US20100287872A1

US20100287872A1 - Open web stud with low thermal conductivity and screw receiving grooves

Info

Publication number: US20100287872A1
Application number: US12/662,822
Authority: US
Inventors: Ernest R. Bodnar
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-05-13
Filing date: 2010-05-05
Publication date: 2010-11-18
Also published as: CN102422069A; WO2010130044A1; AR077444A1; CA2668945A1

Abstract

An open web low thermal conductivity steel stud, having a web defining a longitudinal axis and a web plane, side edges along each side of the web, main web openings formed through the web defining two lengthwise sides parallel to the web axis, ribs extending diagonally across the web between the main web openings, right angle flanges formed around the main web opening and along the ribs, bent away from the web, and having flange extensions formed along the right angle flanges formed along the two lengthwise sides of the main web opening, bent at further right angles into planes parallel to but spaced from the web plane to define three sided parallel spaced apart reinforcing channels extending lengthwise along the web, rib openings at each end of each rib defining narrow throat portions of the web for reduced heat transmission, at least one stud leg formed along at least one web side edge, at right angles to the web, and, two parallel screw receiving grooves formed in the at least one stud leg, the grooves being spaced apart equally on opposite sides of the centre of the stud leg.

Description

FIELD OF THE INVENTION

The invention relates to steel studs for supporting drywall panels, and in particular to an open web stud having low thermal conductivity, and formed with grooves in predetermined spacings, the grooves having a specific shape, for receiving dry wall screws.

BACKGROUND OF THE INVENTION

Steel studs in construction, especially for erecting dry wall, are replacing wooden studs for many reasons. Wood is becoming scarce. Wood will warp, or may rot, or become infested.
Wood is inflammable. Also in many cases it has become less suitable simply because it is heavy, and is bulky for transportation. Wood has however two major advantages over steel studs, namely that wood has low thermal conductivity, and that wood will receive and retain dry wall fastenings such as screws.
Clearly it is desirable to use steel studs to overcome the various disadvantages of wood, provided the two major problems with steel as compared with wood can be solved. In addition, since steel studs when installed are not subject to outdoor environment conditions, they can be made from recycled steel with no loss of performance, thus providing a good outlet for steel recovered from other products. In the past there have been numerous proposals for steel studs of various designs. In many cases steel studs were roll formed as simple C-sections. These simple sections had a web, and two “legs” or side walls, forming a simple rectangular channel in section. Dry wall panels were secured to the two legs with dry wall screws.
These simple C-sections are widely used throughout commercial and high rise construction. They are unsatisfactory for various reasons, most of which are well known.
Simple C-section studs are either of a heavier gauge steel than is desirable or economical, or are of so light a gauge that it renders them too flexible. Where such studs are too thin and flexible, it becomes difficult to install screws to fasten the panels.
Heavier gauge will permit ease of insertion of screws, but it will transmit noise, and has high thermal conductivity. It also uses more steel than is desirable. This may result in excessive floor loadings in multi story buildings. The heavier the gauge, the greater will be the cost of the raw material.
Lighter gauge steel may be considered to be more desirable, but it can be difficult to install. The vertical and horizontal members of a wall framed with such light gauge steel C sections may be too weak and flexible, and the wall may vibrate. Fastening the vertical stud members to the horizontal channels to make a frame is also rendered difficult, since the fastenings, usually special self tapping screws for use in sheet metal, are difficult to secure firmly in the excessively thin steel material of the members. In general the typical dry wall screws used in the trade for securing dry wall to wooden studs, are not suitable for use with sheet metal studs, especially, sheet metal studs of lighter gauge steel.
Self tapping screws for sheet metal are designed with a chisel drill point and a threaded shank. These screws will hold more securely in sheet metal. However when using lighter gauge steel, the steel may be so thin that only one or two threads of the screw shank with grip the metal. Another factor is that the light gauge steel material of such simple C sections is often too flexible to permit rapid insertion of screws. This slows down the men installing the dry wall panels and ends up increasing the overall cost of the work.
Plain C-sections are not suitable for erecting exterior walls, or walls where the thermal gradient across the wall may be significant, or where sound transmission is a problem. Such simple sections have high thermal conductivity and can result in condensation, known as “ghosting”, along lines on the inside walls, where the dry wall panels lie against the sides of the C section studs.
Studs can be made with spaced apart openings, so as to reduce thermal conductivity. Such studs may be classified as “open web” studs. Studs can also be made with various flanges and ridges to make them more rigid. However in the past such studs have been more costly than plain C-section studs and have not found wide acceptance. It is apparent that forming studs with complex openings, ribs, and ridges will be more costly than rolling simple C-sections. Thus if the more complex open web studs are to be competitive with simple C-section studs, it must be on the basis that while the open web studs may cost slightly more to make, they are easier and quicker to install, and can be made of significantly thinner gauge, thus using less material, than simple C-section studs. Open web studs are also beneficial for reducing sound transmission through the wall, and reduce the weight of the building on its footings. For exterior walls, open web studs will be far more attractive the simple C section studs since with exterior walls thermal conductivity is a problem. The lower thermal conductivity of open web studs will mitigate the problem of “ghosting” condensation lines along the inner wall surfaces, which was a problem with plain C section studs. For this reason every effort must be made to reduce thermal conductivity to an absolute minimum, while retaining the other advantages over C-section studs.
Open web studs having low thermal conductivity are also highly desirable for making exterior walls using exterior wall panels which are a composite of thin shell concrete and steel reinforcing studs. Such composite exterior panels have numerous advantages over solid concrete exterior panels. One of the significant advantages is that such composite panels incorporate both the exterior concrete surface, in a relatively thin slab of concrete, and in addition, incorporate steel studs on the inside of the slab for reinforcing. The thinner concrete slab, saves the cost of concrete, and also reduces the weight on the footings of the building. The steel reinforcing studs extend from the interior surface of the thin concrete slab and provide the interior studs within the building to which dry wall can be attached.
Again, the ability to make such reinforcing studs of light gauge steel has several advantages. The cost of material is reduced. The weight of the composite panel is reduced. The thermal conductivity of each stud is also reduced. This last advantage results from the fact that lighter gauge steel, being of reduced mass, will transmit only a reduced number of thermal units, at any given temperature gradient, as compared with a stud of heavier gauge.
Another factor arises from the use of lighter gauge steel in open web studs. Such studs are widely used for supporting dry wall panels. Panels are usually attached to the legs of studs with screws. Where two panels abut edge to edge, then their edges must lie side by side over one leg of a single stud. The leg width may be less than five cms. This leaves only a relatively narrow support for each panel edge. Screws must be secured, through each panel edge, into the same stud leg, to hold the edges of the two abutting panels onto the same leg.
Where light gauge steel is used to make the studs, then there is a tendency for the legs to bend or twist, when screws are being inserted. This tendency becomes more pronounced in the case of open web studs, where the gauge may be even less than the gauge of conventional C-section studs. This tendency can be reduced by incorporating various ribs, webs and flanges in the studs, giving them increased stiffness, in spite of their reduced gauge.
The invention further provides for this situation by the use of two wide screw receiving grooves in each stud leg. The grooves are formed parallel, at a predetermined spacing so as to optimise the insertion of screw fastenings. The grooves have a width greater than the width of the screw points, so as to assist in screw penetration, and to permit the screws to drill into the sheet metal, even where there is some twisting of the steel due to pressure of the screw point, on the metal.
Between and alongside the two grooves the stud legs are flat and planar, so as to support the panel edges, firmly. The grooves also assist in forming tubular recess in the legs, when the screws are inserted. These tubular recesses provide a greater security of engagement for the threads of the screws.

BRIEF SUMMARY OF THE INVENTION

With a view to achieving a solution to these complex and conflicting problems the invention comprises an open web low thermal conductivity steel stud, having a web defining a longitudinal axis and a web plane, side edges along each side of said web, main web openings formed through said web, said openings defining two lengthwise sides parallel to said web axis, ribs extending diagonally across said web between said main web openings, right angle flanges formed around said main web opening and along said ribs, bent away from said web, and having, flange extensions formed along said right angle flanges formed along said two lengthwise sides of said main web opening, bent at further right angles into planes parallel to but spaced from said web plane to define three sided parallel spaced apart reinforcing channels extending lengthwise along said web; rib openings at each end of each rib defining narrow throat portions of said web for reduced heat transmission; at least one stud leg formed along at least one said web side edge, at right angles to said web, and, two parallel screw receiving grooves formed in said at least one stud leg, said grooves being spaced apart equally on opposite sides of the centre of said stud leg.

The Invention Further Comprises

An open web low thermal conductivity steel stud wherein said steel has a gauge of between about 0.88 mm and 1.15 mm, and wherein the thermal transmission values as compared with a standard C-section stud are reduced by between 80% and 94%, compared with a conventional C-section stud. The invention further comprises an open web low thermal conductivity steel stud wherein said stud includes an edge lip formed on said stud leg, said edge lip being formed at an included angle less than 90 degrees.
The invention further comprises an open web low thermal conductivity steel stud wherein said edge lip is formed with a groove indented therein.
The invention further comprises an open web low thermal conductivity steel stud said rib openings are circular, and including annular flanges formed from said web around said rib openings.
The invention further comprises an open web low thermal conductivity steel stud including depressions formed in said web adjacent to or around said rib openings.
The invention further comprises an open web low thermal conductivity steel stud wherein the mass of metal in the locations of said restricted throat portions is reduced by between about 80% and 94% compared with the mass of metal in an equivalent plain C section stud.
The invention further comprises an open web low thermal conductivity steel stud Including an embedment edge formed along said web, said embedment edge defining a root portion, and a mid portion, with concrete flow opening in said mid portion, and Metal portions punched from said openings being bent outwardly normal to said mid portion, and a continuous lip formed on said mid portion, said lip being bent at right angles to said mid portion.
The various features of novelty which characterize the invention are pointed out with more particularity in the claims annexed to and forming a part of this disclosure.
For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

IN THE DRAWINGS

FIG. 1 is a perspective view of a dry wall stud illustrating the invention;

FIG. 2 is a section along 2-2 of FIG. 1;

FIG. 3 is a section similar to FIG. 2 but showing portions of two dry wall panels, and screw fastenings, attached to the stud;

FIG. 4 is a side elevation of a portion of the stud, showing the various areas of reduced thermal transmission;

FIGS. 5, 6, 7, 8, and 9 are sections along 5-5, 6-6, 7-7, 8-8 and 9-9 of FIG. 4;

FIG. 10 is a schematic section of a portion of a stud showing dry wall self tapping screws at two stages of insertion;

FIG. 11 is a perspective of a low thermal conductivity stud for reinforcing a composite concrete wall panel;

FIG. 12 is a perspective of the stud of FIG. 11 from the opposite side;

FIG. 13 is a section along 13-13 of FIG. 11;

FIG. 14 is a side elevation of a further embodiment;

FIG. 15 is a side elevation of a further embodiment;

FIG. 16 is a perspective of a further embodiment;

FIG. 17 is a side elevation of FIG. 16;

FIG. 18 is a section along line 18-18 of FIG. 16;

FIG. 19 is a schematic view of a patin stud showing the heat transmission paths; and,

FIG. 20 is a schematic view corresponding to FIG. 19 showing the heat transmission paths of a stud according to the invention.

DESCRIPTION OF A SPECIFIC EMBODIMENT

As explained above the invention is illustrated in the form of a steel stud of light weight and having low thermal conductivity. The stud has specific features adapted to facilitate the attachment of dry wall panels, in a manner explained below, and further features providing it with low thermal conductivity and low sound transmission, as will be described below.
FIGS. 1 and 2 show a stud (10), which may generally be classified as an “open web stud” for the purpouse of illustrating the invention.
The stud (10) has a web (12). The web (12) defines an outer mounting face (14) and an inner face (16). The web (12) is formed with two parallel longitudinal web grooves (18).
Between the grooves (18) there are main web openings (20) of generally trapezoidal shape. The openings (20) define long and short longitudinal sides parallel to one another, and diagonal sides joining the ends of the long and short sides.
Between the main web openings (20) ribs (22) extend diagonally across the stud. Longitudinal flanges (24) extend along the parallel sides of each opening (20).
Diagonal flanges (26) extend along each side of each rib (22). Flanges (24) and (26) are bent at right angles to the plane of web (12), to reinforce the stud.
Longitudinal flanges (24) are extended, by simply leaving more metal from the web, in the flanges.
These longitudinal flanges (24) are bent at a further right angle (28) so that they define an L-shape in section with the edge portions lying in planes parallel to but spaced from the web itself.
In this way these flanges (24), together with the adjacent portions of the web (12), form three sided rectangular shaped reinforcing channels (30) having centre portions and two opposed walls, extending along the parallel linear longer and shorter sides of each main web opening (20) for still greater strength.
Along each side of each rib (22) the flanges (26) form a generally triangular peak (32), for increased strength. The rib (22) varies in width along its length being narrowest at about its median point.
At each end of each rib (22) there is a circular rib opening (34) surrounded by an annular flange (36). The circular rib openings (34) define two separate narrow throat portions (38) and (40), one on each side of each circular rib opening ( )
The throat portions (38) and (40) at each end of each rib, and the narrow median region of each rib all constitute thermal restrictions or heat transmission barriers. Thus any heat being transmitted across the stud, can pass only through these thermal barriers.
Referring to FIGS. 5, 6, 7, 8, and 9, it will be seen that the throat portions (identified and shown in elevation in FIG. 4), are shown in section. Any heat being transmitted through the stud will have to pass through these throat portions.
Studs of this design, with openings, ribs, and flanges and channels, can be made of light gauge sheet steel. The steel can actually be a lighter gauge than the gauge of conventional simple C-section metal studs, because the various flanges, edges, channels, and openings all contribute to make the stud stronger and with a better load bearing character, than convention simple C-section studs.
It will therefor be appreciated that the function of the throat portions becomes even more effective as a barrier to heat transfer.
The heat transferred, over a given time span, must depend, both on the temperature gradient across the stud, and also the volume of steel providing a heat transfer path. FIGS. 5, 6, 7, 8, and 9 show sections of the various throat portions. These thermal barriers contain the smallest volume of steel, at the locations given, for transmission of heat across the stud. Thus whatever heat gradient exists across the stud, heat transmission from one edge to the other of the stud must be restricted to whatever heat values can be transmitted at these restricted locations.
The reduction in heat transmission values for the present studs compared with plain studs, from one edge to the other of the stud, can be seen from the following figures for various gauges of steel.


Steel gauge	20 (0.88 mm)	18 (1.15 mm)
Plain stud	100%	100%
Open web stud	4% to 8%	20% to 25%

These represent values developed theoretically.
In practice it is more likely that the values will be subject to some margin, one way or the other, depending for example on the heat loss conditions outside the exterior of a wall or building, which may depend not merely on temperature, but also wind chill and the like. There may be some slight variation due to differences in steel formulations, and possibly, the volume an type of thermal insulations placed in the wall between the studs.
It is clear that given a steel gauge which is lighter than the gauge of a conventional plain stud, the actual volume of metal at each throat portion or restriction is radically reduced.
Because the design of the open web stud makes it stronger than conventional plain C-section studs, and because the design of the stud restricts heat transfer to the throat portions only, the stud of the present invention is greatly superior to conventional C-section studs.
Along each edge of the web (12), in this embodiment, the web is bent at a right angle to create web legs (42). Each web leg is formed with an edge lip (44) turned inwardly at an angle of 90 degrees, in this case. In some larger studs the edge lip (44) may be extended as shown in FIG. 2, and formed with a further groove.
Each web leg (42) in this embodiment is formed of planar sheet steel, in which are formed two parallel indented grooves (46). The outer surface of each web leg defines a planar area (48). This permits panels of dry wall to be laid flat on the planar area (48) of the web leg outer surface. Where two panels of dry wall material meet and abut, each panel edge will lie over one half the width of the web leg (42). Dry wall screws will pass through the edges of the two abutting panels of dry wall material, into their respective grooves (46). Where the dry wall panels lie over one the legs (42) of intermediate studs (10), they will be fastened by a single line of dry wall screws. These screws will be received in one of the two grooves (46).
In order to facilitate the location of the dry wall panels, a central scribe line (50) is formed along each web leg (42). The scribe line (50) is positioned along the median of the web leg, equidistant between the two grooves (46). This enables the dry wall installers to line up the panels precisely, so that any two adjoining panels will overlap the web leg of one stud by the same distance. This ensures that the screws will be inserted correctly into the grooves in the web leg.
All formations and indentations in the web (12) are formed from the web outer surface (14) and extend inwardly. This leaves the web outer surface, and the outwardly directed surfaces of the web legs (42) planar and flat so as to accept juxtaposition of other materials. In the case of web legs (42) the material would be dry wall panels,(FIG. 3), which can thus lie flat against the planar outer surfaces (48) of the web legs (42).
Each of the grooves (46) is formed with linear angled sides (52) (FIG. 10). The angled sides are formed in such a way as to facilitate the insertion of self tapping screws (54). Self tapping screws (54) are designed specifically for the fastening of dry wall panels (56) (FIG. 3), to the steel studs (10). For this purpouse the screws (54) have self drilling chisel points (58). The chisel points have tips which are formed as a cone around cone angle. The included angle of the cone point is in the region of approximately 40 to 50 degrees, thereabouts.
The included angle of the angled sides (52) of the grooves (46) is between about 75 and 90 degrees. The grooves are therefor wider, and the chisel points are narrower. In this way the chisel point of each screw can reach completely down into the depth of the groove. This ensures that the screw will start to drill into the steel immediately the screw is rotated by the insertion tool, usually a power driven screw driver.
If the screw is pressed in too hard causing the web leg to deflect and bend, the chisel point will remain trapped in the groove. This prevents the screw from twisting sideways and slipping off the web leg. FIG. 10 shows a left side screw being about to start drilling and a right side screw fully inserted.
Once fully inserted, it will be seen that the screw will form the sheet metal in the groove of the web leg into a generally trumpet shaped metallic tube (60). The metallic tube (60) so formed can be seen to engage several threads of the screw (54), and thus provide a secure hold.
Various different wall systems will require studs with different specifications and dimensions.
Non load bearing studs will usually have web legs which have a width of 0.92 mm to 102 mm.
Load bearing and heavier duty studs may have web legs with a width of 152 mm and up.
Where two panels join the web legs of the studs function to receive the edges of two edge abutting dry wall panels.
Accordingly the grooves (46) shall be equally spaced from each other, on opposite sides of the median scribe line (50). The grooves shall also be equally spaced from the web (12) and from the lip (44).
The parameters of groove spacings B for various studs of varying widths, and web leg widths A are shown below.


Stud Size	Leg Width (A)	Groove Separation (B)

92 mm to 102 mm	41 to 51 mm	17 to 26 mm
152 mm	41 to 64 mm	17 to 39 mm

Studs of the invention, with some changes, can be used in making thin shell composite concrete panels, reinforced with steel studs.
The reinforcement studs (70), FIGS. 11, 12, 13, for this purpouse, will be made essentially as described above, with the same parts given the same reference numbers, with the exception that there is only one web leg (42), bent at a right angle.
In place of the other web leg there is an embedment edge (72) formed for embedment in a thin shell concrete panel (not shown), usually from about 3.25 cm thick to about 4.5 cm thick, although these figures are merely by way of illustration and without limitation. Edge (72) consists of a right angular root portion (74), extending from the web (12). A mid portion (76) extends from root portion (74) at right angles, in this embodiment. Openings (78) are formed through mid portion (76) at spaced intervals to permit flow of concrete therethrough. The metal portions (80) punched out from openings (78) remain joined along one edge and are deflected so as to extend at right angles to one side of mid portion (76).
A continuos lip (82) is formed along the edge of mid portion (76), bent at a right angles and extending in a direction opposite to metal portions (80).
Additional strength can be added to the embodiment of FIG. 1 and FIG. 11, as shown in FIGS. 14 and 15.
In this case the studs (90) have the same features as those described above and have the same reference numbers. Additional strength is provided by forming additional indentations (92), of generally three sided shape, FIG. 14, or indentations (94) of generally linear shape with rounded ends, FIG. 15.
In a still further embodiment, FIGS. 16, 17, and 18, studs (100), indentations (102) of generally irregular triangle shape are formed at the end of each rib (22). Circular rib openings (104) are formed in the indentations (102), which function to define throats for reduction of heat transmission, in the same way as in FIG. 1, and FIG. 10.
The general effect, on heat transmission, of the open web stud of the invention, as compared with a plain stud are shown in FIGS. 19 and 20.
FIG. 19 show a length of plain stud (P). A series of arrows (A 1) represent the heat transmission across the stud. The heat is transmitted simply straight across the stud, without any restriction. The greater the gauge of steel, the more heat will be transmitted.
FIG. 20 shows a length of open web stud (10) according to the invention. A series of arrows (A 2) represent the heat transmission across such an open web stud of the invention.
It is clear that the heat is restricted to a narrow path, in the stud (10) of FIG. 20, so that the actual heat transmitted will be far less than that carried by a plain stud of FIG. 19.
The foregoing is a description of a preferred embodiment of the invention which is given here by way of example only. The invention is not to be taken as limited to any of the specific features as described, but comprehends all such variations thereof as come within the scope of the appended claims.

Claims

1. An open web low thermal conductivity steel stud, having a web defining a longitudinal axis and a web plane, side edges along each side of said web, main web openings formed through said web, said openings defining two lengthwise sides parallel to said web axis, ribs extending diagonally across said web between said main web openings, right angle flanges formed around said main web opening and along said ribs, bent away from said web, and comprising;

flange extensions formed along said right angle flanges formed along said two lengthwise sides of said main web opening, bent at further right angles into planes parallel to but spaced from said web plane to define three sided parallel spaced apart reinforcing channels extending lengthwise along said web;

rib openings at each end of each rib defining narrow throat portions of said web for reduced heat transmission;

at least one stud leg formed along at least one said web side edge, at right angles to said web;

two parallel screw receiving grooves formed in said at least one stud leg, said grooves being spaced apart equally on opposite sides of the centre of said stud leg, and, planar regions on either side of each said groove for receiving wall panel material thereon.

2. An open web low thermal conductivity steel stud as claimed in claim 1, wherein said steel has a gauge of between about 0.88 mm and 1.15 mm, and wherein the thermal transmission values as compared with a standard C-section stud are reduced by between 80% and 94%

3. An open web low thermal conductivity steel stud as claimed in claim 1 wherein said stud includes an edge lip formed on said stud leg, said edge lip being formed at an included angle of about 90 degrees.

4. An open web low thermal conductivity steel stud as claimed in claim 3 wherein said edge lip is formed with a groove indented therein.

5. An open web low thermal conductivity steel stud as claimed in claim 1 wherein said rib openings are circular, and including annular flanges formed from said web around said rib openings.

6. An open web low thermal conductivity steel stud as claimed in claim 1 and including depressions formed in said web around said rib openings, said depressions being of generally irregular triangular shape.

7. An open web low thermal conductivity steel stud as claimed in claim 1 and including depressions formed in said web adjacent to said rib openings, said depressions being of generally linear shape.

8. An open web low thermal conductivity steel stud as claimed in claim 2 wherein said grooves have sides formed whereby to define an included angle of between 75 and 90 degrees.

9. An open web low thermal conductivity steel stud as claimed in claim 8 wherein said grooves are spaced apart from one another by a distance of from between 70 mm and 105 mm.

10. An open web low thermal conductivity steel stud as claimed in claim 9 and including a scribe line formed along said at least one leg equidistant between said two grooves and defining the median line of said leg.

11. An open web low thermal conductivity steel stud as claimed in claim 1 wherein, in a given length of stud, the metal removed from said stud by said main web openings and said root openings, leaves a mass of metal in the locations of said restricted throat portions is between about 16% and 21% of the total mass of metal in said given length of stud.

12. An open web low thermal conductivity steel stud as claimed in claim 1 including an embedment edge formed along said web, said embedment edge defining a root portion, and a mid portion, with concrete flow opening in said mid portion, and metal portions punched from said openings being bent outwardly normal to said mid portion, and a continuous lip formed on said mid portion, said lip being bent at right angles to said mid portion.

13. An open web low thermal conductivity steel stud, having a web defining a longitudinal axis and a web plane, side edges along each side of said web, main web openings formed through said web, said openings defining two lengthwise sides parallel to said web axis, ribs extending diagonally across said web between said main web openings, right angle flanges formed around said main web opening and along said ribs, bent away from said web, and comprising;

said ribs and said throat portions, at a predetermined temperature gradient across said stud, transmitting heat at between 5% and 20% of the heat transmitted across a plain C-section stud.

14. An open web low thermal conductivity steel stud as claimed in claim 13, wherein said steel stud is formed of steel of between about 0.88 mm and 1.15 mm.