Preliminary tests are not always practical, 5 test ties at the bottom of a 20 x 20?
A lot of occasions the material at the bottom if the building differs from the top ie, brick to stone or shop windows ect what is the point were missing?
Morning Baxter
I think the answer is all in the first line of 6.1 Preliminary Tests, "These are carried out wherever there is any doubt"
6.1 also points out that Preliminary Tests are to be carried out in each different base material.
I would guess the point being missed is that over a number of years of developing testing proceedures for the industry there are now guide lines for all to follow thus attaining a systimatical and common set of results upon which to base the suitability of the anchor system chosen for, and or used on a job.
Extract from TG4:11
6.1 Preliminary Tests
These are to be carried out
wherever there is any doubt about the suitability or recommended load capacity of proposed anchors for a particular base material, e.g. if there is no manufacturer’s recommended load data for the base material which is often the case with brickwork, stonework and timber. The approach is to test a series of 5 sample anchors to a load which demonstrates a satisfactory safety margin and thereby, if possible,
avoid testing fixings to failure. If any of these anchors fails to support the test load then, the results should be referred to the responsible designer who should consider the options outlined below.
Procedure
5 tests should be carried out in each different base material of the project. They should be carried out on sample anchors in the same base material but away from areas which will be used and must not be used in the job. The procedure is outlined in more detail in an Article on the CFA website[12].
Test load:
All anchor types except nylon anchors shall be loaded in tension to a load of 2 x the working load as applied to the anchor,
Nylon anchors should be tested to 3 x working load.
If all test anchors take the test load without slip then the anchor may be used in that base material for the proposed working load. Should any anchor fail to meet the required test load then that fact should be referred back to the person responsible for the anchorage design of the project.
Possible courses of action to consider:
(a) use an anchor of the same type but with a deeper embedment depth*
(b) use an anchor of the same type but with a larger diameter*
(c) use a different type of anchor*
(d) redesign the scaffold to reduce the loadings while remaining within the criteria of TG20.
(e) use the original anchor specification but with an allowable load derived from further tests as below:
*For any solution (a) – (c) a new series of preliminary tests must be carried out.
Procedure for option (e) using the same anchor with reduced loads:
— For each anchor in the original series of tests which held the test load take each carefully to failure.
— Determine the allowable load from the lowest of the following values:
For all except nylon anchors
• the average failure load** ÷ 3
or
• the lowest failure load** ÷ 2
For nylon anchors
• the average failure load** ÷ 5
or
• the lowest failure load** ÷ 3
** Calculations are based on all 5 failure loads. The resulting allowable load may not be higher than the manufacturer’s recommended load in similar or stronger base materials.
The failure load is taken as the maximum load reached during the test or the load at approximately 1mm movement in the case where an anchor pulls out of the base material.
Note: Allowable loads determined from tests on one job should never be considered suitable for the design of another job unless the base material is known to be identical.
The new “Allowable load” must then be made known to the designer who should decide on the best course of action.
Depending on the proposed anchor type it may be possible to retain the same number of ties while doubling the fixings per tie by, e.g. changing from a tie using one fixing to a tie that uses two – in this case the manufacturer’s recommendations regarding anchor centre spacings should be followed. Or the scaffold may be redesigned for a reduced working load (no greater than the Allowable Load just determined) being transferred through the same number of ties while remaining within the criteria of TG20.
When brickwork or stonework is weak then a secondary means of restraint should be considered.
I hope this is of some help
regards
Alan
