How to select anchors in accordance with

Transcription

How to select anchors in accordance with
How to select anchors in accordance with
BS 8539:2012 Code of practice for the selection and installation of
post-installed anchors in concrete and masonry.
1
Introduction
BS 8539 is arguably the most significant development in the use of construction fixings since the
advent of European Technical Approvals (ETA)1. It was published as a result of a series of
failures of anchors due to the failure of various stakeholders to properly select or install them
correctly and in order to prevent further such incidents. If fully adopted it will result in a reduction
in failures of construction fixings with consequent savings in injuries, deaths and liabilities.
This “How to” guide provides an overview of the responsibilities of the designer and specifier in
selecting anchors for safety critical applications together with the actions which are key to
satisfying those responsibilities. All stakeholders should acquire a copy of the code2.
1.1
Key themes
Key themes of the code include the importance of using anchors with ETA if at all possible, the
need for one person to carry the overall responsibility for anchors from selection to
commissioning - including temporary works phases, the passing of information between
stakeholders at appropriate stages and a recognition that anchor specifications should not be
changed unless the full selection procedure is undertaken.
1.2
Terminology
Terminology used in the code is based largely on that used in ETA, in ETAGs (the Guidelines
used to award ETA) and CEN Technical Specifications3 for anchors. This has resulted in the
adoption of terms and notation not previously used by the CFA or by some manufacturers. For a
detailed explanation of the differences between the new and old terminology see4.
The term “fixing” – integral to the name of the CFA – does not appear in the code having been
replaced, for consistency and clarity, with “anchor”. Anchors are still fixings.
The term “load”, is used only in the general sense having been replaced by “Action”, - the load to
be transferred from the fixture to the base material, and “Resistance” - the capability of the
anchor in resisting the action.
BS 8539 defines selection5 as the overall process of choosing the anchor including both the type
and size, and within that process design6 is the particular process of determining the size of
anchor(s) required, referred to in conjunction with anchors carrying ETA as the Design Method.
1.3
Responsibilities
BS 8539 defines the designer as the person with overall responsibility for the design of the
structure, which includes the anchorage, and the specifier as the person responsible for the
selection (including the design) of an anchor. Note: the designer and specifier may be the same
person.
CFA
A “How to” guide to BS 8539 for specifiers and designers.
(Issue 1. March 2013)
1 of 4
2
Preliminary considerations (BS 8539 Clause 5.2)
The designer should take into account the preliminary design considerations listed below and
supply all the necessary information to the specifier.
a) Ability of the structure to support the applied actions.
The design process will determine whether or not the local area of a structure is capable
of accepting the applied actions but this process does not verify that the overall structure
has sufficient integrity to sustain these actions. This must be verified independently by the
designer. It should be noted that tests carried out on site to the procedures outlined in BS
8539 will not contribute to this verification.
b) Concrete condition. The designer should determine if the concrete is cracked or noncracked. The default would be to assume it to be cracked.
c) Robustness, redundancy and progressive collapse. Consideration must be given as
to whether or not there is sufficient built-in redundancy to allow for load distribution in the
case of individual anchor failure, i.e. is it statically determinate or indeterminate. This may
affect the choice of ETAG to which the ETA of an anchor has been awarded. Clause 5.2
gives several helpful references on this subject.
3
Information to be provided by the manufacturer/supplier to the specifier (BS 8539
Clause 6.2)
Full technical data should be provided by the manufacturer/supplier to the specifier, including
the following:
•
•
•
•
•
•
•
•
•
designation of anchor including size and type
material type used to manufacture the anchor, e.g. stainless steel/carbon steel
design method to use e.g. ETAG 001 Annex C or CEN TS 1992-4 Parts 1 – 5
performance data (preferably given in an ETA), including:
• characteristic resistance
• design resistance (or partial safety factor, for the material, to enable it to be
calculated)
• recommended resistance (or appropriate safety factor to enable it to be calculated)
effective embedment depth,
minimum and maximum fixture thickness
edge and spacing criteria
minimum thickness of base material
setting details including
• temperature limits and curing times if appropriate
• installation equipment
• installation instructions
The anchor manufacturers generally provide the required information in one (or all) of the
following formats, usually available from the company website:
o technical manuals
o anchor design software
o any associated information on anchor technology and design, ETAs and safety data
sheets (SDS)
CFA
A “How to” guide to BS 8539 for specifiers and designers.
(Issue 1. March 2013)
2 of 4
4
Information to be provided by the designer to the specifier (BS 8539 Clause 6.3)
If the designer is not also the specifier, they should supply all the necessary information to
enable the specifier to select and specify the anchor. This information should include, for
example:
The application details, including;
•
design actions, their directions and nature (static and/or dynamic)
•
whether the application is statically determinate or statically indeterminate
•
confirmation that the structure is capable of sustaining the characteristic action
•
details of the base material type, thickness and likely strength at the time of
installation
•
in the case of concrete - its status i.e. cracked or non-cracked
•
edge distance and anchor spacing used for the design
•
details of the base plate, material and thickness
•
environmental conditions, including elevated temperatures or fire rating
requirements, corrosion conditions and required durability or life expectancy
•
special requirements (e.g. need for through fixing, immediate loading, flush fixing for
ease of removal, etc.)
5
Selection process
BS 8539 considers the selection process in two phases – determination of the anchor type (BS
8539 Clause 5.3) and determination of the anchor size (BS 8539 Clause 5.4) this part of the
process being referred to as the “Design” of the anchor, see section 6 below.
Determining the anchor type means considering – among other things - anchor reliability, base
material, positioning, environmental considerations (temperature and corrosion) and various
aspects of the loading.
Anchor reliability BS 8539 recommends that anchors should be selected which will function
reliably in all potential site conditions and that one way of achieving this is to select an
anchor with an ETA.
Base material considerations include its type (concrete or masonry and whether solid or
hollow) its nature (if concrete is it cracked or uncracked?), structural thickness, etc.
Loading aspects include the nature, magnitude, direction and duration of the applied load.
Nature: - is the overall loading statically determinate or indeterminate? (as discussed in 2
c) above) and is it a static or non-static action? (BS 8539 Annex E).
The magnitude of the characteristic action (a.k.a. applied [unfactored] load) is
fundamental to the anchor selection and handled within the design process (See BS
8539 Annex A).
Direction. Different anchors have different capabilities in tension and shear and a special
approach is needed for combined tensile and shear loads. Few anchors cope with
bending loads very well.
Duration. Is the action applied over the short or long term?
Temperature considerations may influence the type of anchor in terms of limitations placed
on service temperatures (plastic anchors and resin bonded anchors) or installation
temperatures (resin bonded anchors).
Corrosion aspects are discussed within BS 8539 in clause 5.3.5.2 and Annex F.
CFA
A “How to” guide to BS 8539 for specifiers and designers.
(Issue 1. March 2013)
3 of 4
6
Design method (See BS 8539 Annex A)
The design method for an anchor with an ETA will be referred to in the ETA itself and will
commonly be to ETAG 001, Annex C, for mechanical anchors or TR 029 for bonded anchors, the
details of which may be downloaded from the EOTA website at http://www.eota.eu. Some ETA
may refer to design methods of CEN TS 1992-43. The design would, in all cases, require
reference to the ETA for the particular anchor to obtain the relevant design data.
The practical approach is to download design software from the anchor manufacturers’ website,
which will provide an accurate and efficient design along with the widest choice of suitable anchor
types to select from. Most companies also offer technical support to assist in this and engineers
who can visit your office/site to discuss the application.
The manufacturers also provide technical manuals that allow “simplified” designs to be carried
out by hand, but these are usually much more conservative than the software and the data is
updated far less frequently.
7
Changing specifications (BS 8539 Clause 10)
Anchor specifications are frequently qualified with the phrase “or equivalent” or “or similar
approved”. With or without such a phrase changes to specifications should only be made
following a full change management process carried out by the specifier.
It might be necessary to change the specification of an anchor for a variety of reasons including:
• unavailability of the specified anchor;
• contractor has a preferred supplier;
• a change in the design loading information means the current anchor specification is
inappropriate;
• economic reasons.
Whatever the reason for the change request, the alternative specification should be determined
by either the original anchor specifier or by a responsible engineer on site who assumes the role
and responsibilities of the specifier and has access to the original design data. The full process
must be completed.
It is not sufficient to change the specified anchor for one which appears to be similar, or to
compare headline performance figures quoted in catalogues of the proposed alternative anchor
with those of the originally specified anchor. To do so might not take into account the way in
which the performance of different anchors changes with particular application factors . Nor is it
acceptable to carry out proof load tests of the proposed alternative anchor on site, as this does
not validate the required safety margin.
1
For more information on ETAs see CFA Guidance Note: ETAs and design methods for anchors used in
For copies of British Standards relating to construction fixings, at a discount of 15%, contact the CFA via
our website www.the-cfa.co.uk.
3
Design methods for anchors with ETA may be contained in CEN Technical Specifications in the series
CEN TS 1992-4 Parts 1 – 5. (Otherwise they are as the relevant ETAG.)
4
CFA Guidance Note: Anchor terminology and notation
5
BS 8539 Clauses 3.1.39 & 5. CFA Guidance Note: Anchor selection.
6
BS 8539 Clause 5.
2
CFA
A “How to” guide to BS 8539 for specifiers and designers.
(Issue 1. March 2013)
4 of 4