Advisory Desk
NSC 31
Technical Digest 2019
2. Smith, A L, Hicks, S J, Devine P J, Design of floors for vibration: a new approach,
Revised edition, February 2009, SCI publication P354
3. Willford, M R, Young, P, A design guide for footfall induced vibration of structures,
Concrete Centre, November 2006
Contact: Callum Heavens
Tel: 01344 636555
Email: advisory@steel-sci.com
AD 434
Validity rules for hollow section
joints
This AD note concerns a significant typographical error in Table 7.8
of BS EN 1993-1-8. The table presents validity limits for welded joints
between hollow section brace members and RHS chord members.
For the common case of a K or N overlap, there is a limitation under the
“Gap or overlap” column expressed as:
bi /bj ≤ 0.75
where bi is the width of the overlapping bracing member, and bj is
the width of the overlapped member. Thus the limit precludes bracing
members of the same size, for which bi /bj = 1 , and this is clearly wrong.
In fact, the limit should be expressed as:
bi /bj ≥ 0.75
which prevents a narrow overlapping bracing being welded to a wide
overlapped brace, but permits bracing of the same width to be used.
This limit is correctly expressed in literature published by Tata Steel, and
has been corrected in the draft revisions to EN 1993-1-8.
As an aside, it may assist designers to note that definitions of some
factors that appear in the joint verification expressions, such as β, λov, n and
γ are found in Section 1.5 of BS EN 1993-1-8, not in Section 7 as might be
expected. Similarly the definition and dimensions of gap and overlap joints
are found in Figure 1.3, rather than in the section concerned with hollow
section joints.
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steelconstruction.org
AD 435
Beams supporting precast planks:
checks in the temporary condition
The purpose of this note is to remind designers of their responsibility
for basing their design on a safe method of erection. This is particularly
necessary if structural stability in the part-erected condition is not evident.
The CDM (2015) regulations consider this in Regulation 11 where “(1) The
principal designer must … ensure that, so far as is reasonably practicable, the
project is carried out without risks to health or safety. …
“In fulfilling the duties in paragraph (1), the principal designer must
identify and eliminate or control, so far as is reasonably practicable,
foreseeable risks to the health or safety of any person –
(a) carrying out or liable to be affected by construction work; …”.
BS EN 1090-2:2018 addresses this issue more directly in paragraph 9.3.1
which states that the design basis method of erection shall consider amongst
other things the following: “d) stability concept for the part-erected structure
including any requirements for temporary bracing or propping”.
SCI publication P401: Design of composite beams using precast concrete
slabs in accordance with Eurocode 4 states in Section 3.6 “The stability of
the steel beams during the erection of the floor units and the placement of
the structural topping must be considered. The designer should take due
account of the floor erection process (which will usually require erection in
‘bays’ to avoid excessive re-siting of the crane). Should a particular sequence
of erection or temporary support be necessary, this should be noted in the
specification and on the drawings. The placement of the precast concrete
units should be carefully controlled in order that out of balance construction
loads are kept within the limits assumed in the beam design …”.
Section 4 of the publication discusses the checks for torsion which should
be carried out in the event that an out-of-balance load results from the
assumed erection sequence. Such conditions may result from:
1. The assumed erection sequence;
2. Unequal plank spans on either side of the beam;
3. Planks spanning in different directions on either side of the beam;
4. The sequence of placing the in-situ topping.
Other relevant issues are the effectiveness of the lateral restraint provided
by the precast planks and the specification of additional restraint if the
planks are inadequate by themselves. (See P401, Section 3.6).
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com
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