R' Chama b'R Chaninah asks "What does the posuk mean when it says "Atzei shitim Omdim"?" Maharsha explains the question as "Isn't it obvious that the boards had to be standing upright? After all the Torah describes the "adonim", the number of beams, and the dimensions of the Mishkan. We can therefore infer quite simply that the boards had to be standing upright and not laying horizontally". The Gemarah answers that "Omdim" means that they had to be positioned so that the top of the beam corresponded to the top of the original tree and the base corresponded to the bottom.
However Rashi in Terumah (26:15) explains that "omdim" means that the beams had to be vertical and not horizontal. Why would Rashi adopt this approach when it is obvious in Peshutei shel mikrah that the beams could only be vertical? Why didn't Rashi quote our Gemorah here?
Shmarya Richler, Montreal, Canada
The Panim Yafos Al ha'Torah (from the Hafla'ah) also states that our Gemara did not give Rashi's explanation because it is obvious from the Peshuto Shel Mikra. However, it seems likely that Rashi did not bring our Gemara because our Gemara reaches the novel conclusion that all Mitzvos must be "Derech Gedilasan," which is not seen from the Pashtus of the Pasuk (see Sfas Emes in Sukah (45b) who remarks that this is indeed quite a Mechudash conclusion). One might counter that Rashi should still have quoted the Gemara solely regarding the Atzei Shitim (that they must be Derech Gedilasan). It is possible that Rashi held the Pasuk needed to tell us that this form of building the Mishkan with the Atzei Shitim horizontal could not have been done, even b'Dieved, for the Mishkan to be called a Mishkan.
I don't think Rashi in Chumash deals with Halachah as you theorize. Rashi is concerned with Peshutei shel Mikrah.
Having said that your answer only reinforces my question. The word Omdim in Terumah is superfluous precisely because a few psukim later the Torah describes the adonim and how they fit into the kroshim and how they stood and their number. Therefore the word omdim is superfluous, hence Rashis question. The problem is why did Rashi choose an answer that doesn't dovetail with peshutei shel mikrah? The Gemorah in Yoma (72b) asks the same question and provides a much clearer answer (derech gedolosan). As far as this being a novelty as you mention, I think that peshutei shel mikrah calls for an answer that isn't shown to be untenable a few pesukim later. That is, it is impossible that the kroshim were horizontal. After all, the posuk describes the adonim and yadim that were carved into the end of the kroshim.
Perhaps you misunderstood my answer. Of course Rashi does not come to tell us the Halachah, and is only concerned with Peshuto Shel Mikra (as he himself says many times, for example see Bereishis 4:8). However, Rashi will definitely use a Halachah as the background which maintains and justifies the explanation he gives as Peshuto Shel Mikra. Rashi always prefers to give us the direct translation of the words, as this is the highest form of Peshuto Shel Mikra. I suggested above that Rashi understood that there were grounds to justify the Peshuto Shel Mikra meaning in the above Pasuk. These grounds are to say that this is absolutely necessary, even b'Di'eved. Rashi does not even mention this aspect of "b'Di'eved," as he is merely occupied with being able to transmit the Peshuto Shel Mikra. This is something one finds in the Mefarshei Rashi when people like you ask a good question, which then mandates an explanation of how this can possibly be Peshuto Shel Mikra.
Regarding the impossibility, I believe my answer covers that as well. Of course it was not the Pasuk's intent that the Mishkan should be made this way. But what would be the Halachah if it was? If the Menorah was made out of any metal, it was kosher b'Di'eved, even though the Pasuk clearly states it should be made out of gold (see Menachos 28b). A Pasuk is therefore necessary to tell us that this is not a Mishkan, even b'Di'eved.
Gemar Chasimah Tovah,