I am not sure why this stirs up so many arguments, and also not sure why so many people when it comes to car chassis building want to argue on this, but let's set the record straight, and also give a few of my opinions which may or may not be gathered in fact. I am begging @Aloppen
to stop by and tell me if I am right on, or out to lunch on this? Unibody vehicles are built in many different ways, but by definition if the main chassis structure and the body are integrated that is a unibody (GM calls the Hummer EV an Integrated body and structure). The reason for doing it on the Hummer EV is clear, you get a much stiffer chassis, without having to massively over build the frame, making it heavy and expensive to build. Case in point on stiffness, how many people have noticed a typical pickup has a gap between the cab and the bed, do you wonder why? In a typical body on frame pickup the frame has quite a bit of flex when you are maneuvering through certain terrain and if there was not an "joint" in the body you would wrinkle the panels when you twist the truck. Modern trucks have made this better with fully boxed frames, but everyone knows they could build them into a unibody they would be stiffer, lighter and cheaper, so why do OEM's build pickups body on frame? From my understanding this is because body on frame is more flexible in the factory when you have multiple wheelbases, cab configurations, and bed lengths all being built on the same lines. They can just roll the frame upside down to install certain components, and then roll it upright to marry the body. A body on frame pickup is what Elon Musk made fun of in the Cybertruck presentation, and when he stated that the cab and bed are just extra cargo along for the ride, he was right, the cab and bed do not add significantly to the structural integrity of the chassis in certain bending moments. Now back to the Hummer EV, we know it has a very heavy battery adding weight, so to try to save weight and get a stiffer chassis I think GM engineers decided to integrate the structure and body, there are sound engineering reasons for this, as any engineers know, when you build a structure to transfer loads, often the taller the structure can be the more load you can transfer without unsatisfactory deflection, or heavy / expensive materials. On a pickup truck frame you do not have much height to work with unless you want to lift the truck extra high (or have low ground clearance), but in the case of the Hummer EV, you can use the structure of the cab, and bed to reinforce the understructure, and also use the battery enclosure for further reinforcement you get an incredible stiff structure. I am pretty certain from looking at it the Hummer is the stiffest chassis GM builds, both front to back and side to side. Now, there is one reason a Unibody is not ideal for a low production vehicle like the Hummer EV, and that is factory setup cost... Unibodies are mostly put together using automated systems in the body shop, and those systems are expensive and not flexible for changes, so how does GM amortize the cost of the body shop setup for the Hummer EV program, step up and let me introduce you to the Silverado EV. Now this is my opinion, but I think the Silverado EV will sit on the exact foundation that is under the Hummer EV. Of course the Silverado EV will not have all the cool off road gizmos, and likely a cheaper suspension system, but I am going to guess that the internal structure, from the cowl down are nearly identical. I am now going to attach some pictures and comment on what I see...
Let's start with the painted Hummer EV Body. Interestingly GM seems to be painting the bottom of this structure, this is great news as it should be more resistant to rust, and other failures, and well for an anal guy like me that details my cars top and bottom, it just looks a lot nicer. Will the painted underside make production? I sure hope so. In this picture you can clearly see the structure is massive.
The next picture shows Mark Reuss looking at the underneath of the structure, I think he is looking at the rear suspension mounting hard points, and clearly you can see that this is a prototype part by the "X's" marking bends and weld points.
The next picture is further back from that same chassis, this shows the backbone I think will be shared between Hummer and Silverado EV. I was assuming the Silverado EV will have a slightly shorter wheelbase and obviously the top hat (windshield frame, roof, and rear window) will be unique. When you look at this structure, its obvious that is going to be stiff as heck.
The last picture shows a closer image of the firewall, again, that is a structural part bracing the chassis in side to side, and twisting moments, this is exciting to see. Damn, I cannot wait for mine.
GM showed the Silverado EV at the Barclays conference, it's obvious it is on the same platform, and I venture to guess shares a lot of parts with the Hummer EV.