www.wwtonline.co.uk | WWT | sepTember 2014 | 35
In the know
Technically speaking: BS5911 Part 3:2014
depth of both rectangular and circular
manholes for a defined strength class
as the boxed example demonstrates.
The final significant change to the
standard is to Table 8, which con-
cerns test loads for covers, reducing
and landing slabs to fit chambers
greater than 1250mm diameter and
corbel slabs. This change has been
introduced to align the standard with
Eurocodes.
As a result the 150kN and 100kN
wheel loads from adjacent lanes (EC1
Pt2 Load Model LM1 TS) has now been
simplified to a single load of 250kN
over a 400mm x 400mm contact area
for testing purposes.
These changes to the standard
should now enable users to take full
advantage of circular, off-site manu-
factured manhole solutions. This will
allow them to reduce installation time
by up to 50% and installed costs by
15-30%, predominantly when the man-
hole is installed without a concrete
surround. And because the units are
manufactured under controlled condi-
tions, quality is also assured.
For more information visit
www.concretepipes.co.uk/page/quality-
and-standards
Example
Consider a manhole designed to be in
accordance with sewers for Adoption
7th edition at a maximum depth
of 3.5m in good ground (D=20kN/
m3, ø'=30˚) with a road surcharge
of 12kN/m
2
. In accordance with
bs5911-3 Annex A, bs eN 1992-1-1,
bs eN 1997-1
Option A – 1250 x 1250 rectangular
Strength Class 24 / Fn = 30kN/m
From 3.1 M(Rd,c) = 3.24 kNm
From 3.2
σO = 49.5kN/m
2
From 3.3 M(Ed,ext) = 8.08 kNm
external corner gives critical moment
From 3.4 3.24 / 1.15= 2.82 < 8.08 Unit not satisfactory
select higher strength class and recheck
Option B – 1200ø circular sha Strength Class 25 / Fn = 30kN/m
Compressive stress under installed conditions
fc =
σO D / 2t where D = average diameter, t = wall thickness
= 49.5 x 10^
-3
x 1290 / 2 / 90= 0.35 N/mm2
From bs eN 1992-1-1 cl 3.1.6
Design compressive strength fcd = αcc fck / γc = 0.8 x 30 / 1.5 = 16N/mm
2
16 > 0.35N/mm
2
Unit satisfactory
select Class 25 circular unit
•Total Thermal Oxidation
•Filter
Free
•Ozone Free
•Catalyst Free
German
Engineering
•ATEX Zone I and II Options
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