referring in detail to the drawings , fig1 shows a portion of an elongated structural member 10 having a longitudinally extending axis designated as 11 at the geometric center or centroid of the member . the cross - section as shown in fig2 shows the substantially triangular cross - section 12 of the member . a continuous passage of circular cross - section defined by inner surface 13 of passageway 14 is located at or near the geometric center or centroid of the substantially triangular cross - section of the member 10 . the member 10 includes three outer faces 15 , 16 and 17 that form the outer surface of the member . the points of intersection or corners 18 of the member may be truncated as a practical expedient to avoid chipping of an otherwise sharp corner or , at greater extents , an accommodation of other design objectives , but shall not violate the substantial triangularity of the cross - section until the extent of truncation produces a cross - section of regular , hexagonal shape or intersects the passageway . fig3 shows an embodiment of the member having a cluster of passageways 19 , 20 and 21 defined by their respective inner surfaces 19a , 20a and 21a . the cluster of passages of circular cross - section have their centers located at or near in surrounding relationship the centroid of the substantially triangular cross - section of the member . the centers 19b , 20b and 21b of the passages are positioned on the locus of a circle having a radius extending from the centroid of the member 22 . all of the other features of the cross - section of member 22 are identical to those identified above with reference to the embodiment of fig2 . referring to fig4 and 5 , a wide selection of materials is presently available and a wider selection of materials will be available in the future from which to compose a structural member as specified herein . whenever the material chosen requires the incorporation of a reinforcing element or elements , the reinforcement is installed entirely within the substantially triangular exterior boundary of the cross - section of the member . a structural mix of portland cement concrete is one particularly useful choice of material for the implementation of the invention . fig4 shows a chosen arrangement of a structural member 23 made of portland cement concrete having a passageway 24 defined by the inner surface 24a . tensile elements 25 , 26 and 27 extend longitudinally through the member 23 and are spaced from the centroid of the member . the tensile elements 25 , 26 and 27 may be either prestressed or non - prestressed with the elements located at the corners of the member 23 . closed stirrup - shaped tensile elements 28 are attached to the tensile elements 25 , 26 and 27 and add additional strength to the member . a force may be applied to the longitudinal reinforcing elements 25 , 26 and 27 and concrete poured around the elements so that release of the force after the concrete has cured will provide a prestressed member . the stirrup - shaped elements 28 interconnecting the rods carry transverse tensile forces . in fabricating the structural member , construction of the forms , placement of the reinforcing elements 25 , 26 , 27 and 28 , placement of the concrete 23 and stripping of the forms can be accomplished by conventional procedures for either precast or cast - in - place construction . construction of the forms may be simplified as forms may need to be only placed on surfaces 29 and 30 without placing a form on surface 31 . the passageway 24 may be formed by any of various procedures , the choice of which is influenced by the type of construction of the member , that being either precast or cast - in - place . for precast construction , the passageway 24 may , for example , be formed by a mandrel and sleeve procedure . this procedure is appropriate if the sleeve is not strong enough to support the pressure of the wet concrete . in such a case , a mandrel of sufficient strength is inserted in the sleeve before the concrete is placed . when the concrete is cured , the forms are removed , the mandrel is withdrawn , and the sleeve remains in place . for cast - in - place construction , the passageway 24 may , for example , be formed by a cast in sleeve procedure wherein the sleeve is of sufficient strength to alleviate the need of a supporting mandrel and is left inside the member . the concrete member constructed as above may be installed to serve either as a beam or as a column . as a column , the member is erected with its longitudinal axis oriented in a generally vertical direction . in this application , the passageway 24 or multiple passageways 19 , 20 and 21 as shown in fig3 may serve as a substitute for conduits for ventilation , plumbing , wiring and the like . utility of the passageway 24 or passageways 19 , 20 and 21 may be enhanced by providing access , such as by opening , to them through the faces 29 , 30 and 31 of the member . the angle α ( see fig4 ) of the faces 29 and 30 of the member 23 from an axis perpendicular to the longitudinal axis of the member provides an efficient surface for reflecting light or sound into the space surrounding the member . as a beam , the member may be installed with its longitudinal axis oriented in a generally horizontal plane . in this application the passageway 24 or passageways 19 , 20 and 21 as in fig3 may also serve as conduits for ventilation , plumbing , wiring and the like . again the utility of the passageways may be enhanced by providing access to them through the faces of the member . also the angle α ( fig4 ) of the faces of the member from a vertical axis perpendicular to the longitudinal axis of the member may be used to provide an efficient surface for reflecting light or sound into the space below or to the side of the member in like manner as when the member is used as a column . referring to fig6 a column 32 is formed from a cluster of six triangular structural members 33 , 34 , 35 , 36 , 37 and 38 which are constructed as proposed by this invention . the truncated edges of the triangular members provide a central passageway 32a . beams 39 , 39a , 40 , 40a , 41 and 41a are connected to the column 32 at 71 and may be secured thereto or formed as a single unit when cast - in - place . in particular , all the above structural members have a substantially equilateral triangular cross - section with one passageway located at the center of the cross - section of each member and all are composed of a structural mix of portland cement concrete using light weight aggregates reinforced with non - prestressed reinforcing steel tensile elements such as shown in fig5 . this system is composed of cast - in - place , steel reinforced , light weight concrete . illumination elements 42 and 43 may be attached to beams 39a and 40a with an opening in the beam connecting with the passageway 44 in the beam 39a and passageway 47 in the beam 40a serving as a conduit for the wiring to the illumination elements 42 and 43 . sprinkler elements 45 and 46 may be attached to faces on the beams 39 and 40 with the passageways in these beams ( not shown ) serving as conduits for water to the sprinkler heads . ventilating system grills 48 and 49 may be connected at openings in the beams 41a and 41 leading to the beam passageways ( not shown ) in beam 41a and 41 such that the passageways in the beams serve as ducts for heated and cooled air . electrical outlet 50 may be placed on the column 33 with the electrical wiring for the outlet in passageway 51 . it is understood that the use of the passageways in the columns and beams would depend upon the requirements for their application . the above structural system , by incorporating all features of the invention in all members of the superstructure of a building would provide an extremely economical and efficient alternative to a traditional cast - in - place concrete system composed of traditional members of solid convex cross - section . an example of a building structure in which the structural member of the invention has application is shown in fig7 . the building , generally designated 51 includes a plurality of triangular columns 52 , 53 , 54 , 55 , 56 and 57 and a central column 58 formed of six triangular structure members joined together as in fig6 . beams 59 , 59a , 60 , 60a , 61 and 61a are joined with the columns to form the framework for a structural system that forms the skeleton of a building . illumination elements 42 and 43 may be positioned on the beams 61 and 59a as discussed above , sprinkler heads 45 and 46 may be positioned on beams 59 and 61a as discussed above , and ventilation outlets 48 and 49 may be positioned on beams 60a and 60 as also discussed above . electrical outlets 62 , 63 and 64 may be located on the columns as desired in the particular application . the floor structure 65 may be formed of cast - in - place reinforced concrete as is known in the art . the ceiling structure 66 may be the lower surface of a floor structure in the case of a multiple story application and may be typically a suspended ceiling as is well known in the art . since a suspended ceiling is typically used to cover the additional conduits for electrical , plumbing and ventilation systems it may no longer be needed since the systems are incorporated in the framework structure of the beams and columns . a roof structure 67 is shown supported by the beams and columns . a wall structure 68 , one of which is shown , having window structures 69 and 70 may be connected between the columns in a known manner . it is understood that such wall structures could also incorporate doors and the like as needed in each particular application . while the structure shown in fig7 is a six sided structure , many variations in the arrangement of columns and beams would be apparent to those skilled in the art . it is also contemplated that multiple stories and multiple sections would be joined together or formed together to provide larger multistory building structures . the joint j may be formed by making the beams into an integral unit using a cast - in - place method . alternatively the beams could be interconnected and attached to the column structure using the precast method . the foregoing disclosure and description of the invention are illustrative and explanatory thereof , and various changes in the size and materials of the generally triangular structural member having a generally circular passageway therethrough may be made without departing from the spirit of the invention .