Connections in Precast Buildings: The Weakest Link in the Chain

1. A two-sided record

İTÜ’s 6 February report says two opposite things about precast buildings. Among the causes of collapse it lists, for industrial buildings, missing lateral stability connections and inadequate beam–column joints (§3.14 item 7; see DEP-03). The same report notes a good record for precast systems in post-earthquake housing: the Bingöl housing with moment-resisting (MAB3) precast frames came through the 2003 earthquake undamaged, and an 8-storey building in Van made monolithic by post-tensioning (MAB2) suffered no structural damage in 2011 (p. 168).

What unites the two observations is a single word: connection. A precast building is sound factory-made members joined on site; the weakest link is usually not the members but what joins them.

2. The connection type sets the behaviour factor

TBDY 2018 Chapter 8 defines two families. Pinned connections (MFB) transfer no moment, only shear and axial force: pinned (MFB1), welded (MFB2) and slotted (MFB3). Moment-resisting connections (MAB) must transfer the forces at the capacity of the connected members without loss of strength or ductility (8.4.1.2): wet (MAB1), fully post-tensioned (MAB2), wet-top/welded-bottom (MAB3) and sleeved-pinned (MAB4).

Table 4.1 ties this choice directly to the behaviour factor: a precast moment frame takes R = 7 with wet or post-tensioned connections, but R = 5 with welded or sleeved ones, and is limited to lower buildings. The typical industrial building, a single-storey system with columns pinned at the roof, has R = 3: it must carry most of the seismic force without reduction.

3. The forbidden detail

What the report calls "missing lateral stability connections" is explicitly forbidden by the code:

• Clause 8.3.1.3: in pinned connections, horizontal members may not be seated without connection on other horizontal members, columns or walls.
• Clause 8.3.1.4: friction at supports is ignored; friction cannot be relied on to keep a beam in place.
• Clause 8.3.1.8: forces in pinned connections are computed amplified by the overstrength factor D.

The details of a pinned connection are prescribed too: at least a 10 mm steel-reinforced elastomer pad where no steel plate is used (8.3.1.5); the pin hole at least 6 pin diameters from both the beam end and the free edge of the column support (8.3.3.1); hole diameter at least twice the pin diameter (8.3.3.2); d/φ > 6 in the shear calculation (8.3.3.7). In slotted connections the slot cheeks are at least 15 cm thick and reinforced for 1.2 times the overturning moment (8.3.5).

4. Stricter general rules

• The minimum concrete class is C30 (8.2.3.2); for cast-in-place members resisting earthquakes the lower limit is C25.
• In high-seismicity zones (DTS 1a, 2a), for precast buildings of BYS 2–3, strength-based design with I = 1.5 is only a preliminary design; the building is further assessed by deformation for Controlled Damage under DD-1 and Limited Damage under DD-3 (8.2.2.3).
• An undefined connection type may be used only if tests show adequate stiffness, strength and ductility, stable hysteresis and energy dissipation equivalent to a monolithic connection (8.2.1.3).

5. Lessons

1. Precast is not the problem; the connection is. The same technique produced undamaged buildings with the right connection and collapses with the wrong one.
2. No member simply "sits". Every beam is tied to its support by a pin, weld or slot.
3. The ductility of the connection sets R. Choosing a less ductile connection means a larger seismic force.
4. An industrial building is a structure too. Single-storey pinned systems are designed with R = 3; they leave very little of the seismic force to ductility.