Again, sketchup seems to be adding or subtracting random 64ths to my dimensions; as they were even numbers when I laid them down. the top is intended to be 27" wide; the center part of which should be 9". It should be 48" long, 34" high (before you add height adjusting feet to the bottoms of both front legs, and either the center of the back leg, or to both uprights... you can adjust the leg length to suit your preferences), and the straight sections of the the top shown in the plan view should each be 19.5" long, with the angled sections being 9" long and 9" wide.
The primary structure is all 4x4 dimensional lumber milled down to 3", with 3/4" plywood top, shelves, and gussets. All wood joins are glued and screwed (face screwed, pocket screwed, or lag screwed, depending on the join). The benchtop is finished with tempered hardboard, glued and screwed, with thin sheetmetal edging, supplied by drywall corners.
The main structural member, is a T-beam, created by jointing, then gluing and lag screwing three, four foot lengths of 4x4 milled down to 3" (tip: mill to 3-1/8" before gluing up the three across pieces, then run the glueup through your planer after it's dry... presuming you are using a soft resin glue that wont damage your planer of course... to smooth and true up both sides and get down to 3" thickness); with a fourth length of the 3" lumber glued and lag screwed to the center piece, forming the T shape.
In these two views, I have shown only the front plywood gusset, the rear being obscured; but it should be obvious that it is a simple rectangle, extending to the bottom of the lower shelves subframe.
Also the front gusset is shown as ending above the shelf, but this was just for clarity, to show the 3" crossmember between the front legs, underneath the shelf. You could leave it this way, or you could extend the gusset to the bottom of the crossmember, for improved looks, and added rigidity.
In these views it is hidden, but the front benchtop crossmember is one piece, extending the width of the top, and saddle joined over a half lap notch cut in the cross-t section of the front of the T-beam; then glued and screwed.
In these views there are also three frame members you cannot see.
There is a long frame member running from between the rear leg uprights and the bottom of the rear gusset; glued and screwed to the shelf, and lagbolted into the front crossmember, under the front gusset.
There are also two diagonal braces under the benchtop; running from the end of the front upper top supports, back to the T-beam (to which they are pocket screwed), at a 45 degree angle.
Oh, and on the back legs, there are two little shelf cleats, just to keep that back end of that shelf stable. You can make them out of the scrap cutoffs from the 3" stock pretty easily.
You may want to forgo the metal top coaming, and perimeter edge the entire top with thin trim stock (1x2" pine trim board works fine). It will add somewhat to strength and rigidity, plus improve the longevity of the plywood top, while also dramatically improving the looks of the thing.
The dimensions of this bench are 27"x48". It could easily be extended out to as wide as 48", and as long as 72".
If you extend the width of the top beyond 32", I recommend you double the width of the center section to 18" with a six timber wide main beam section, and two timber wide center "t"; and instead of just a diagonal cross brace on each side of the top, run a full perimeter frame on each wing, with a second top crossmember across the point where the cutout begins.
You may also want to run a support brace, from the outer corners of the wings frame, down to the lower crossmember on the front legs; pocket screwing them into the legs at that point. If you do so, I recommend making the front plywood gusset, extend to at least half the width of the bracing frame; for additional rigidity.
Alternately, if you extend all the way out to 42" or 48" you could take vertical members straight down from the wings; making them the primary front legs, and meeting up with the splayed central members, as crossbracing (miter cut the center angled members to meet the vertical face of the legs, and pocket screw them into place.. or even rabbet them or birdsmouth them in). In this configuration, you could add an additional crossmember across the braces if you wanted additional support and rigidity.
Frankly, this is a little overkill structurally. It's going to weigh something like 150lbs as is; but it should be extremely stable, and last until the pine rots.
The question was asked, why use 4x4s, and why mill them down to 3x3?
Good question, and I never did explain it in the last few posts.
You mill the 4x4s down, to get them flat, square, and true; something most dimensional lumber is most definitely NOT from the lumber yard.
It's an old cabinet makers trick for getting clean and relatively cheap framing wood. 4x4s, 4x6's, and 4x8s (if you can find them anymore) are generally pretty decent wood, just ugly, and twisted, bowed, skewed, knotted etc... by milling and resawing them you can get good, even dimensioned wood.
Also, it's very convenient to work with square, evenly dimensioned stock. It makes for good corners, good miters, mice face joins etc...
The problem with using 2x4s is their true dimensions are 1.5"x3.5" and by the time you've got them flat, straight, and true, you're probably below 1.25"x3.25"; which is an awkward dimension, and not particularly strong for primary frame members.
If I were looking to not overbuild here, I would make the primary t-beam and the legs from 4x4 stock milled to 3x3; and then do all the secondary framing with 2x4s milled to 1x3.