Connections critical to building strength

Many modern farm buildings built with wooden framing are covered by a skin or cladding of ribbed-metal sheeting. The combined strength of the framing and metal skin contributes to the economy of that type of building.

Many modern farm buildings built with wooden framing are covered by a skin or cladding of ribbed-metal sheeting. The combined strength of the framing and metal skin contributes to the economy of that type of building. Some primary structural members such as wood trusses or laminated-wood posts are pre-fabricated. The majority of the connections between primary structural members, secondary structural members and the building skin are made in the field, but are critical to the strength of the assembled building. Attention to those connections in design, construction and maintenance will enhance building strength and longevity.

Applied loads

need resistanceWhen buildings are subjected to wind, snow and dead – building-material weight – loads, the resulting forces must be transferred to the ground or foundation by the building members. Designers may refer to the transfer of forces as creating a continuous load path to the ground. Snow and dead loads generate downward vertical loads on building members. Wind-induced loads may generate downward, upward and horizontal forces on building members.

Movement – such as deflection, bow, sway or lean – of the building from those applied forces must be resisted by the stiffness of the building. In metal-clad frame buildings, the overall stiffness is a combination of frame stiffness and stiffness provided by the metal skin.

Carrying the applied building loads safely to the ground requires properly designed and field-installed fasteners at critical building connections.

Post embedment – When building posts are set in the ground, the embedment depth and the surrounding soil type and conditions determine the resistance provided to prevent the post from being pulled out of the ground or leaned to the side. Soils with more sand or gravel provide greater load-bearing strength than those with predominantly silt or clay. Undisturbed soils provide more strength than fill material. Support provided to the post by connection to a rigid building floor can provide additional lateral and uplift resistance. Post footings must be properly sized and rest on undisturbed soil rather than fill or loose material. Confirm that all aspects of the post embedment and soil strength comply with the building design.

Post-to-foundation attachment – When posts are attached to the top of a foundation, the connection must resist wind uplift along with horizontal wind forces and bending forces in the post. Those connections must include either steel plates embedded in the concrete foundation that extend an appropriate distance up the side of the post, or a base plate of adequate design bolted to anchors embedded in the concrete. Both the embedment of the connectors in the concrete and the attachment of the connectors to the post are critical in the design and function of those connections. Ensure that anchors are properly placed as the foundation is installed, and that all connections to the post are made as specified in the design.

Figure 2. Post-to-foundation attachment

Stud-wall-to-foundation attachment – When stud-wall framing is connected to a foundation, the connection must resist wind uplift along with horizontal wind forces. That type of connection cannot resist bending forces, so wall and building lean must be resisted by other means.

Nails or screws driven into the end grain of wood studs have minimal withdrawal resistance, which is not counted in building design. Uplift resistance must be provided by either metal straps – called tie plates, stud ties or hurricane clips – that attach with nails or screws to the side of the stud, and the edge of the base plate or foundation. Or lap sheathing or siding materials over the base plate and connect securely to both the studs and the base plate. Follow the manufacturer's recommendations for fasteners required for metal-tie plates, and building designer's specifications for sheathing or siding attachment to the plate.

When the base plate is providing the structural connection between the wall framing and the foundation, it must be securely fastened to the foundation with anchors embedded in the concrete. Follow the building-design specification for the length, size and spacing of anchors. Ensure the base plate is protected from moisture damage that can weaken the ability of the wood to hold the required fasteners. Monitor the base plate and attached sheathing or siding for signs of moisture damage, rust and fastener deterioration.

Truss-to-wall attachment – Forces acting on the building roof are transferred to the roof trusses, and from the trusses to the building posts or wall framing. The connection must resist downward snow and dead loads along with wind uplift and horizontal forces. Adequate bearing area between the truss and wall must be provided to carry snow and dead loads. Bearing area is usually provided by resting the heel of the truss directly on the wall plate or on a notch in the post. If trusses rest on bearing blocks attached to the side of a post, there must be sufficient fasteners to carry the full load from the block to the post – following the design specifications.

Figure 3. Truss-to-wall attachment – A truss-bearing block illustrates a likely-inadequate three-nail connection to a post.

Figure 4. Truss-to-wall attachment – knee braces

Horizontal and uplift wind forces must be carried from the truss to the wall. Toe-nailing, common in house and garage construction, is not adequate to carry those loads. Bolts or metal tie plates are required to carry the loads. Connectors should tie the truss to the post or studs rather than to the top plate, unless the top plate is also connected to the studs with load-bearing connectors specified in the design.

In post-frame buildings, the connection may also carry bending forces from the post to the truss. The bending-force transfer may be carried through diagonal knee braces at the truss-to-post connection, or by bolts through the truss and post with a specially designed deep-heel truss. Closely follow design specifications for knee-brace connections and deep-heel-truss connections. Do not allow field modification or omission of knee braces or truss connections without express approval of the building designer.

Roof-purlin attachment – Wind uplift forces applied to roofing materials are transferred to the trusses through the roof purlins. When purlins are nailed on top of the truss, fasteners must provide adequate embedment depth and size for withdrawal resistance from the truss. Toe nailing is likely not adequate. Special ring-shank pole-barn nails or screws may be required. Metal tie straps provide additional uplift resistance. When purlins are hung between the trusses in metal hanger brackets, adequate fasteners are required into both the purlin ends and the truss. Increases in wind loads around the edge of the roof add to fastener requirements at the edges of roof sections.

Figure 5. Roof-purlin attachment – Purlin nailing is shown with inadequate nail embedment.

Figure 6. Roof-purlin attachment – Metal tie plates or straps

Diagonal wind bracing – Building framing assembled in rectangular patterns, such as stud walls and roof trusses with purlins, requires bracing to prevent racking – leaning or twisting the rectangles into diamond shapes. The steel skin on the building can provide much of the diagonal bracing effect, but particularly during construction diagonal frame bracing may be required. That bracing may be temporary, or may be permanently included in the structure.

Ensure diagonal bracing is installed according to design specifications. Do not cut or omit diagonal bracing for doors, windows or equipment without approval of the building designer. Connections at the ends of diagonal bracing are particularly important to brace function. Ensure that connections to posts, plates or foundations are properly installed and protected from moisture damage. When bracing carries compression loads – squeezing along the length – lateral bracing of the member may be required to prevent sideways buckling. Follow design specifications.

Metal-sheeting attachment – The metal sheeting of the building must carry loads from wind and snow to the structural framing. Wind uplift and outward forces, along with induced diagonal-bracing forces, are significant design factors for sheeting connections.

Uplift and outward force resistance requires adequate fastener spacing and embedment depth. Follow design specifications. If screw fasteners are over-tightened and strip the threads, replace them with longer fasteners – or move to a new fastener location and seal the hole from the stripped screw. Wind-uplift forces are greater at the edges of roof and wall surfaces due to both the movement of wind around the corners and the load on roof overhangs, when present. Additional fasteners – closer fastener spacing – are justified in those edge zones. Edge zones of greater wind pressure are generally assumed to be at least 3 feet wide from the edge of the roof or wall section.

As mentioned in the section on diagonal wind bracing, the building steel sheeting can also provide resistance to wind forces that would cause racking of the building frame. The metal sheeting attached to the framing forms a rigid diaphragm that provides resistance to racking. In resisting racking, shear forces are created in the connections between the steel sheets, and between the sheets and the framing. Those forces are larger around the edges of each diaphragm – that is, around the edge of each individual wall or roof section. Those additional diaphragm forces add load to the connections between steel sheeting and the underlying framing, and between that framing and the trusses, posts or foundation.

Building-failure investigations frequently point to connection failure around the edges of the roof surface as the beginning point for structural failure. Figure 7 shows roof panels that have been peeled off the edge of the roof as a result of inadequate strength of the connections at the roof edge. Figure 8 shows a section of roofing – roof sheeting and purlins – separated from a building frame by wind forces. Note that the sheeting remained attached to the purlins, but purlins were not adequately connected to trusses.

Figure 7. Metal-sheeting attachment – Building wind-load failure happens at the edge of the roof.

Figure 8. Metal-sheeting attachment – A roof section is separated from a building frame by wind.

Inspecting connections – Inspection during and after construction should verify that the connections at critical locations were properly installed. Verify with the construction plans that the proper size and number of connectors were installed. Check screw or nail size, spacing, length and embedment into framing members.

Fasteners in sheeting ribs do not carry as much shear load as fasteners in the flat part of the sheeting. Fasteners too near to the edge or end of a wooden-framing member will carry less load. Overtightening of fasteners that damages the metal sheeting or under-tightening that leaves a loose fit will reduce connection strength.

Verify the condition of the framing members at the edges of roof and wall sections. Defective framing members – roof purlins, eave purlins, sidewall girts, top plates or bottom plates – can lead to connection failure between the metal sheeting and the frame.

Do not allow building modifications that are not part of the original building design and plans. Insist that any on-site modifications be confirmed in writing from the building designer.

Connection maintenance – Critical connections should be inspected every few years, and also following any severe windstorms. Areas near the edge of the roof and the base of the wall are also prone to deterioration and maintenance issues as buildings age. During maintenance inspections and repairs, pay particular attention to connection and framing-member condition along the roof eaves, roof ridge, foundation, wall corners and end-wall rake – the joint between the roof and end wall.

Roofing materials near the eaves of livestock buildings are particularly prone to moisture condensation and deterioration of roof metal. Watch for signs of rusting or corroding of roof metal at those locations. Corroded or rusted metal sheeting is prone to fasteners breaking through the metal when subjected to load.

Figure 9. Connection maintenance – Roof sheeting rusts around fasteners.

Replace rusted or corroded metal roofing. Reduce the risk of future metal rusting or corrosion through means such as modified ventilation attic air intakes, improved insulation or condensation control. Refer to Iowa State University-Extension and Outreach's publication "Gable End Attic Air Intakes for Swine Building Ventilation – AE 3545. Visit store.extension.iastate.edu and search for "ae 3545" for more information.

Repair or replace building-framing members that show signs of deterioration or damage. Use a metal probe, knife point or small screwdriver to test wood members for integrity. Use a wood-moisture meter to check for excess moisture content. Design connection strength is reduced when wood-moisture content in use exceeds 19 percent. Look for visible streaking as an indication of past condensation dripping and running. If necessary, remove soffit panels or insulation to gain better access for inspection. Use a flexible inspection camera to access restricted areas for investigation. Consult a qualified engineer or building professional if damage is found that requires repair more complex than fastener or member replacement. Truss-design and inspection are not the focus of this article, but if inspection reveals truss damage or truss-plate teeth that are not firmly embedded in truss members, seek professional advice for repair strategies.

Figure 10. Connection maintenance – Truss-plate tooth withdrawal is shown.

Safety First – Be cautious when investigating and repairing building connections. Hazards include sharp metal edges and points, raw wood edges, blowing insulation and debris, and falls from ladders. Wear protective eyewear and gloves, and disconnect power before inspecting areas around electrical wiring.

Authors – Shawn Shouse, Kapil Arora, Brian Dougherty, Kris Kohl and Kristina TeBockhorst – field agricultural engineers with Iowa State University-Extension and Outreach; reviewed by David Bohnhoff, emeritus professor with University of Wisconsin-Biological Systems Engineering

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