The connections that hold a building up: rigid joints, pinned joints and why they get overlooked

Most people judge a building's structure by its parts: the thickness of the wall, the size of the beam, the grade of the concrete. A structure is also a chain of connections, and the load path from the roof to the footing is only as strong as its weakest link. The connections are small, usually covered within days, and rarely inspected again.

This article is for owners, committees, builders and developers. It explains what a structural connection does, the difference between rigid and pinned joints, how the issue shows up in different construction systems with precast walls as the worked example, and what to ask for. It is general information. Connection design is engineering work and belongs to the structural engineer of record.

What a connection is for

Every structure has to carry loads from where they are applied to the ground: the weight of the building, the people and contents, wind, and in some places earthquake. The members (slabs, beams, columns, walls, rafters) carry the load along their length. The connections pass it from one member to the next. A strong member with a weak connection does not carry more load. It is limited by the connection.

Connections also provide:

• Stability: keeping walls and frames upright and stopping them racking or toppling.
• Lateral load resistance: passing wind and earthquake load from walls and roofs to bracing, shear walls or frames, and down to the footing.
• Robustness: tying the structure together so that damage to one element does not bring down a disproportionate part of the building.

Rigid, pinned and movement joints are different things

These three terms are often mixed up.

• Rigid (moment) connection: a joint designed to transfer bending as well as shear and axial force. The connected members stay at a fixed angle to each other as the structure deflects. Examples are a welded or heavily bolted steel beam-to-column connection, or a reinforced concrete beam-column joint cast monolithically.
• Pinned (simple) connection: a joint designed to transfer shear and axial force but to allow rotation. The structure then relies on bracing, shear walls or other members for stability. Many precast panel and steel connections are designed this way on purpose.
• Movement joint: a deliberate gap, sealed with a flexible sealant over a backing rod, that lets elements expand, contract and settle without cracking. It is not a structural connection and should not be bridged by grout, render or rigid fill.

A rigid connection is not automatically better. A rigid joint attracts bending, and a structure designed with pinned joints can be damaged if a joint is stiffened without an engineer's check. The point is that the connection must behave the way the designer assumed. If a pinned joint locks up, or a rigid joint is poorly made and acts as a pin, the loads go to places the structure wasn't designed for.

How the issue shows up in different structures

• Precast and tilt-up concrete: panels are cast separately and joined by welded or bolted cast-in plates, grouted dowels, starter bars and in-situ joints. The panels stand on temporary braces until the permanent connections replace them. The connections hold the panels upright, pass wind and earthquake load to the floor, roof and footing, and tie adjacent panels together.
• Steel frames: the beam-to-column connections decide whether a frame resists sway itself or relies on bracing. Bolt grade and tightening, weld size and quality, and the base plate and holding-down bolts are critical.
• Reinforced concrete frames: beam-column joints and reinforcement lap splices carry load between members. Congested or poorly detailed joints are a well-known source of damage in earthquakes.
• Masonry: wall ties, bond beams and connections to floors and roofs stop walls separating, rocking or falling outward.
• Timber framing: nailed and bolted joints, bracing fixings, hold-downs and tie-downs resist wind uplift and racking. Our article on carpentry standards and structural integrity covers part of this.

In every system, the roof-to-wall, wall-to-floor and wall-to-footing connections form the load path.

A well-known example

In 1968 at Ronan Point, a London tower of large precast panels, a gas explosion on the 18th floor blew out a load-bearing wall panel, and the panels above it lost their support. A corner of the building collapsed and four people died. The panels were not the main problem. The connections did not tie the structure together well enough to survive the loss of one element. Building standards for robustness changed afterwards, in the UK and elsewhere.

Where connections get overlooked

These are patterns drawn from engineering reasoning and site experience. They are not a measured failure rate.

During construction

• Connections incomplete when temporary support comes off. Welds not finished, bolts not tightened, grout not placed or not cured. The element looks fine, but the connection does not yet exist.
• Wrong grout or poor placement. Grouted joints and dowel pockets need the specified non-shrink grout, full fill with no voids, and proper curing.
• Cast-in items or holes in the wrong place. When fixings are out of position, the site fix is often an unplanned weld, packing or extra hole that nobody has checked.
• Substitutions. A different plate, bolt grade, weld size or bar swapped on site for convenience, without the engineer's review.

During finishing and fit-out

• Connections buried by linings and services. Once a ceiling, lining, cladding or fire-rating system goes over a connection, nobody inspects it again. That is why connection checks should be hold points before it is covered. See our article on hold points and the handover record.
• Penetrations and cuts. Core drilling or cutting near a connection to run services can remove reinforcement or damage cast-in items.
• Sealing or filling the wrong joint. Movement joints are sometimes filled with grout during a repair, which locks them and moves the stress elsewhere. Structural connections are sometimes sealed over and treated as movement joints.

Over the life of the building

• Corrosion. Steel plates, bolts and dowels corrode where water reaches them: at failed sealed joints, at the base of walls and columns, and in coastal or wet locations. Corrosion swells and cracks the surrounding concrete, which exposes more steel.
• Water at the base. Failed damp-proofing or blocked drainage can corrode the connections at the footing, which is often the most heavily loaded point.
• Alterations. New openings, removed walls, added roof loads and mezzanines can change the load path. Each change needs an engineer to confirm the original connections still work.
• Deferred maintenance. Failed sealant lets water into the joint. By the time rust staining or spalling shows on the face, the connection behind it may have lost section.

Signs to look for

None of these proves a structural problem. Each is a reason to ask an engineer to look.

• Rust staining, cracking or spalling concrete at joints, connection pockets or the base of a wall or column.
• Cracks that follow the line of a connection, or that open and close with the weather.
• Elements that have moved out of line, or joints that have widened unevenly.
• Grout at dowel pockets or bases that is cracked, crumbling or missing.
• Water staining at the base of walls or at junctions inside the building.
• Alterations nearby that nobody can show engineering sign-off for.

What the code and standards say

• NCC structural provisions (Part B1) require a building to be designed with appropriate reliability against structural failure, having regard to the consequences of failure. The code sets the outcome. The standards and the engineer's design supply the detail. Check which NCC edition applies to your building, because the structural provisions were revised in the latest edition.
• AS/NZS 1170.0 is the general structural design standard for actions and combinations. It also addresses robustness, which is the principle behind tying elements together.
• Material standards cover connection design for each system, for example AS 3600 (concrete structures), AS 4100 (steel structures), AS 3700 (masonry structures) and the AS 1684 series (residential timber-framed construction).
• AS 3850 covers prefabricated concrete elements. Part 2 deals with building construction and, from its published contents, includes handling and erection, temporary bracing of vertical elements, joint widths, footings, connections (including restraint of vertical elements) and tolerances. It replaced AS 3850-2003, which was titled "Tilt-up concrete construction". The publication status of the 2015 editions has changed over time, so confirm the current edition with Standards Australia.

We have not reproduced clause text here. If you need a specific requirement for a project, it comes from the engineer's design documents and the current published standard.

What to ask for

For a new build, an alteration or a precast or tilt-up project:

1. Does the engineer's drawing set show every structural connection, with sizes, grades, weld details, grout type and cure requirement, and does it say which are rigid and which are pinned?
2. Has the engineer reviewed the shop drawings, including cast-in items, for any prefabricated or fabricated elements?
3. Who checks the connections before temporary support is released, and is that a recorded hold point?
4. Are grout placement and curing records kept (product, batch, date, who placed it)?
5. Are welds and bolt tightening inspected, and is the inspection recorded?
6. Which joints are movement joints, what sealant and backing rod are used, and what is the maintenance interval?
7. Before any alteration, has an engineer confirmed the effect on the load path?

For an existing building, add a condition inspection of the connections at the bases of walls and columns and at joints, particularly if you see any of the signs above.

How we approach it

At Precision Projects Group we work across new construction, renovation and strata building maintenance, so we see both when connections are built and when they have been neglected for twenty years. We treat structural connections as hold points: they are checked and recorded before they are covered, and the handover record shows what was checked and by whom. For existing buildings we arrange inspection by a qualified engineer where there are signs of distress, and coordinate the repair so that the load path and the weatherproofing are both put right.

If you are planning a project, or you manage a building with these signs, get in touch or see our services. Our article on material compatibility, sequence and junctions covers a related problem at the finishes level.

Sources

• NCC Volume One, Part B1 Structural provisions, Australian Building Codes Board
• AS 3850.1:2015 Prefabricated concrete elements, General requirements, Standards Australia
• AS 3850.2:2015 Prefabricated concrete elements, Building construction, Standards Australia
• Tilt-Up Concrete Construction to Prefabricated Concrete Elements: the changes to AS 3850, National Precast Concrete Association Australia
• AS/NZS 1170.0, Structural design actions: General principles; AS 3600, Concrete structures; AS 4100, Steel structures; AS 3700, Masonry structures; AS 1684, Residential timber-framed construction, Standards Australia
• Ronan Point collapse, 16 May 1968, London: widely documented in UK building-regulation history

This article is general information, not technical or legal advice for your building. Structural connection design, inspection and repair depend on the design, the building class and the site, and must be done or confirmed by a qualified structural engineer. Full website disclaimer