warehouse is said to be
the largest building in
the South West
A range of big spans
David Brown of the SCI discusses some of
the features of large distribution centres.
Apart from the overall size, The Range
distribution centre is of conventional
portal frame design, illustrating
the economic dominance of this form of
construction when enclosing large volumes.
The spans of 36.5m and centres of 8m
are quite orthodox, and 15m clear to the
underside of the haunch is not uncommon for
this form of distribution facility.
The sheer size of the structure does
bring one rather unusual issue, which is the
contribution of drag to the overall wind
loads. Clause 18.104.22.168 of BS 6399-2 covers the
frictional drag component, which comprises
a frictional force from the walls (Clause 2.4.5)
and a frictional force from the roof (Clause
2.5.10), both depending on the type of surface
and thus the frictional drag coefficient from
For walls, the frictional drag is assumed to
act over all of zone C (the most downwind
zone, away from the turbulence that causes
high local suctions at the windward edges).
For roofs, when wind is blowing parallel to the
ridge(s), frictional drag is again assumed to act
over zone D, the most downwind zone.
For small buildings, the effect of drag is
often rather insignificant, but for The Range
distribution centre, the unfactored contribution
of drag, from the roof alone, was 1200 kN.
In contrast, the overall load from the
pressure and suction on the end elevations was
approximately 2800 kN, so the drag from the
roof alone increased the overall force by over
40%. With larger plan buildings, the lesson is to
never neglect drag – it can be considerable.
Distribution centres, as their name implies,
are almost certain to have many openings on
the elevations. Depending on the direction
of the wind and the location of the openings,
much increased pressure or suction inside the
building could be the result. This is the effect
of a dominant opening, covered in Clause 2.6.2
of BS 6399-2.
The designer must carefully consider if
assuming the doors to be shut in the event
of a severe storm is realistic. BS EN 1991-1-4
requires that if an opening would be dominant
(almost certainly) yet is assumed shut, the
accident of the door being open should be
A second challenge with distribution centres
is that the large doors can often mean that
locating vertical bracing on the elevations can
be problematic. This was the case at The Range
distribution centre, so portalised bracing
was adopted around the door openings and
conventional diagonal bracing above.