Civil Engineering Reference
In-Depth Information
alone is insufficient to meet the requirements of the Building
Regulations and Approved Document A, and therefore those
requirements of BS 8110 that are not covered by Eurocode 2 are
published in PD 6687-1:2010 (BSI, 2010) as non-contradictory
complementary information. Eurocode 2 requires that verti-
cal ties are provided only in panel buildings of five storeys
or more, which is at odds with Approved Document A which
requires vertical ties in all Class 2B buildings (unless alterna-
tive loadpath analysis is used). Vertical ties should be continu-
ous over the full height of the building. All precast stairs and
stairs incorporated into
in situ
concrete construction should be
effectively anchored to their supports whether or not they are
used as ties, the anchorage being designed to be capable of car-
rying the weight of the unit (and imposed load corresponding
to use for escape) to the tying system.
In post-tensioned slab design, the pre-stressing tendons should
be fully grouted such that the risk of loss of tension in fire or
explosion is eliminated. In unbonded construction, tendons
should not be considered part of the tying system, and tying pro-
vided wholly with normal reinforcement. Bonded tendons pro-
vide excellent horizontal tying due to the absence of laps. Where
possible, pre-stressing tendons should pass directly over the col-
umn heads between the vertical reinforcement in the columns.
The tendons should be inclined such that they drop down through
the slab to the bottom flange towards midspan. This gives a sub-
stantially greater resistance to punching shear and thereby greater
robustness than if the tendons pass either side of the column face
or outside the shear zone (Brooker, 2008; Pinho Ramos
et al
.,
2008). Where in pre-stressed construction it is not possible for
the tendons to pass through the columns, the tendons should
be as close as possible to the column heads and additional bot-
tom steel should be provided to lap over the duct line. A similar
arrangement should be adopted in non-pre-stressed slab construc-
tion whereby the bottom bars pass through the columns and are
anchored securely into the surrounding slab. Both pre-stressing
tendons and bottom reinforcement in non-pre-stressed construc-
tion should be designed for 100% of the post-failure load.
The principal remaining challenge in pre-stressed concrete
is achieving sufficient interaction between horizontal and ver-
tical ties.
Approved Document A as an alternative to effective horizon-
tal ties. Standard details are given in BS 5268-2:2002 (BSI,
2002a), although in reality these details are just a nominal con-
nection that provides only a very modest level of robustness.
In large-panel timber panel construction robustness is typ-
ically relatively modest, while advances in timber technol-
ogy have led to an acceleration in the height of large-panel
timber construction (Wells, 2011) so that such buildings are
now approaching a Class 3 risk classification. BS 5628-2:2002
(BSI, 2002a) recommends that internal ties should be designed
for a maximum of 3.5 kN/m if distributed, an order of mag-
nitude lower than in reinforced concrete or steel construction
that, while the all-up building weight in timber construction
is lower, cannot be easily justified. A continuous rim beam at
each floor level capable of supporting the floor above in the
event of removal of the panel below provides a continuous
peripheral tie and can be an effective means of enhancing the
robustness of the structure. A rim beam at eaves level in single-
storey timber panel construction fulfils a similar function.
12.12.4 Robustness in load-bearing masonry
construction
Historically, many load-bearing masonry buildings were
exempt from robustness requirements because the height
of construction was limited, but the extension of the robust-
ness requirements in 2004 to almost all buildings changed
this. Consequently, in almost all load-bearing masonry build-
ings except for single residential dwellings it is necessary to
provide, as a minimum, effective horizontal ties or effective
anchorage of slabs to walls. In Class 2B construction, the
emphasis is usually on alternative loadpath analysis to dem-
onstrate suitable bridging capacity, although in the 1970s the
Brick Development Association developed design recommen-
dations in response to the fifth amendment, showing how hori-
zontal and vertical tying can be successfully incorporated into
load-bearing masonry (Morton, 1985). More recent guidance
(Brick Development Association
et al
., 2005) gives lighter, less
robust design solutions based on the standard design details in
BS 5628-1:2005 described as effective anchorage rather than
effective tying (BSI, 2005a), though the design solutions in
Morton (1985) are preferable and closer to the intent of the
robustness requirements.
It is again noted that effective horizontal anchorage achieved
in Class 2A buildings using the standard design details given in
BS 5628-1:2005 is very modest, these details in reality being
relevant only to providing simple lateral restraint to horizon-
tal movement of load-bearing walls, rather than to sustain
the forces necessary for effective tying. The designer should
implement effective horizontal tying in all Class 2A and Class
2B buildings rather than 'effective anchorage' in the former,
and alternative loadpath analysis if vertical tying cannot be
achieved in the latter. While this may add modest cost to the
scheme, the engineer has a responsibility to demonstrate that
all reasonable measures have been adopted in the design to
12.12.3 Robustness in timber construction
In timber construction, the main design guidance is from the
UK Timber Frame Association (2008). Due to the nature of
timber construction, vertical ties are not usually a practical
design option, and the usual approach in design is for the
structure to be designed to bridge over missing elements, or to
be based on design of key elements. Horizontal ties are more
easily achieved: flitch plate connections in timber frame con-
struction can accommodate the horizontal tie forces relatively
easily, although splitting along the grain usually limits the cap-
acity of the connection.
In domestic timber-framed construction, reliance is usu-
ally placed on 'effective anchorage of floors to walls' given in
