The species of wood must be the same as wood shrinkage and may cause splitting. preservative-treated or natural decay- that for which the connector was rated by Solution: Smaller discontinuous side resistant wood products. (If preservative- the manufacturer. Manufacturers test and plates transmit forces and allow wood treated wood is used, preservative treatment publish allowable design values only for movement. needs to be applied after all field cuts.) certain species of wood. For other species, Use end caps and flashing. Allow for air consult with the connector manufacturer. Beam-to-beam connections: For beam flow and drying—e.g., by detailing drain hangers, fasteners placed near the top of the holes or slots in box-type connectors u The wood must not split when the supported beam can restrain wood shrinkage or maintaining a gap of at least ½ in. fastener is installed. A fastener that splits and may cause splitting. Solution: Fasteners between wood and concrete or masonry the wood will not provide enough capacity placed near the bottom of the supported construction. for the design load. If wood tends to split, beam can minimize the effects of shrinkage consider pre-boring holes using a diameter while top tabs provide lateral restraint. SELECTION AND INSTALLATION not exceeding ¾ of the nail diameter. Pre- Ensuring that structural connectors boring requirements for screws and bolts are Beam-to-wall connections: If bolts are perform as intended involves a number of provided in the NDS. placed high on the beam, or high and low, considerations for both the designer and tension perpendicular to grain stresses may installer. In the ASD/LRFD Manual, AWC u Regarding shrinkage, most connectors cause splitting. Solution: Bolts near the highlights them as follows: are manufactured to fit common dry lumber bottom of the beam, preferably with slotted dimensions. Other dimensions may be holes, allow for wood shrinkage. (A variety Wood Members available from the manufacturer. of pre-engineered connections are available Specific details related to the wood members with pre-drilled slotted holes.) being connected will have an impact on the u Where built-up members (multiple

6 CONNECTION OPTIONS FOR WOOD-FRAME AND HEAVY TIMBER BUILDINGS

members) are installed in a connector, the other. Therefore, the choice and installation Beam-to-Concrete or CONTINUING EDUCATION members must be fastened together prior to of the fasteners is critical to the performance Masonry Wall Connections installation of the connector so they act as a of the connection. The following are Design concept: Concrete is porous and single unit. important considerations: “wicks” moisture. Good detailing never permits wood to be in direct contact with u The dimensions of the supporting u All fasteners specified by the manufacturer concrete. member must be sufficient to receive must be installed in accordance the specified fasteners. Most connectors with the manufacturer’s installation FIVE STORIES AND HIGHER are rated based on full penetration of all recommendations to achieve the published With the exception of height, the outward specified fasteners. Refer to the connector value. appearance of mid-rise wood-frame manufacturer for other situations. buildings has not changed a lot over the u The size of fastener specified by the years; however, the quality and precision u If a connection is designed to transfer manufacturer must be installed. of design and construction has increased load by bearing, the bearing capacity of the considerably. Connection-related design wood members should be evaluated. u The fastener must have at least the same aspects that are more critical in a five- or resistance as the connector. six-story wood-frame building include u Local stresses in connections using accommodating greater dead, live, wind multiple fasteners should be checked to u Bolts must generally be structural quality and seismic loads, which increase with the ensure adequacy. Where a fastener group is bolts, equal to or better than ANSI/ASME additional height. composed of closely spaced fasteners loaded Standard B18.2.1. parallel to grain, the capacity of the fastener 1 Source: American Wood Council group may be limited by wood failure at the u Bolt holes must be a minimum of ⁄32 in. 1 net section or tear-out around the fasteners and a maximum of ⁄16 in. larger than the bolt caused by local stresses. One method for diameter. Beam on shelf in determining local stresses is provided in wall. The bearing plate distributes Appendix E of the NDS. u Fasteners must be installed prior to the load and keeps loading the connection. the beam from direct contact with Connectors the concrete. Steel The condition of the connector is critical u Power-driven fasteners may deflect angles provide uplift resistance and can to how it will perform. The following are and injure the operator or others. Nail also provide some important items regarding the connector: tools may be used to install connectors, lateral resistance. The end of the beam provided the correct quantity and type should not be in u Connectors may not be modified in the of nails are properly installed in the direct contact with field unless noted by the manufacturer. manufacturer’s nail holes. Nail tools the concrete wall. Bending steel in the field may cause fractures with nail hole-locating mechanisms at the bend line, and fractured steel will not should be used. Follow the nail carry the rated load. manufacturer’s instructions and use the Similar to appropriate safety equipment. detail A u Modified connectors may be available with a steel bearing plate from the manufacturer. Not all modifications DESIGN GUIDANCE FOR only under are tested by all manufacturers. Contact the EFFECTIVE CONNECTIONS the beam. manufacturer to verify loads. Connections obviously need to provide the structural strength necessary to u In general, all holes in connectors should transfer loads. As discussed above, be filled with the nails specified by the well-designed connections also manufacturer. Contact the manufacturer mitigate potential adverse effects of regarding optional nail holes and optional moisture ingress. Finally, well-designed loads. connections minimize the potential for excessive tension perpendicular to grain Similar to u Different environments can cause stresses—under design conditions detail A with corrosion of steel connectors. Always as well as those related to unusual slotted holes to accommodate evaluate the environment where the loading conditions. Chapter 10 of the slight lateral connector will be installed. Connectors are ASD/LRFD Manual provides details movement of the beam available with different corrosion resistances. for a variety of design configurations under load. This Contact the manufacturer for availability. (e.g., beam-to-concrete or masonry detail is more commonly used wall connections and beam-to-beam when the beam Fasteners connections). Many begin with a brief is sloped as Most wood connections rely on fasteners to discussion of the pertinent design opposed to flat. transfer the load from one member to the challenges. For example:

7 CONNECTION OPTIONS FOR WOOD-FRAME AND HEAVY TIMBER BUILDINGS

Going from four to six , for example, Construction Details (Form T300) by APA – CROSS LAMINATED TIMBER increases the gravity loads by 50% but lateral The Association. These Cross laminated timber (CLT) is an forces due to wind or seismic forces may details are consistent with the construction engineered wood building system, actually double. requirements outlined in IBC section 2304.10. part of a new product category While the size and capacity of connections They are representative of those you would known as mass timber. CLT is made might change and designers may use more find in existing historic buildings, and are from several layers of solid-sawn or specialized, high-capacity connectors, the useful when retrofitting or restoring historic structural composite lumber, stacked connectors themselves are largely the same as structures. crosswise (usually at 90-degree angles) they are for any light wood-frame building— and glued together on their wide nails, bolts and lag screws used with and faces. This cross lamination provides without proprietary connectors. dimensional stability, strength and Continuous tiedown systems are often rigidity, which is what makes CLT a used in multi-story wood shear wall viable alternative to concrete, masonry construction for resisting wind or seismic and steel in many applications. It can overturning loads. be used for an entire building, as both the lateral and vertical load resisting HEAVY TIMBER CONSTRUCTION system, or for select elements such as Heavy timber construction utilizes many of the roof, floors or walls. the standard connections described above, There are numerous including bolted connections, timber rivets, available to designers interested in split rings and shear plates. Innovative connections related specifically to proprietary connections are also widely used, CLT and other mass timber products. and today’s designers have a tremendous These include the U.S. CLT Handbook, range of options. Innovative examples include, which is available as a free download among many others, grid plates and concealed at www.rethinkwood.com, and the kerf plates with self-drilling . Where 2015 NDS available from the American fire endurance is required, connectors and Wood Council. fasteners shall be protected from fire exposure. Some methods of providing protection Common connections for CLT include wood, fire-rated gypsum board or fire assemblies include wall-to-foundation, resistant coatings. wall-to-wall (straight or junction), floor- Because heavy timber buildings lend to-floor, wall-to-floor, and wall-to-roof. themselves to innovative design, many Panels may be connected to each other require custom connections to arrive at the with half-lapped, single or double sensitive balance between safety, efficiency, splines made from engineered wood serviceability and aesthetics. products, while metal brackets, hold- Traditional heavy timber details are downs and plates are used to transfer available in the publications, Heavy Timber forces. Mechanical fasteners may be Construction Details by AWC and Glulam dowel-type (e.g., nails, screws, dowels, bolts) or bearing-type (e.g., self-drilling or self-tapping screws). In Europe, self-tapping screws are Additional Resources: used extensively for the connection of • National Design Specification (NDS) for Wood Construction, American Wood Council CLT panels. Benefits include ease of • ASD/LRFD Manual for Engineered Wood Construction, Chapter 10 – Mechanical Connections, American Wood Council installation, high lateral and withdrawal • DES310: The Wood Connection Session (e-course), American Wood Council, APA – The Engineered Wood Association, Canadian capacities, ability to take combined Wood Council, Wood Council of America axial and lateral loads and the fact that, • Technical Note: Glulam Connection Details, 2007, APA – The Engineered Wood Association most of the time, they don’t require • U.S. CLT Handbook, 2013, FPInnovations pre-drilling. Glued-in rods are also • Structural Design in Wood, Stalnaker & Harris, Kluwer Academic Publishers becoming increasingly popular both in mainstream heavy timber construction and, increasingly, for CLT. This is being driven by recent developments in CNC reThink Wood represents North America’s lumber industry. We share a passion for wood and the it comes from. Our goal is to generate machining technology, wood materials awareness and understanding of wood’s advantages in the built environment. and the desire for a high level of Join the reThink Wood community to make a difference for the future. Be part prefabrication to reduce assembly time of the message to “rethink” wood use, address misperceptions and enhance and cost. awareness of wood’s benefits and choices. Visit reThinkWood.com/CEU to learn more and join.

Reprinted from the October 20/27, 2014 issue of News-Record 8