the description which follows , and the embodiments described therein , are provided by way of illustration of an example , or examples of particular embodiments of the principles of the present invention , these examples are provided for the purposes of explanation , and not of limitation , of those principles and of the invention . in the description which follows , like parts are marked throughout the specification and the is drawings with the same respective reference numerals . the drawings are not necessarily to scale and in some instances proportions may have been exaggerated in order more clearly to depict certain features of the invention , referring to fig1 a , 3 a , 3 b , and 3 c , a sheet metal structural member is indicated generally as 20 . it has a web 22 and a pair of generally opposed , longitudinally extending parallel main flanges 24 and 26 that extend perpendicularly out of the plane of web 22 to form a generally ‘ c ’ shaped channel section , with the toes of flanges 24 and 26 having an inwardly bent lip 28 , 30 . in the example of fig1 member 20 has width and depth dimensions to simulate a ( nominal ) 2 ″× 6 ″, although 2 ″× 4 ″ studs ( seen in the background of fig1 and indicated as 32 ), 2 ″× 8 ″ beams or other stud or beam sizes can also be formed . web 22 can be termed the back of the c - channel section , and lies , generally , with its outer face 34 in a web plane indicated as ‘ w ’. the inner face 36 of web 22 lies to one side of web plane ‘ w ’, and the legs of the section , main flanges 24 and 26 , extend to the same side of plane ‘ w ’, the longitudinal direction is indicated by an ‘ x ’, the axial direction perpendicular to ‘ x ’ and lying in plane ‘ w ’ is indicated as ‘ y ’ and the axial direction perpendicular to plane ‘ w ’ is indicated as ‘ z ’, the width of web 22 , and the overall depth of structural member 20 , are measured in the ‘ y ’ direction , and the width of main flanges 24 and 26 is measured in the ‘ z ’ direction . web 22 of member 20 has been blanked and formed to leave an array 38 of alternating diagonal struts 40 and 42 that extend between web margins 44 and 46 whence flanges 24 and 26 extend . openings , of which one representative example is indicated as 48 , are defined in the spaces bounded by respective diagonal struts 40 and 42 , and margins 44 and 46 , as the case may be . a one - piece , snap fit grommet 60 is mounted to member 20 by being pushed into opening 52 in the insertion direction indicated by arrow ‘ p ’, opening 52 is , for the purposes of this description , identical to any one of openings 48 , 50 or 54 , such that grommet 60 could be mounted in any of them , interchangeably , grommet 60 provides a protected port for services , such as a water pipe or electrical conduit or cable ( not shown ) to traverse web 22 . opening 48 , being typical , will now be described in detail . it has a generally triangular shape , having a first radiused corner defined at the altitudinal vertex , or apex 62 , lying between struts 40 and 42 , a second , radiused base corner 64 lying between strut 40 and margin 44 , and a third , radiused corner 66 lying between strut 42 and margin 44 . in general , the triangular shape so defined is an isosceles triangle the radius at base corners 64 and 66 is smaller than the larger , more generous radius of apex 62 . as shown , the base 68 of the triangular shape lies along margin 44 . bases 67 and 69 of the adjacent triangular openings , indicated as 50 and 54 , lie along the margin 46 , each triangle alternating in orientation relative to its neighbours . flangework , in the nature of a continuous rim , or lip flange 70 is formed out of the plane of web plane ‘ w ’ in the direction away from outer face 34 , and extends fully about the periphery of opening 48 . lip flange 70 includes left and right hand strut flanges 72 , 74 that extend along the sides of struts 40 , 42 and a base side flange 76 formed on mar & amp ; i 44 ( similar flanges extend along the sides of the adjacent opening , 50 ). in each of base corners 64 and 66 there is a corner flange portion 80 , 82 whose free edge 84 , 86 lies at a shallower depth than the free edge of flanges 72 or 74 , and at a shallower depth than the free edge of the medial portion 88 of flange 76 . lip flange 70 also includes an apex flange 87 which runs along apex 62 and smoothly into strut flanges 72 and 74 . each of struts 40 and 42 has a medial portion 90 and root portions 92 , 94 at either end . in each of root portions 92 , 94 there is a generally triangular indentation 96 , 98 stepped to one side of plane ‘ w ’ of web 22 generally . each of indentations 96 , 98 has a generally straight base side 100 , another side 102 facing the altitudinal vertex of the adjacent opening , and a curved side 104 sharing a center of curvature with base corner 64 or 66 as the case may be . in section , medial portion 90 has a back 106 lying in the plane of web 22 generally , and a pair of formed legs , being flanges 72 and 73 in the case of strut 40 , and flanges 74 and 75 in the case of strut 42 . flanges 72 and 73 ( and 74 and 75 ) lie at an out - of - plane true angle to the plane of web 22 generally that is indicated by α such that flanges 72 and 73 present a tapered wall that has a root at back 106 and a distal edge lying away from web ‘ w ’. by comparison , corner flange portion 80 ( and 82 ) is formed on an angle β to extend away from plane ‘ w ’ and presents a tapering surface with a distal edge 84 , β being less than α . grommet 60 also is shaped to conform , generally , to the shape of opening 48 ( or 50 , 52 , 54 as shown ). grommet 60 has a peripheral wall 110 having a base side 112 , base corners 114 and 116 , a pair of sides 118 and 120 rising toward an altitudinal corner , or apex 122 . as such it has a general shape in plan view of an isosceles triangle , with truncated corers , attachment fittings 124 , 126 and 128 are located at respective corners 114 , 116 and apex 116 . peripheral wall 110 has a forward or leading portion , 130 , that is , the portion that is first introduced into opening 48 during insertion of grommet 60 , and a rearward or trailing portion 132 that follows the leading portion during insertion . each of sides 112 , 118 and 120 has a cross - section as shown in fig3 a . each has a side wall or leg , 134 that is generally rectangular in section , and a trailing edge flange 135 extending perpendicularly relative to leg 134 . leg 134 is slightly tapered , having a draft angle to facilitate removal from the mold on manufacture . leg 134 has an inner face 136 that faces inwardly toward , and defines a portion of the boundary of , an internal passageway 140 . internal passage 140 extends generally in the direction of insertion indicated by arrow ‘ a ’, and is formed to carry services through web 22 , such as plumbing wiring , vacuum hoses , or other hardware . leg 134 also has an external , or outer face 138 that faces outward relative to passageway 140 . corners 114 and 116 are right and left hand images of each other , if sides 112 , 118 , and 120 were extended they would meet at a sharp corner , instead they are truncated at the respective corner fittings . at the points where sides 112 , 118 and 120 are truncated in this way , grommet 60 has corner webs 142 , 143 , and 144 which form part of , and complete , the boundary of passageway 140 formed by peripheral wall 110 . for the purposes of description , attachment fitting 128 at apex 122 differs from attachment fittings 124 and 126 only in size , corner radius , aspect ratio and other dimensions rather than in nature . thus , although only one fitting is described , the description is applicable to the fittings as well . each attachment fitting has an archway 146 having a pair of left and right hand columns 147 and 148 surmounted by an overspanning member in the nature of a lintel 150 that surmounts the distal ends of columns 147 and 148 and has a forward surface lying flush with the leading edge 152 of peripheral wall 110 . columns 147 and 148 stand outwardly from peripheral wall 110 . that is , the inner edge of columns 147 and 148 back on the adjacent sidewall portions while their outer left and right hand edge facings 153 and 154 stand outwardly of the planes defined by outer faces 138 of the respective wall portions . in side view , as seen in the detail of fig2 d , the leading portion of column 147 ( and 148 ) has a rake angle ψ such that column 147 ( or 148 ) is wider at its base than at its distal end . a straight edge placed across a pair of columns at either end of any of side portions 112 , 118 or 120 will stand clear of their respective outer faces , 138 . that is , a rebate 156 formed between outer surface 138 and the columns at either end thereof further , facings 153 and 154 , and the outward edge of lintel 150 are formed to conform to an arcuate surface , such that the leading end of grommet 60 can be introduced into opening 52 , and so that facings 153 and 154 seat in the smoothly curved corners of , for example , opening 48 , 50 , 52 or 54 . each attachment fitting 124 , 126 , or 128 includes a catch member 160 that depends from lintel 150 . it has an outwardly oriented tapered facing , in the nature of a fluted cam face 162 , the taper being such that the depending edge 164 is biased to stand further outwardly than facings 153 and 154 . a shoulder , in the nature of a rearwardly facing abutment 166 extends inwardly from edge 164 to terminate at a rearwardly extending depending skirt , or tab 168 of catch member 160 , in operation , when grommet 60 is moved forward along the insertion direction , cam face 162 will tend to engage the arcuate tapering surface of one or another of corner flange portions 80 , 82 and apex flange 87 . this will in turn tend to cause catch member 160 , and hence abutment 166 to deflect inwardly into the alcove , or chamber , defined inside archway 146 . catch member 160 is resilient , and is biased outwardly . when grommet 60 is advanced sufficiently far for edge 164 to clear the distal edge 169 of the respective corner flangework , abutment 166 is able , under the bias of catch member 160 , to move outwardly to engage distal edge 169 . in this outward position abutment 166 discourages rearward movement of grommet 60 , preventing its removal from opening 52 . each attachment fitting 124 , 126 or 128 also includes a seating element 170 in the nature of an outwardly extending cantilevered wing 172 that has a pair of left and right hand flange extensions 174 and 176 that lie flush with , and are smoothly rooted into flanges 135 , of peripheral wall 110 . an arcuate bridging member 178 lies in the same plane as , and connects the distal ends of , flange extensions 174 and 176 . as thus arranged the trailing edge of web 142 , 143 , or 144 as the case may be , the inside edge of flange extensions 174 and 176 , and the inside edge of bridging member 178 define a port 179 through which mold extensions may move during fabrication of grommet 60 to form the underside of lintel 150 and the inward or rearward face of catch member 160 . on the leading face of each seating element is a forwardly protruding nub 180 that engages the outer face 34 of web 22 when grommet 60 is inserted . seating element 170 is resilient . when the trailing face of grommet 60 is pushed forward enough to cause abutment 166 to snap into position , the forward motion also causes nub 180 to seat against web 22 , and , in so doing , a pre - load is applied to seating element 170 , in this position abutment 166 sits in a position that is generally opposed to seating element 170 and the space between them , measured in the direction of insertion , is sufficient to permit capture of apex flange 87 ( or flange portions 80 or 82 as the case may be ) and web 22 . when grommet 60 is left in place , the preload in seating element 170 will tend to bias peripheral wall 110 rearward relative to the insertion direction , and in so doing , will tend to exert a force to urge abutment 166 more firmly against distal edge 169 , thus tending to immobilise grommet 60 . this preload may also tend to help grommet 60 resist loosening and rattling under modest vibration . when grommet 60 is seated in this way in opening 48 , 50 , 52 or 54 , the outer faces 138 of leg 134 of each of the three sides of peripheral wall 110 will stand clear of the corresponding facing portions of flangework lip flange 70 . this increases the likelihood that vibration loads will not be transferred to or from the mid sections of the struts , but rather will be carried to the corners at margins 44 and 46 . in alternative embodiments , other geometric arrangements of seating element can be used . for example , flange extensions 174 and 176 could be curled forward to present a leaf - spring - like arrangement for deflection on insertion . not all of the seating elements need be resilient . in some cases it may not be desirable to have any pre - loaded seating elements , in others a single pre - loaded element or spring would suffice to pre - load all of the fittings . for example , although the configuration shown in fig2 a , 2 b and 2 c is preferred , a grommet having a pair of abutments ( or seating elements ) located at the mid - portion of the long sides 118 and 120 could be opposed by a pair ( or more ) of seating elements ( or , abutments ) arranged in the corners , or one at apex 122 and one along the middle of the base side 112 . a single spring loaded element , among a total of four seating elements and abutments , would then serve to preload all of the others . in an alternative embodiment as shown in fig4 a , a grommet 181 has three internal dividers 182 , 184 , and 186 that , subdivide the space enclosed within a peripheral wall 188 , similar in other respects to peripheral wall 110 described above . an allowable maximum outer pipe diameter φ is defined by the largest circular section that can be inscribed within side wall portions 192 , 194 and 196 , and is shown in dashed lines in main passageway 198 . ( it will be appreciated that a slightly larger pipe can be carried , or the enclosure of peripheral wall 188 can be made slightly smaller , if an interference fit is desired .) as illustrated , each of dividers 182 , 184 and 186 is located outside the circle of diameter φ , and the subsidiary passages so formed , indicated as 200 , 202 and 204 , are available to carry other services , dividers 182 , 184 , and 186 need not be straight walls , or webs , but could be formed on a circular arc , as shown at 206 in fig4 b . alternatively , as shown in fig4 c a spider 208 having a circular center 210 , such as may be suited to a water pipe , and outwardly radiating partitions in the nature of dividers 212 could be used . one or two dividers could be used rather than three as shown , or more dividers could be used depending on the number , shape and size of the passageways to be formed . in fig4 d a single divider 214 divides the internal passageway into a minor section 216 for a pipe 217 ( shown in dashed lines ) and a major portion 218 for other services . divider 214 is bent so that the long sidewalls of the grommet can be squeezed inward to flex divider 218 , and , when released it holds snug against pipe 217 , grommet 60 is a molded pant made from a polypropylene homopolymer plastic . it is considered to be and may tend to function as an electrical insulator . the edges of grommet 60 that face inward toward passageway 140 , or that stand rearwardly of web 22 when installed , have a small radius ( typically 0 . 047 ″) to decrease the likelihood that service conduits or cables passing through grommet 60 will encounter a sharp edge . in the specific embodiment illustrated in fig2 a , grommet 60 is of a size to fit an opening in a ( nominal ) 2 × 6 member . alternative sizes of grommet can also be made , as typified by grommet 220 shown in the foreground of fig1 a , and installed in the ( nominal ) 21 ″× 4 ″ structural member 38 lying in the background of fig1 a . grommet 220 is shown in top , bottom and sectional views in fig5 a , 5 b and 5 c respectively . it has attachment fittings 222 , 224 , and 226 of the same nature and features as attachment fittings 124 , 126 and 128 . openings 230 , 232 , and 234 defined in web 236 of structural member 38 have the same pitch between centers as openings 48 , 50 , 52 and 54 described above , but a shallower height , reflecting the shallower depth of section of member 38 as compared to member 20 . grommet 220 differs from grommet 60 primarily in the ratio of its altitude , or height , in plan view , as measured from base sidewall 238 to apex 240 . thus sidewalls 242 and 244 are each shorter than base sidewall 238 , whereas base sidewall 112 is the shortest side of grommet 60 . a similar lengthening or shortening of aspect ratio permits a family of grommets to be manufactured suitable for a family of structural members sharing the same pitch between adjacent through - web openings . although structural member 22 , as shown in fig1 a , 1 b , and 1 c is preferred , other arrangements of flangework could be presented . for the purposes of the present invention it is not necessary that the flangework extend fully and continuously about the entire periphery of the opening . the flangework could , for example , extend only along the sides of the struts , and in the strut root portions , the flangework can be of varying height , or depth , about the periphery . the flangework can taper from a deepest section at the mid span of the struts to shallower portions adjacent the roots , or to taper into no flange at all , or could be formed only at the roots . for example , in fig6 a and 6 b a structural member 250 has a lip flange 252 that is less sharply angled from web plane ‘ w ’ that that of structural member 20 . the openings defined in structural member 250 are more sharply triangular than opening 48 , having the form of equilateral triangles with a common radius of curvature at each corner . the radius at the free edge of the corner flange portions 254 is larger than at the plane of web ‘ w ’, yielding a relatively large , relatively shallowly angled corner web 256 . in fig7 a and 7 b a structural member 260 is similar to structural member 250 , except insofar as it employs a steeper corner flange portion 262 . in fig8 a and 8 b a relatively heavy section structural member 270 had formed flanges 272 and 274 , and a deeper lip flange 274 , than member 250 . in fig9 a a structural member 276 has a lip flange 278 that is formed outwardly of the section , rather than inwardly as in fig3 a ., such that the insertion direction of grommet 60 would tend to be reversed . in fig9 b a structural member 280 has asymmetric main flanges 282 and 284 . in fig9 c a structural member 286 has one flange 288 formed to one side of web plane ‘ w ’, and another flange 290 formed to the other side . grommets such as grommet 60 or grommet 220 , depending on the size required , can be installed to co - operate with each of the illustrated sections noted above . further , although the preferred form of opening , and of grommet , is a form that is generally similar to an isosceles triangle with rounded corners , other shapes could be used , for example , a trapezoidal opening between the opposed margins of the web and a pair of diagonally inclined struts is also possible . as the radius of curvature of the apex corner increases , the resultant opening tends to take on a ‘ d ’- shaped appearance in plan view , particularly as the width of section increases relative to the base dimension of the opening , or as the radius of the apex approaches half the length of the base side . in general , the opening has a base portion adjacent to one margin of the web , and a pair of side portions that extend away from the base portion to meet at an apex adjacent to the other margin of the web . another type of roll - formed structural member is shown in fig1 a and 10 b as 300 . it has a web 302 having wide portions 304 alternating with narrow portions 306 to give a saw - toot , or zig - zag appearance . one edge 308 of web 302 is straight while the other edge 310 undulates . along edge 308 member 300 has a longitudinal main flange structure 312 . along undulating edge 310 member 300 has a lip , or edge reinforcement flange 314 extending perpendicular to the plane of the web . triangular openings 316 are defined centrally in each of the wide portions of web 302 , each triangular opening 316 having a peripheral flange 318 and smoothly radiused base and altitudinal corners 320 , 322 and 324 . each triangular opening tends to divide its respective wide portion into a pair of tapering triangular struts 326 and 328 that meet at a point 330 . the flange 314 acts as an embedment formation for point 330 , but in addition point 330 also has defined in it , adjacent flange 314 , an aperture 332 through which a cast material such as concrete can flow . further , as shown in fig1 c , the embedment formation can include a tongue 334 that extends laterally from the web in the opposite direction from flange 314 , and can be formed from the blanked - out material removed to form aperture 332 . as illustrated in fig1 a , the tip of point 330 is embedded in cast material 336 to a level lying between flange 314 and the attitudinal corner of flange 314 . a grommet , such as grommet 60 or grommet 220 , with suitable adjustments for size of opening and depth of flange to be captured , can be seated in opening 316 . as such services carried through the grommet are carried in a controlled position , through the web , at a position between main flange 312 and flange 314 , through the controlled geometry of opening 316 , the geometry of opening 316 is controlled in the sense that it is determined by the shape of dies in a rotary press as the point of manufacture of the structural member . there may tend to be much less control over the dimension between flange 314 at the narrow throat 338 and the cast material . main flange 312 can be formed in a variety of configurations . as shown in fig1 a the main flange can be a triangular element 340 , or other tube shape , whether circular of polygonal . as shown in fig1 b , the main flange 342 can be more or less channel - shaped , with a relatively large outboard leg 344 . as shown in fig1 c a main flange 346 can be a main flange 348 can be formed in the shape of an angle iron with a main leg 350 parallel to the cast material , and a small lip 352 at the outboard distal extremity . in fig1 a and 1 b , a pair of structural members 360 and 362 having undulating edges are mounted back - to - back to form a beam having a pair of parallel main flanges 364 and 366 . as above , each has an opening 368 with a formed lip 370 to which a grommet such as grommet 60 or 220 can mount , thereby providing a controlled dimension protected passageway for carrying services . various embodiments of the invention have now been described in detail . inasmuch as changes in and or additions to the above - described best mode may be made without departing from the nature , spirit or scope of the invention , the invention is not to be limited to those details , but only by the appended claims .