referring in more detail to the drawings , fig1 illustrates a portable ramp assembly 10 in a secured position between a vehicle trailer 12 and a ground surface 14 to facilitate loading and unloading cargo 16 into and from the trailer , such as with the use of a hand truck 18 , readily available from magline , inc ., of pinconning , mich . as shown in fig3 , the ramp assembly 10 has a plurality of panels 20 extending between opposite side rails 22 , 23 to provide a generally planar , upwardly facing support surface 24 to facilitate transporting the cargo 16 into and out of the trailer 12 . the panels 20 are reversible in end - over - end fashion and interchangeable or individually replaceable to allow worn or damaged sections of the support surface 24 to be replaced in position to extend the useful life of the ramp assembly 10 . the side rails 22 , 23 are constructed from a rigid , high strength material , and are preferably formed from a high strength polymeric or metallic material , and more preferably are extruded from aluminum . as shown in fig2 - 4 , the side rails 22 , 23 are preferably formed as generally flat beams with opposite inner and outer sides 26 , 27 . the outer sides 27 preferably have a recessed channel 28 to facilitate weight reduction , and the inner sides 26 preferably have a panel supporting flange 30 ( fig2 , 5 and 6 ) extending laterally outwardly therefrom generally along the length of the side rails 22 , 23 . the flanges 30 are preferably formed as one piece with the side rails 22 , 23 , though they could be constructed separately from the side rails 22 , 23 , and thereafter , attached thereto , such as through a weld joint or fasteners , for example . the flanges 30 have a plurality of openings 32 ( fig2 ) spaced a predetermined distance from one another to facilitate attachment of the panels 20 to the side rails 22 , 23 . the side rails 22 , 23 preferably have one end 34 with a convergent configuration , and shown here by way of example , as having converging opposite edges 36 to form a triangular nose portion 38 facilitating making a flat engagement between the nose portion 38 and the ground surface 14 . as shown in fig2 and 3 , a relatively rigid ladder - like frame 38 is constructed by maintaining the side rails 22 , 23 laterally spaced from one another by a plurality of cross members 40 . the cross members 40 are preferably equal in length and fixed between the cross rails 22 , 23 , such as , by way of example and without limitations , through a weld joint or fasteners to maintain the cross rails 40 in a fixed , generally parallel relation to one another . the cross members 40 are preferably spaced equidistant from one another along the length of the side rails 22 , 23 to define panel receiving openings or spaces 42 of equal size between the cross members 40 to receive a predetermined number of panels , and represented here , by way of example and without limitations , as a pair of panels 20 within each space 42 . as best shown in fig5 , to facilitate receiving and maintaining the drop - in panels 20 within the spaces 42 , the cross members 40 preferably have leading edges with an outwardly extending protrusion or tongue 44 , and trailing edges with an flange or stop surface 46 extending outwardly and generally perpendicularly from the trailing edge , such that the stop surface 46 is generally laterally offset and parallel to the support surface 24 . the cross members 40 are preferably constructed having a hollowed core 48 , and are preferably extruded from aluminum . it should be recognized that the cross members 40 can be solid , and constructed using other rigid , high strength materials , such as high strength polymeric or fiber reinforced materials , for example , and that other manufacturing methods , i . e . molding , stamping and bending , could be used . the ladder - like assembly 10 has a lower end cross member 50 with upper and lower surfaces 51 extending divergently so as to be generally triangular in cross section and conforming to the shape of the triangular nose portions 37 of the side rails 22 , 23 . as shown in fig5 , the lower end cross member 50 is preferably constructed from extruded aluminum stock , and preferably has ribs 51 a forming a receptacle 52 traversing a back side to facilitate assembly of the panels 20 within the opening 42 between the lower end cross member 50 and the next closest cross member 40 . the panels 20 may be die - cast from magnesium or aluminum , though other materials and processes , such as steel or high strength polymeric material and molding or extruding could be used to construct the panels . the panels 20 have opposite upper and lower grid sides 54 , 55 preferably having an identical tread configuration extending between opposite ends 56 , 57 defining opposite leading and trailing edges 58 , 59 . the opposite sides 54 , 55 preferably have raised , outwardly extending protrusions or nubs 60 over their upper and lower surfaces to facilitate maintaining traction . in addition to , or in lieu of the nubs 60 , the sides 54 , 55 may have recessed or scalloped areas , thereby rendering the planar surface as the traction surface . as best shown in fig4 , to further increase the traction , promote drainage and reduce the weight of the assembly , the panels 20 preferably have a plurality of through openings 62 . the through openings 62 are bounded by web portions 64 arranged in a honeycomb or grid configuration . as shown in fig4 - 6 , the leading edges 58 of the panels 20 are preferably formed having an outwardly extending tongue or protrusion 66 for positive supporting engagement with the flange or lip stop surfaces 46 of the cross members 40 , and for interlocking receipt in the trailing socket or groove edge 59 of the adjacent panel 20 . the protrusions 66 may extend intermittently or substantially across the leading edge 58 . the trailing edges 59 of the panels 20 are formed with a lateral recess or channel 68 sized for receipt of the leading tongue edges 44 of the cross members 40 and the tongue 66 of an adjacent panel 20 . the leading and trailing edges define a width of the panels , wherein the width is preferably defined so that a pair of panels 20 are received within each of the spaces 42 between a pair of the cross members 40 . the opposite ends 56 , 57 of the panels 20 define a length of the panels 20 which is such that the panels 20 preferably fit between the side rails 22 , 23 , but overlie the opposite flanges 30 on the side rails 22 , 23 . the panels 20 have fastener openings 70 ( fig3 ) adapted to align with the openings 32 in the flanges 30 and / or cross members 40 to facilitate attachment of the panels 20 to the side rails 22 , 23 and / or cross members 40 . each panel 20 is shown here , by way of example , as being attached to the side rails 22 , 23 via a pair of fasteners 72 ( fig4 ), such as standard bolts , adjacent the opposite ends 56 , 57 of the respective panel 20 , though more fasteners could be used in combination with more through openings in the side rails , if desired . in assembly , the panels 20 are individually attached to the frame 38 via the fasteners 72 . within each space 42 , a pair of panels 20 is disposed such that the channel 68 of a first panel 20 is disposed to receive the tongue 44 of one cross member 40 , while the protrusion 66 on the opposite end of the first panel 20 is disposed within a channel 68 of a second adjacent panel 20 . the protrusion 66 of the second panel 20 is then lowered for supporting engagement with the stop surface 46 of another cross member 40 . this process is continued until all the panels 20 are attached to the frame 38 . in the space 42 between the lower cross member 50 and the next closest cross member 40 ( fig5 ), the protrusion 66 of a first panel 20 is disposed in the receptacle 52 of the lower cross member 50 , and the channel 68 of a second panel 20 receives the protrusion 66 of the cross member 40 . thereafter , the protrusion 66 of the second panel 20 is guided for receipt within the channel 68 of the first panel 20 as the first and second panels 20 are lowered into engagement with the flanges 30 of the side rails 22 , 23 . thereafter , the fasteners 72 are inserted through the fastener openings 70 and into the openings 32 in the flanges 30 to secure the panels 20 in their releasably attached positions with conventional nuts . with the adjacent panels 20 being interlocked with one another , and with the panels 20 engaging respective ones of the cross members 40 , any load applied to an individual panel 20 is uniformly distributed among the interlocked panels 20 , and the cross members 40 . as such , the individual panels 20 may be constructed having further reduced material content , as the entire applied load is not being borne by any single panel 20 at one time . additionally , as one of the sides 54 , 55 of the separate panels 20 becomes worn , damaged , or otherwise rendered undesirable for continued use , the respective panel 20 may be removed from the frame 38 by removing the associated fasteners 72 , and then the panel 20 can be flipped end - over - end to bring the unused or fresh side 54 , 55 from its idle position to the support position . as shown in fig7 , a ramp assembly 110 is constructed according to another presently preferred embodiment of the invention . the ramp assembly has a pair of side rails 122 , 123 constructed generally the same as in the previous embodiment . the side rails 122 , 123 have flanges 130 ( fig8 and 9 ), on which cross members 140 are fixed or attached , such as through a weld joint or fasteners , for example . the flanges 130 have openings ( not shown ) spaced therealong at predetermined locations to facilitate attaching a plurality of panels 120 between the side rails 122 , 123 . the cross members 140 have opposite sides 141 extending between the side rails 122 , 123 constructed for mating cooperation with the panels 120 , and are shown here , by way of example and without limitations , as having recessed channels 143 therein . the channels 143 are sized for receipt of protrusions 166 extending from the sides of the panels 120 to interlock the panels 120 and cross members 140 . as such , loads applied to the panels are distributed uniformly to the cross members 140 . as shown in fig7 and 8 , by way of example and without limitation , three grid panels 120 are received within spaces 142 between adjacent cross members 140 . the outer panels 120 adjacent the cross members 140 have one of the protrusions 166 inserted into the channels 143 in the cross members 140 . the middle panel 120 is set between the outer panels 120 , with the protrusions 166 of the middle panel 120 and the adjacent outer panels 120 being slightly spaced from one another to accommodate clamp assemblies 145 . the panels 120 are preferably fastened to the flanges 130 of the side rails 122 , 123 via fasteners , as in the previous embodiment . as shown in fig9 , the clamp assemblies 145 have opposite elongate upper and lower plates 147 , 149 arranged for attachment to one another and preferably to the flanges 130 of the side rails 122 , 123 to facilitate maintaining the panels 120 in a releasably fixed position relative to the side rails 122 , 123 . the plates 147 , 149 are arranged in overlying relation to portions of opposite sides of the adjacent panels 120 and are drawn toward one another to clamp on the portions of the panels 120 upon being attached to one another . as such , at least a portion of the adjacent panels 120 are received between the opposite upper and lower plates 147 , 149 . the plates 147 , 149 are preferably extruded or molded from aluminum , though they could be formed from any suitable high strength resilient material , including high strength plastics , for example . the plates 147 , 149 preferably have inwardly extending curled edges 151 that overlie the protrusions 166 , preferably in a mating and generally conforming relation , and capture or clamp about the protrusions 166 extending from the edges of the adjacent panels 120 . the plates 147 , 149 are represented here , by way of example and without limitations , as being attached to one another via bolts 153 passing through axially aligned openings in the plates 147 , 149 , with standard nuts 155 being threaded onto the ends of the bolts 153 . the upper plates 147 preferably have a counter bore or recessed channel 157 sized to receive the enlarged heads of the bolts 153 such that the bolts 153 remain generally flush or slightly recessed relative to the traction surface of the panels 120 . when a panel or panels 120 become worn on one side , the respective nuts 155 and bolts 153 may be easily removed and the worn panels 120 may be flipped to bring a fresh side upward . the panels 120 are then reattached between the side rails 122 , 123 , at least in part , via the bolts 153 and nuts 155 . it should be recognized that upon reading the disclosure herein , that one ordinarily skilled in the art of would readily recognize other embodiments than those disclosed herein , with those embodiments being within the spirit and scope of the invention . accordingly , the disclosure herein is intended to be exemplary , and not limiting . the scope of the invention is defined by the following claims .