Method Statement for the fixing of Flooring and the subsequent building-up
of Party Walls for use with Cullen JH/JHI Masonry Hangers
We act as Consulting Engineers for Cullen Building Products and further
to your request for information pertaining to the above, we comment below
on the performance of Cullen JH/JHI masonry hangers where a partial cure
exists in the supporting mortar beds, and no top courses are present above
the hangers. The intention is to help in developing a method statement
for the fixing of flooring and the subsequent building-up of party walls
where Cullen masonry hangers are used. Before this is done, pertinent
background data is reviewed below under the following headings: (i) hanger
capacity at full cure with top courses, (ii) hanger capacity with no mortar/top
courses, (iii) mortar bed capacity at full cure without top courses, (iv)
mortar bed cure development, (v) site tests (vi) conclusions, (vii) recommendations.
(i) Cullen JH/JHI masonry hanger capacity at full cure with top load
Requirements for load testing and assessment of safe working loads of
masonry hangers are given in BS6178: 1990 'Joist Hangers - Part 1. Specification
for joist hangers into masonry walls of domestic dwellings'. This states
that joist hangers should be tested at least 28 days after the test walls
have been constructed, traditionally this having been done with a minimum
of 3 masonry courses above the hanger. Cullen JH/JHI masonry hangers were
tested and assessed to the requirements of this standard by both CERAM
and Coventry University during the period May-October 2001. A total of
60 tests were conducted on hangers of various size, and blocks of various
crushing strengths. These showed the dominant failure mode was crushing
of the block beneath the hanger, with a characteristic hanger failure
load (5% probability of failure) of 15kN. A corresponding safe working
load of 7.54kN was recommended for all masonry crushing strengths greater
than 2.8N/mm2 on all hanger widths. Because these BS6178 loads are inextricably
linked to a 28 day cure period, and at least 3 courses of cured masonry
being present above the hangers, they effectively relate to the hanger
capacity in the finished structure, rather than being of any great relevance
during the build process, where construction loads are applied with neither
a full cure in the masonry or a top load being present.
(ii) Cullen JH/JHI masonry hanger capacity with no mortar/top load
To establish a hanger capacity more appropriate to the construction process,
tests were later carried out at Cullen Building Products premises with
JH/JHI masonry hangers sitting on loose 3.5N/mm2 blocks with no mortar
present. These blocks were free to rotate; i.e. no other masonry tied
in on either side, and no masonry above. The mean failure load established
from 5 such tests, was 6.6kN (approx 45% of the BS6178 value), giving
a corresponding safe working load of approx 3kN. The failure mode on this
occasion was bending/straightening of the hanger top flange.
(iii) Mortar bed capacity at full cure without top courses present
Having established both lower and upper bound capacities for the hanger
itself, attention then shifted to the mortar bed capacity immediately
beneath the block upon which the hanger is placed. This is subject to
compression from the joist reaction itself, and to bending since this
load is applied near the edge of the block rather than on its centreline.
When no top courses exist above the hanger, this bending component dictates
the capacity of the bed joint. Our calculations indicate that a 600mm
length of designation III mortar in flexure on 100mm blockwork will sustain
a characteristic failure load (5% probability of failure) at full cure
of 5kN - 6kN, applied to the inside face of the blockwork. This would
equate to a safe working load of approx 1.6kN if a safety factor typical
of that used in masonry design were to be applied. When top courses are
later added, this mortar bed capacity will increase.
One assumption adopted in deriving this mortar bed capacity is that the
hanger is fitted tightly to the blockwork. If a small gap exists between
the hanger face and the wall, then the load eccentricity will increase
and the capacity of the mortar bed will drop dramatically. To ensure the
hanger is fitted tightly to the wall, it is therefore recommended that
return style masonry hangers are used, i.e. Cullen JHR/JHIR masonry hangers.
(iv) Mortar bed cure development
Since the mortar bed capacities discussed above relate to full-cure, it
is important to establish how quickly they develop. We have only been
able to locate one reference (BRE Digest 360) giving indicative bond strength
development values for various mortars with cure time. In the absence
of more specific data, it is assumed that compression strength will follow
a similar trend. The data relates to mortar curing in laboratory temperatures
at 20°C. No data has been located for cure development at lower temperatures,
although we do know it will be slower. The Digest suggests Designation
III mortar will develop over 70% of its 28-day bond strength after 3 days
at 20°C, with over 95% of its strength being attained after 7 days
at 20°C.
(v) Site tests
In order to validate the above estimates of the strength of Cullen JHR/JHIR
masonry hangers and of the mortar beds beneath during the build process,
practical site tests to destruction were conducted in June and August
2002 on floor sections constructed on two sites managed by Bovis Homes
Ltd. On both sites, the final two courses of masonry immediately below
the hanger locations were built-up during the late afternoon of the day
before the tests. On both occasions the weather conditions were dry and
warm throughout. Load testing to failure was then carried out the day
after, some 18-21hrs after the top courses had been laid.
On both sites a pair of joists were installed 600mm apart, simply supported
on Cullen return style 75x225 deep JHR masonry hangers. Chipboard was
laid on top of the joists adjacent to the load-bearing walls, and once
props had been installed 50mm beneath the hangers to arrest any collapse,
palletised packs of concrete blocks were then loaded onto the joists near
to the wall junction via a telescopic loader until failure occurred. On
each site, this operation was first conducted adjacent to the party wall
bearing (constructed using 100mm high density concrete block), and then
repeated at the opposite external wall bearing (constructed of 115mm lightweight
concrete block) once the failed party wall bearing had been temporarily
reinstated to its original level and supported on props from beneath.
This procedure was then repeated on a second section of wall, using two
new joists, yielding a total of 4 test results for each site (2 party
wall junctions and 2 external wall junctions). By measuring the average
weight of the concrete blocks used to load the joists, the spans of the
joists involved, and the positions at which the load was applied in each
case, the load applied to each hanger when failure occurred could be calculated.
The results are summarised below :
Site Hanger failure load - party wall junction (100mm dense conc. block)
Hanger failure load - external wall junction (115mm lightweight block)
Failure mode
Test 1 Test 2 Test 1 Test 2
Lowestoft (June 2002) 4.7kN 5.2kN 3.8kN 3.8kN Unbending of the hanger
top flange
Gosport (Aug 2002) 4.5kN 4.1kN 3.9kN 4.0kN Unbending of the hanger top
flange
As can be seen, failure in all cases was by unbending of the hanger top
flanges, at somewhat lower loads than had been determined previously in
laboratory tests (see section ii). Contrary to the indications given by
calculations (section iii) and the relatively early cure of the mortar
(section iv), no failure of the mortar was observed. Tests on joists which
were both tightly fitting between the walls, and with a gap of up to 10mm
in the hanger, suggested this had no influence on the failure loads/modes
observed.
In summary, the site testing carried out suggests that Cullen JHR/JHIR
masonry hangers with no top courses present will fail by unbending/straightening
of the metal top flanges at a load of approx 3.8kN on lightweight block,
and slightly higher values on denser blocks. Provided the masonry courses
beneath the hangers are cured for a period of at least 18hrs, this will
be the maximum load the joist platform can support before additional masonry
courses are added.
(vi) Conclusions
In summary, we believe loads of approximately 3.8kN (per joist) are required
to unbend/straighten the top flange of Cullen JHR/JHIR hangers when no
masonry courses exist above the hanger. Where sufficient top courses do
exist above the hanger, and the masonry cure is fully developed, the load
capacity of the hanger connection will ultimately be dictated by masonry
crushing at a value of approx 15kN.
Two situations exist during construction for which the development of
method statements would be desirable. The first relates to the time at
which the floor decking might be fixed after the supporting walls have
been built and the joist platform has been installed. The load which we
estimate one carpenter (0.95kN) carrying one sheet of 22x600x2440mm chipboard
(0.24kN) will impose when he stands directly on a joist near the hanger
is 1.2kN. Site tests indicate that the corresponding safety factor against
the top flange of the joist hanger unbending/straightening is approximately
3, and that provided dry/warm conditions prevailed during the initial
mortar cure, this operation can be safely carried out the day after the
masonry courses beneath the floor have been laid. To comply with the 1.2kN
maximum load requirement on any joist, boards should be stacked with their
2.4m direction perpendicular to the joists (laid across the joists), with
no more than 5No. 22x600x2440mm boards stacked on any one joist.
The second situation for which a method statement is desirable is the
time at which the sheathed floor can then be used as a working platform
from which to construct further masonry courses, and what the corresponding
maximum applied construction loads can be. From site tests it is known
that a maximum construction load of approx 1.3kN should be imposed on
Cullen JHR/JHIR masonry hangers, if a factor of safety of approximately
3 is to be maintained against the hanger top flange unbending/straightening.
Assuming one operative carrying and laying blocks will impose no more
than 0.95kN per joist, this suggests the loading out of blocks ready for
installation must impose no more than 35kg/joist (60kg/m run of wall).
This indicates that only a single course of high density (1500kg/m3) concrete
blocks can safely be loaded on the floor in a single line across the joists
before being built into the wall. In this case the safety factor against
the hanger top flanges unbending/straightening will again be approximately
3. To assure the necessary confidence in the degree of cure that exists
in the masonry at the time, it is recommended that this operation should
only be carried out when the masonry beneath the floor has cured for a
minimum of 3 days. Should it be necessary to load any more than one course
of blockwork in a single line across the floor at any one time, or wall
construction is necessary before 3 days, then it is recommended that independent
support is provided to the floor, or an alternative working platform is
constructed.
(vii) Recommendations
From the analysis and testing described above the following recommendations
can be given :
Fixing of Flooring
Site tests have indicated that fixing of flooring can safely be carried
out on floors supported using Cullen JHR/JHIR masonry hangers the day
after the masonry has been laid, provided dry/warm conditions have prevailed.
Should the temperature drop below 15°C for any significant length
of time during this initial cure period, or installation on the same day
is desired, then independent support should be provided to all joists
by means of temporary props designed to carry the requisite loads. Boards
should be stacked no more than 5 high, with their 2.4m direction perpendicular
to the joists (laid across the joists), and no more than one stack being
placed on any one joist.
Building-up of party walls
Site tests indicate that construction loads no greater than 1.3kN should
be applied to Cullen JHR/JHIR masonry hangers if a factor of safety of
3 is to be maintained against the hanger top flange unbending/straightening.
In addition to the loads imposed by a site operative laying masonry courses,
this is sufficient to enable one course of high density concrete blockwork
to be loaded on the floor at any one time, in a single line across the
joists, before they are installed above the hangers. This operation should
only be carried out when the masonry beneath the floor has cured for a
minimum of 3 days. Additional masonry courses can then be loaded-out in
a similar manner and constructed in single courses. Should it be necessary
to load any more than one course of blockwork in a single line across
the floor at any one time, or party wall construction needs to proceed
before 3 days have elapsed, then it is recommended that suitably designed
independent support is provided to the floor, or an alternative working
platform is constructed.
In both situations given above, it is essential that return-style Cullen
'JHR' or 'JHIR' masonry hangers are used throughout. These will ensure
the hangers are seated flush with the inside face of the blockwork, and
that the safety factor against hanger failure is the maximum attainable.
Joists must be seated firmly against the back of the hanger, and under
no circumstances should this gap exceed 6mm. The above statements have
specifically been developed for use with Cullen JHR/JHIR masonry hangers,
and are only valid where these hangers are used. No responsibility is
accepted for the use of this information for any other type of masonry
hanger.
