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Sunday, June 24, 2018

Concealing of PVC pipes in RC Columns


In modern building construction, PVC pipes (plumbing works) on the surface of buildings is not always very desirable. In a country like Nigeria, PVC surface pipes deteriorate quickly due to weather conditions thereby leading to increased maintenance costs. On the other hand, they are usually not aesthetically pleasing.

Fig 2: Surface piping in a building




To solve this problem, architects normally provide ducts for MEP services (which is the best practice) during the design of a building. Another option that is normally considered is to conceal the pipes in walls or structural members.

Two things are actually involved;

(1) If it is an intentional design, or
(2) if it is an afterthought.

When it is part of the design, the structural engineer takes into account the effect of the pipes before producing working drawings. But when it is an afterthought, there is need to carry out checks and evaluate the effect of the plumbing work on the structural element before signing off.

Fig 3: PVC pipe concealed in masonry wall
We have always seen situations in buildings where structural members are compromised in order to allow pipes and other services pass through. This should not be so because starting from the onset, we should realise that services are part of a building, and should be considered during the planning and design stage. The flow of services in a building should not be an afterthought.

Let us use this example below to highlight the effect of installing PVC pipes in reinforced concrete columns.

Example
What is the effect of passing a 75 mm diameter pipe longitudinally through an axially loaded short reinforced concrete column with the following data?

Size of column = 230 x 230 mm
Grade of concrete = 25 Mpa
Grade  of steel = 410 Mpa
Reinforcement provided = 4Y16
Design axial load on column = 593 kN




Solution
From equation (39) of BS 8110-1:1997;

N = 0.35fcuAc + 0.7fyAsc

From the data provided above;
Asc = 804 mm2 (4Y16)
A=  (230 x 230) - 804 = 52096 mm2
N = [(0.35 × 25 × 52096) + (0.7 × 410 × 804)] = 686588 N = 686.588 kN

686.588 kN > 593 kN (Therefore column is adequate without the pipe)

On introducing the 75mm PVC pipe;
Area of pipe = (π × d2)/4 = (π × 752)/4 = 4417.86 mm2

Hence;
A=  (230 × 230) - 804 - 4417.86 = 47678.14 mm2
N = [(0.35 × 25 × 47678.14) + (0.7 × 410 × 804)] = 647931.725 N = 647.931 kN
647.931 kN > 593 kN (Therefore column is still adequate with the 75 mm pipe passing through it)

However, we should anticipate more complex interaction and verifications when the loading of the column is complex. Apart from the reduction in load carrying capacity of columns, care should be taken to ensure that the concrete is well consolidated to avoid honeycombs especially around the pipes. Also adequate care should be taken to ensure that the pipes are not leaking (inclusive of the joints) by pressure testing before concreting is done. Leakage of pipes might compromise the reinforcements by corrosion.

Summarily, adequate design and planning for pipe network in a building is the best solution - all options available should be evaluated. As far as possible, it is best to let the structural members be.


4 comments:

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