Excessively sagging roofs, floors, beams

Most of us would remember the Curtin University roof collapse incident which happened in October 2020. Tragically, an apprentice carpenter, died after falling about 20 metres while working on the glass roof. The West Australian newspaper reported that a contractor, who previously worked on the project, refused to continue as it was unsafe. The contractor said: “I’m talking about 80mm deflection… it was crazy… Even after the glass got, I wasn’t comfortable at all.” He said he “refused to work under while I was there.” Excessive sagging of roofs, floors or slabs during or after the construction stage can be the sign of an upcoming structural failure. People in charge should always be fully attentive to any related notification of concern.

In terms of structural design there may be two different situations yielding excessive deflection of a structural member:

• Member is under designed.
• The member’s strength is adequate to avoid failure, but serviceability check is not done correctly. Though structurally safe, this may create an unbearable condition to the habitants of the structure due to the visually objectionable sag, noticeable vibration, damage to non-structural elements, etc.

Common reasons of structurally safe but excessively sagging roofs, floors or slabs which are adequate in terms of strength capacity:

• When the structural engineer uses a wrong deflection limit and accompanying load combinations during the serviceability check.

• When the structural engineer performs the deflection check on single members and ignores the total deflection of the overall floor, slab or roofing system. To estimate the amount of sagging realistically, the designer should sum up the deflection of the main beam, secondary beam and joist/slab to project the overall deflection under serviceability load combinations. For the beams connected to the columns, axial displacement of the columns to be excluded from the overall deflection. However, absolute vertical deflections may also be needed to figure out the storey lining with the neighbouring buildings and the storey height from the pavement level, etc.

• When the structural engineer neglects the long-term/creep deflection effects of timber beams. As per AS 1720.1-2010, for bending, compression and shear members, duration of load factor for creep deformation is 2 and 3 for seasoned and unseasoned timber, respectively.

• When the structural engineer neglects the long-term/creep deflection effects of reinforced concrete beams and slabs. As per AS 3600:2018, k_cs shall be used to increase the load factors of relevant serviceability load combinations. Effective second moment of inertia Ieff shall be used both for short term and long-term deflections. Ieff can be directly calculated from first principles or with the simplified calculation given in AS3600:2018. Regarding non prestressed slabs, k_cs can be between 0.8 and 2. This is affected by the thickness of the slab and the cover distance of the reinforcement as well as the reinforcement configuration. A structural engineer should be able to determine the location of the neutral axis because there is substantial difference between 0.8 and 2.

• For large span structures with roof pitches ≤ 3° ponding of rainwater can be a serious problem accompanied by additional ponding due to increased deflections. As per Woolcock et al., vertical deflection limit for industrial buildings with pitches ≤ 3° is advised to be smaller than L/500 for rafters under dead loads with the consideration of ponding effect during calculations (2011). This is applicable not only to the large commercial or industrial structures, but also to green roofs, to roof top floorings, etc. AS 3600:2018 clearly identifies the problem as: “Deflection limits given may not safeguard against ponding”. ACI 318 – 19 states “(Deflection) Limit not intended to safeguard against ponding. Ponding shall be checked by calculations of deflection, including added deflections due to ponded water, and considering time-dependent effects of sustained loads, camber, construction tolerances, and reliability of provisions for drainage”.

• Construction issues, such as loose bolt fixings and/or larger than nominated bolt holes of steel floor systems, etc.

Excessive sagging of structural members is a crucial problem during construction and during service life of the structure. An experienced structural engineer should inspect and structurally analyse the excessively sagging roof, floor or slab to determine if there is a possibility of failure and afterwards propose a feasible solution to the problem.

References:

Australian Standards, 2018. AS 3600:2018 - Concrete structures. Standards Australia.

Australian Standards, 2010. AS 1720.1-2010 Timber structures Part 1 – Design methods.

S.T. Woolcock, S. Kitipornchai, M.A. Bradford and G.A. Haddad. Design of Portal Frame Buildings including crane runway beams and monorails, 4th Edition, Australian Steel Institute, 2011.

American Concrete Institute (ACI). 2019. ACI 318-19: Building code requirements for structural concrete. American Concrete Institute.