1.
INTRODUCTION.
2.
PRODUCT CHARACTERISTICS
2.1.
General
2.2. Dimensional Tolerances
2.3. Materials
3.
BEHAVIOUR UNDER LOAD.
4.
STANDARDS AND REGULATIONS
REFERENCES
1. INTRODUCTION.
Welded hollow sections manufactured at the Laminoirs de
Longtain display excellent buckling warping and torsion characteristics and have a
streamlined profile with the option of utilising the inner space. To this, we would add,
for example, the great ease of assembly that it facilitates.
As we have shown, the benefits gained from the use of the
welded hollow-sections produced by the Laminoirs de Longtain are not to be ignored.
The use of hollow-sections in metal construction brings
with it numerous advantages, both from the strength aspects and from the utilisation
aspects. In particular, we would cite the following :
- Excellent buckling characteristics,
in warping and in torsion due to the excellent distribution and
of the material about the longitudinal axis and the closed properties
of the section.
- Their aerodynamic characteristics
giving rise to a much lower drag coefficient than for sharp edged
sections.
- The use of the enclosed volume, either in order to
increase the carrying capacity of the column by replacing concrete, or, in terms of
fire-protection, by implementing a spray system, or, again, by balancing the various
thicknesses available in each size range in order to hold the overall column measurements
constant, while changing the carrying capacity from floor to floor.
- The absence of any sharp edges, which encourages the use
of hollow section columns in places to which the public has access, schools,
- Esthetics and ease of maintenance and cleaning.
For all these diverse reasons, tubing today holds an
important place in metal structures such as buildings and works of art, pylons, etc.
Should one limit oneself to tubes of square or rectangular
section - easier to manufacture, the following is available on the market :
- Seamless, hot formed tube.
- Welded tube/pipe produced from a round rough forging.
- Welded profile hollow-sections (directly
formed).
The profile hollow-sections in the first category -
seamless - are hot formed and, in tubular structures, are the equivalents of 'H' or 'I'
sections, in more traditional metal construction work. Nevertheless, being manufactured by
a hot broaching process, they have relatively high wall thicknesses which, in numerous
applications, do not permit of the optimum economies.
The tube in the third category, square
or rectangular sections manufactured by direct forming operations,
followed by welding the lips, does in fact constitutes an extension
to tubular structures utilising cold-formed open-form metal sections
whose popularity in metal structures is still, today, quite widespread.
They benefit as much from the experience already acquired in this
domain, whether from the more than fifty years of research on
cold-formed metal sections, as by over twenty-five years of research
in tubular structures (1). Together with a high-frequency weld
quality, this manufacturing technique - selected by the Laminoirs
de Longtain - this gives rise to a metal structural product of
a high quality.
The tubes of the second category are also manufactured by
forming operations.
Nevertheless, being formed into a square or rectangular
shape after welding the round rough forging, this manufacturing technique gives rise to
setting the residual elastic stresses in situ, to which, those stresses resulting from the
plastic-deformation process utilised in shaping the corners must be added.
The purpose of this note, on the basis of experimental
results obtained in a number of university laboratories and, in compliance with the
regulations which hold sway in this field, is to demonstrate that the tubes manufactured
by direct forming operations at the Laminoirs de Longtain constitute a high quality
structural product whose specific characteristics are fully controlled.
2. PRODUCT CHARACTERISTICS.
2.1. General
Instead of forming a round section, welding it and then
reshaping it to a square or rectangular section, at the Laminoirs de Longtain, the square
or rectangular section is obtained by successively bending the strip, followed by
high-frequency welding followed by a slight calipering which corrects the dimensions to
the flatness and squareness tolerances required. The direct consequence of this process is
to concentrate the cold-hammering effect only in the edge folds and a change in the
grain size only in the weld area. Due to the careful choice of the fold radii, the
level of this cold-hammering effect remains quite moderate, the flat areas conserving
practically the same mechanical characteristics as those possessed by the steel
originally.
The choice of steel allows any changes
to the mechanical characteristics in the weld area to be markedly
reduced.
2.2. Dimensional Tolerances
With respect to dimensions and tolerances, the Longtain
range comply with European Standard EN 10219/97. Nevertheless, the manufacturing processes
utilised gives rise to products whose actual tolerances are markedly lower than the limits
specified in the Standard.
In view of their method of manufacture,
the tube manufactured by Longtain is to remarkably small dimensional
tolerances. In fact, the thickness firstly is governed by that
of the basic strip and, as a consequence, the variation of same
over the cross-section of the profile section is thus extremely
low. Secondly, the direct forming operation - without passing
through a round forging stage - allows for very strict dimensional
control.
These very low tolerances, as compared to the tolerances
obtained by the hot-formed products, demonstrate the high degree of dimensional control
obtained with the aid of the profile forming manufacturing process.
2.3. Materials
The steels used by the Laminoirs de Longtain comply with
European Standard EN 10025/93.
The introduction of the continuous casting process was a
determining factor for the "Longtain" tubing. Rolling coils at a controlled
temperature provides us with the assurance that compliance with the mechanical
characteristics of the steel will be obtained.
The Al-killing, the low silicon, low carbon steel - the
process generally used - reduces the sensitivity of the steel to ageing. Nevertheless, on
request and subject to the use desired, other killing processes could be considered.
Alternative to grade S355J2H, a steel of higher quality could be used (on request), either
a dispersoid steel (addition of Niobium, Vanadium or Titanium).
This fine-grain steel ensures excellent ductility, even
subsequent to the forming process, as well as excellent weldability. This type of steel
comes within the "high elastic limits" family.
With regard to its galvanisability, all types of tubing
will accept a standard galvanisation process. In fact, where the Si content = 0.035%, then
the rule based on :
Si + 2.5 Ph = 0.080 % facilitates a guarantee that it can
be hot-dip galvanised.
For a rectangular tube of 200 x 150 x 5.8 mm, Figures 1 and
2, below, illustrate the typical distribution of mechanical characteristics of the
Longtain tubes. Figure 1 illustrates the tensile limit, fy, based on tensile
testing, while Figure 2 provides the ultimate tensile strength.
Figure 1 : 200x150x5.8
mm tube : distribution of the elastic limit in the finished
product
[P 10]
Figure 2 : 200x150x5,8
mm tube : distribution of the rupture point limit while tensile
testing on the finished product.
The beneficial effect of cold-hammering by comparison with
the basic product can be established. It has also been observed that the weld does not in
any way constitute a weak point in the product. Finally, tensile testing has demonstrated
that the profile forming process maintains the excellent ductility properties of the
steel, with elongations measured at the point of failure of :
- 40 to 50 % in the faces, and
- 24 to 35 % in the rounded edges.
These measurements confirm those obtained on tubes
manufactured by Longtain, in reference (2).
Sharply testing has also been carried out in the
factory, in order to test the brittle fracture strength of "Longtain" tubes. The
results obtained based on an S235J2G3 steel to EN10025/93 are recorded in Table 1.
The results set out in Table 1 are in
fact the means of the measured values.
| Location |
Location
Number of Test Piece
|
Impact
test at -20° C
|
| |
|
Joules/cm²
|
|
Basic metal
|
|
290.9
|
|
Flat faces
|
11
|
259.7
|
| Edges |
12
|
261.1
|
| Transversal |
6
|
184.5
|
Table 1 : Impact test at -20° C
With respect to the requirements
of the Standards which lay down a minimum of 27 J/cm² at -20°
C, for an S235J2G3 steel to EN 10025/93, it is established that
the value obtained in this test is, by a large margin, superior
to the minimum laid down in this Standard.
A recent study carried out in Federal Germany (3)
confirms that the type of steel selected by Longtain and the careful choice of fold radii
facilitates welding in the cold-hammered areas (edges).
Finally, a problem often raised with regard to cold-formed
profile sections is that of the high level of residual stress induced by the manufacturing
processes.
Figure 3 illustrates the residual stress level taken into
account in hot-formed profile-section manufacture. Measurements carried out on the
Longtain products (2) have given rise to residual stress values, induced by the
profile forming process, of from 0.22 to 0.56 of the value of the elastic limit. Based on
Figure 3, it was easy to establish that these residual stresses are scarcely any higher
than those considered in the hot-formed profile sections. It should also be noted that,
for cold-formed profile sections, the highest residual stresses are located in the edges
where, through cold-hammering the greatest increases of the elastic limit can also be
determined. This results in an at least partial compensation of one effect, which is
negative, by the other, which acts positively on the behaviour of the profile section.
Figure
3 : Residual stresses in hot-formed profile sections,
stated as a proportion of the elastic limit.
3. BEHAVIOUR UNDER LOAD
It is often
considered that, as a result of their residual stresses, the cold-formed
profiles have markedly inferior characteristics subject to buckling
than do hot-formed profile-sections. Proper analysis of the problem
demonstrates that this is untrue. Figure 4 illustrates this topic
based on a column made up of a hot-formed tube and a cold-formed
tube, the failure load being calculated using the calculations
drawn up by RONDAL and MAQUOI (6), used in Standard NBN B51002
(7) and in Eurocode No. 3 (4). Figure 3 demonstrates that, for
low and medium slenderness columns, the cold-formed sections have
a load-bearing capacity higher than for the hot-formed profile
sections, but this trend is reversed for very slender columns.
This conclusion fits in falsely with the standard view. Similar
examples can be worked out for other types of instability. It
is equally usual to consider that the fatigue behaviour of cold-formed
hollow sections is markedly inferior to that of hot-formed hollow
sections. A very large number of tests carried out by the CIDECT
(Comite international pour le Developpement et l'Etude de la construction
Tubulaire - The International Committee for the Development and
the Study of Tubular Structures) have demonstrated that this is
untrue. In fact, the fatigue behaviour of a metal structure is
fundamentally conditioned by the stress concentrations created
in the assemblies and has no relationship with the type of profile
section utilised (8). Moreover, certain tests have demonstrated
that the increase in the elastic limit, through cold-hammering,
in the cold-formed profile sections could, in certain cases, lead
to a slight increase in the fatigue-based life-expectancy, by
comparison with a structure built using hot-formed sections.
- cold-formed
tube
- hot-formed tube Steel AE 235
Figure
4 : Failure load in buckling mode.
4. STANDARDS AND REGULATIONS
Cold-formed
profile sections, such as "Longtain" tubes, fully comply with
the conditions of application of the most advanced regulations,
currently, for metal structures, i.e. Eurocode No. 3 (4) and the
American A.I.S.I. specifications (9). This results in a guarantee
of safety and durability, for structures built using these profile
sections, which can be favourably compared with that obtained
via the use of hot-formed sections, more traditionally used in
metal structures.
5. CONCLUSIONS
The goal of
this technical note has been, without going into too much detail,
requiring long theoretical reports, to demonstrate that the tube
manufactured in the Laminoirs de Longtain, utilising a profile-forming
process constitute a high-quality product, advantageously comparing
with other products utilised in metal structures. The examples
provided, based on measurement and testing carried out in well-known
laboratories, even though, in view of the concise character of
this technical note, limited, clearly demonstrate that the use
of "modern" steels allied with the cold-formed direct forming
techniques, make the "Longtain" tubing a high quality product
for metal structures.
REFERENCES
1.
Construire avec des profils creux en acier (Build using steel
hollow profile sections) Soditube, Paris 1994.
2.
BRAHAM, M., J.P. and RONDAL, J. Flambement des profils creux a
parois minces (the buckling of thin-walled hollow profile sections).
Cas des profils rectangulaires charges axialement (Axially-loaded
rectangular profile section case). Commission of the European
Communities, Technical Research on steel. Convention No. 6210.SA.301,
Final Report, 1979.
3.
BATHKE, W. Schweissen in Kaltverformten Bereichen im Stahlbau
(Welding in Cold-Formed ...
4.
Eurocode No. 3. The Design of Steel Structures. Commission of
the European Communities, Industrial Processes, November 1989.
5.
Manual on the stability of Steel Structures. European Convention
for Constructional Steelwork, 1976.
6.
RONDAL, J. and MAQUOI, R. Formulations d'Ayrton-Perry pour le
flambement des barres metalliques (Ayrton-Perry formulae for the
buckling of metal bars). Construction metallique No. 4, 1979.
pp. 41-53.
7.
NBN B51-002. Charpentes en acier (Steel frames). Institut Belge
de Normalisation, August 1988.
8.
Fatigue behaviour of Welded Hollow Section Joints CIDECT, Monograph
No. 7, Constrado, April 1982.
9.
Specification for the design of Cold-Formed Steel Structural Members.
The American iron and Steel Institute, August 1986.
