First floor
Transfer beam
Ground to first-floor column
Figure 4.1 Transfer beam under wall.
4.3.2 Option 2: Lowest level of wall acts as transfer beam
Where the columns can be placed near to the ends of wall panels, or where an interstitial plant zone would not need a corridor opening, the resulting solid panel can act as a storey deep transfer beam, sometimes with a thicker section. The wall acts as a deep beam to spread the loads from above to the supporting columns (see Figure 4.2). There are particular Figure 4.2
Lowest level of wall used as transfer beam.
First floor
Wall acts as transfer beam
Ground to first-floor column
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The third option is to use the strut and tie design method to reduce the depth and/or width of the transfer beam. This technique is not widely used in the UK, but Eurocode 216 offers more guidance than was provided in BS 811017. It is outside the scope of this guide to explain the principles and application of this method.
The fi gures below show how the strut and tie method could be used in a variety of situations to produce an economic design. However, many buildings do not have lintels across the corridors due to low ceiling height or services distribution along the corridor ceiling void;
a lintel or beam is critical for adopting this stability concept.
Figure 4.3 shows lintel beams across the corridor together with a strong transfer beam. In many cases this can provide the required lateral stability.
Figure 4.4 shows no lintel beams and no transfer beam. The pinned struts across the corridor result in a mechanism, hence stability cores are required. Checks are also required to ensure the strut action has a valid load path; for example, vertical service risers often punch through the strut load path.
Figure 4.5 shows the consequences of offset openings. In these cases, the out-of-balance forces require an additional column and/or stability cores.
4.3.3 Option 3: Strut and tie design
4 Structural support
design considerations to consider in this option, as listed below:
At 200 mm or less the ‘deep beam’ will be particularly narrow, therefore the layout of the reinforcement should be considered at the early stages to ensure that the required reinforcement can be fitted within the element.
Eurocode 216 includes some particular rules for the design of deep beams which should be followed.
There will be some load from the adjacent floor that is carried at the lowest part of the beam and which may require additional link reinforcement (often referred to as ‘hang-up steel’) to resist the tension forces this imposes at the bottom of the beam.
The bearing area at the supports will be small and consequently the local stresses will be high and should be considered in more detail.
The construction sequence is important and should be clearly conveyed to other members of the team, especially the contractors.
The additional reinforcement will slow down the first lift of the construction using tunnel form. However, overall it is probably less time-consuming than constructing a transfer beam before starting the tunnel form construction.
With crosswall, this method is only practical when one panel can span between the sup-ports; even then careful consideration as to how the lower floor is supported is required.
It is unlikely that this method can be used with twinwall construction due to the fact that there is insufficient space for the flexure reinforcement.
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Structural support 4
Compression in columns Tension in beam
Vierendeel action above openings
Compression in wall evenly distributed to beam
Figure 4.3 Transfer structure using strut and tie with
beam under wall.
Tie Strut
High compression in narrow width
Structure above openings acts as prop/tie, i.e. pinned
Balanced compression forces
Low compression in narrow width Significant vierendeel
action above openings
Strong couple to provide lateral stability Poor lateral stability
requires strong cores Below: Figure 4.4
Design using strut and tie to minimise transfer structure.
Below right: Figure 4.5 Transfer structure – infl uence of door openings.
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Where a column or wall is supported at its lowest level by an element other than a foun-dation, alternative load paths should be provided in the event of the accidental loss of this element. In in-situ reinforced concrete the reinforcement can generally be used to tie the structure together. Where ties are not or cannot be provided, either:
the vertical member should be demonstrated for ‘non-removability’. Non-removability may be assumed if the element and its connections are capable of withstanding a design action at a limit state of 34 kN/m2 in any direction over the projected area of the member together with the reactions from attached components, which themselves are subject to a loading of 34 kN/m2. These reactions may be limited to the maximum reaction that can be transmitted; or
each element should be considered to be removed one at a time and an alternative load path demonstrated.
Further guidance on designing the ties for crosswall construction is given in section 5.10.
4.4 Robustness
4 Structural support
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