Mass timber construction has moved from demonstration projects to standard practice across North America. Cross-laminated timber (CLT) panels, glulam beams, and heavy timber columns arrive on site as finished structural members, and crews assemble them much faster than equivalent steel or concrete systems. The building only performs as designed when the connections between members transfer load reliably. Every beam seat, panel edge, and column base depends on the same structural timber engineering principles that apply to sawn lumber, glulam, and heavy timber construction: the joint, not the member, usually governs the design.
Connector makers have answered with code-approved hardware built specifically for mass timber. Structural wood screws carry rated withdrawal and shear values, concealed beam hangers hide steel inside the wood section, angled washers spread bearing across connection plates, and surface splines join CLT panels edge to edge for diaphragm action. This article covers how each family works, what code approval means in practice, and how to match connectors to the load path.
How Mass Timber Members Transfer Load
Mass timber members are large and stiff, so gravity loads rarely strain the wood itself. CLT floor and roof panels, glulam beams, and heavy timber columns carry compression and bending well within their rated stresses. The vulnerable points sit at the connections, where bearing, uplift, shear, and moment all concentrate where one member meets another.
Load paths in CLT and glulam assemblies
A typical floor-to-wall connection in a CLT building sees several load cases at once. Gravity pushes down through the bearing surface. Lateral loads from wind and seismic events push the floor diaphragm into the walls. Uplift at building edges pulls floor panels away from the wall below. Each case demands a different connector behavior: bearing for gravity, shear transfer for lateral loads, and tension anchorage for uplift. Designers track these as separate load paths even though they share one physical joint.
Mass timber also pairs with other materials in hybrid structures. Steel link beams, concrete cores, and post-tensioned elements show up in taller designs, and each interface needs its own connection strategy. These advanced construction materials combine engineered timber with fiber-reinforced polymers, smart components, and other systems, so connector selection must account for different stiffness, thermal movement, and fire behavior at every material boundary.
Structural Wood Screws: The Workhorse Connector
Structural wood screws have replaced many nails, bolts, and lag screws in mass timber assemblies. A modern structural screw combines a deep thread that bites into wood fibers, a hardened shank that resists shear, and a head designed for both the driver and the load. Rated capacities come from testing rather than rule of thumb. Manufacturers publish withdrawal, lateral, and head pull-through values for each diameter, length, and embedment depth, and engineers use those published values directly in design.
What makes a screw structural
- Thread geometry. An aggressive thread pitch increases withdrawal resistance along the grain, so the screw holds when loaded in tension.
- Point design. Self-drilling points with cutting flutes reduce or eliminate pre-drilling and clear material as the screw advances.
- Head style. Washer heads spread bearing over a wider area; combo heads accept hex, Phillips, or square drivers so one screw serves multiple tools.
- Coating and material. Corrosion protection matters in exterior, coastal, and treated-wood applications where a failed fastener would hide behind cladding.
Screw geometry and thread design
The ratio of threaded to unthreaded shank does most of the work. A screw with a long threaded portion engages more wood fibers along its length, which raises withdrawal capacity. Screws intended for steel-to-wood connections use a smaller shank with a deeper thread so the screw can pass through a steel plate and still grip enough wood. Point geometry controls installation speed and quality. Auger-style points pull the screw into the member, while cutting flutes clear chips in dense CLT and glulam.
Washington state has been a proving ground for tall wood buildings, and the story of how Washington builds the way for mass timber construction shows how testing programs and code amendments turned connector data into approved practice.
Concealed Beam Hangers and Hidden Connections
Hangers connect beams to columns, beams to beams, and floors to walls. A conventional hanger wraps the beam end in a steel seat with visible side flanges and fastener holes along the face. Concealed hangers tuck the steel into the wood itself. The hanger sits in a recess or pocket routed into the beam or column, so the exterior face stays clean and the timber reads as solid material.
Why concealment matters
Architects specify concealed connections for appearance, and the choice has structural consequences. A hanger pocketed inside the wood protects the steel from direct fire exposure. When fire attacks an exposed steel hanger, the thin metal heats quickly and loses strength while the wood around it chars at a predictable rate. Concealed steel stays cooler longer inside the wood section, which helps the assembly hold its rated fire resistance.
Concealed systems also change erection sequencing. The beam must lift into the pocket rather than drop into a seat, so the rigging crew needs more clearance and the setting crew needs a positive stop to confirm full bearing. The material properties that make mass timber a viable structural system include predictable char rates and a high strength-to-weight ratio, and connection detailing has to respect them in both fire and gravity design.
CLT Diaphragms, Angle Washers, and Surface Splines
CLT floor and roof panels act as diaphragms that distribute lateral loads to the shear walls. Panel-to-panel joints must transfer those forces across the full building width. Two connection families dominate: angle brackets and screws at panel edges, and surface splines that join panels along their faces.
Diaphragm connection patterns
Edge connections use steel angle brackets screwed to both panels, or long screws driven at an angle across the joint. The bracket resists shear at discrete points, so the engineer spaces them to match the diaphragm shear demand. Surface splines distribute load differently. A spline is a long strip that fits into routed slots on the edges of two adjacent panels, connecting them in the plane of the diaphragm. Screws driven through both panel faces into the spline create a joint that behaves like a continuous panel for shear purposes.
Spline joint detailing
Spline thickness, slot depth, and fastener spacing determine capacity. A spline that is too thin for the slot flexes and loses stiffness; a spline too thick for the slot jams during installation. Splines sized for common CLT thicknesses run the full panel length and take screws on both faces. The result suits long-span floors where a row of brackets would interrupt the ceiling plane.
The same scaling logic carries into mixed projects. LVL and CLT systems now appear in mixed-use building construction, where retail floors, office levels, and residential units share one structural frame, and each occupancy demands different diaphragm and connection treatment.
Code Approvals and Installation Practice
A connector is only as good as its approval. Code-approved hardware carries an evaluation report or manufacturer data reviewed under the model codes. The report lists allowable loads, fastener schedules, edge distances, and installation requirements. Using a connector outside its published conditions voids the design assumption, whether the error is a shorter screw, a wider spacing, or a missing coating.
Structural innovation keeps raising the bar. The structural innovations shaping modern mass timber construction include post-tensioned walls, timber-concrete composites, and larger panel formats, and each one puts new demands on the connection hardware underneath.
Reading an evaluation report
An evaluation report is a contract between the manufacturer and the code official. It names the product, the materials it may connect, the loads it may carry, and the conditions under which those loads apply. Engineers pull allowable values straight from the report, and inspectors check field installation against the same document.
| Connector family | Typical use | What the approval covers |
|---|---|---|
| Structural wood screws | Beam-to-column, panel-to-panel, steel-to-wood | Withdrawal, lateral, head pull-through, embedment |
| Concealed beam hangers | Beam seats in exposed timber | Allowable load, fire encapsulation, fastener schedule |
| Angle washers | Bolt and screw bearing on connection plates | Bearing area, taper angle, plate thickness range |
| Surface splines | CLT diaphragm panel joints | Shear capacity per foot, slot depth, screw pattern |
| Post bases and caps | Column-to-foundation, column-to-beam | Uplift and compression values, anchor bolt size |
Field installation rules
- Verify the connector and fastener match the approved drawing before lifting any member.
- Drill pilot holes wherever the report requires them, and only to the prescribed depth.
- Drive screws to the marked embedment line; under-driving and over-driving both reduce capacity.
- Confirm full bearing at every beam seat and hanger pocket before releasing the rigging.
- Apply the specified corrosion protection to exposed steel after installation.
These five steps look simple, but they catch most field failures. Connector problems in mass timber rarely come from the engineering; they come from substitutions and shortcuts during erection.
Selecting Connectors for the Project
Selection starts with the load path and ends with the erection crew. The checklist below covers the decisions that matter on most projects.
A short selection checklist
- Identify every load case at each joint: gravity, lateral, uplift, and any combination that controls the design.
- Confirm the exposure class before choosing coatings or stainless steel.
- Check the fire rating and whether encapsulation protects the connection.
- Match connector depth to panel thickness so fasteners get full embedment.
- Plan the erection sequence around concealed pockets and spline joints.
Builders choosing mass timber often weigh the environmental case for mass timber alongside cost and schedule, since the carbon story only holds if the building performs for its full service life. Connections are where that performance starts.
Mass timber’s growth in North America depends on hardware that designers trust and crews can install fast. Structural screws, concealed hangers, angle washers, and splines each solve a specific problem in the load path. When the connector matches the demand, the joint disappears into the building and the structure works as drawn. When it does not, the joint becomes the first place a building complains.