Standing Acacia Wood in the Mishkan

וְעָשִ יתָ אֶת־הַקְרָשִ ים לַמִשְ כָן עֲצֵּי שִטִים עֹמְדִ ים :טושמות כו,

1. (Yoma 72a) says in the name of Rabbi Choma b"R Chanina that this means that the beams (kerashim) should be positioned so that their end that was close to the roots when growing should also be at the bottom.
2. Alternatively, Rashi offers that this type of wood was impervious to worm damage, thus their gold cladding would not fall off because of internal rot.
3. Alternatively, the Gemara offers that they give permanence to their cladding, i.e. they keep their plating standing. Rashi d.h. "shemaamidin" explains that this means that their gold plating (verse 29) which was very thin and not able to stand by itself should be permanently fastened to the wooden beams with nails. Thus, the kerashim made stand the gold.
4. Panim Yafos explains that the word "shemaamidin" indicates precious and beautiful. This was accomplished by placing the gold over the wood. Thus, the gold made beautiful the kerashim
5. The Gemara has a 3rd explanation that the acacia beams are permanent, that they will remain forever. Even though they are now hidden from us, they will be shown to us at the end of days.
6. Rashi and Sforno on our verse say that OMDIM teaches us that the beams should not be placed horizontally one on top of the other to create walls, but rather, they should be placed vertically, OMDIM
7. OMDIM means taking the wood from growing trees that are alive and standing, and not from dead trees that have fallen, as they are subject to rot and might already have hidden interior deterioration. (Hadar Z'keinim)
8. Tosfos Hasho'leim writes that they did not change their color with aging, nor did they deteriorate. Change of color is not a factor in the cosmetic aspect of the Mishkan because they were gilded with gold, "V'es hakrashim t'tza'peh zahav," (verse 29). However, it explains why Hashem chose this material.
9. OMDIM means from PREPARED trees. Medrash Tanchuma #9 on our parsha says that Yaakov planted acacia trees in Egypt and told his descendants that they should be harvested and taken along upon departure for use in the Mishkan. This has a mathematical allusion. "Hakrashim laMishkan" is the same as "Yaakov Avinu nota lahem arozim b'Mitzrayim." Another indication to this is the word "Hakrashim," with a definitive letter Hei, the specific beams that Yaakov planted to be brought with them. (Kli Yakar)
10. According to the Medrash the world was unworthy of cedar trees; nevertheless, HaShem created them for the sake of the Mishkan and later the Batei Mikdashim. The wood of the cedar tree is extremely hard, symbolic of strength and inflexibility. However, this very attribute was created to be used for a positive purpose, i.e., that a Jew must never be deterred by those that mock him (Avnei Ezel).
11. Cedar trees allude to the concept of strong as a rock, fortitude that can stand against the struggles of the world. Just as only a permanent knot (kesher) is forbidden on Shabbos, so too the kerashim are symbolic of the common values that connect and unifies the Jews. Even within oneself, one needs to be strongly connected with proper values. Kesher signifies true connect whereas one needs to be careful about false and fake (Sheker –same letters) connections (Rabbi Zevin L’Torah and L’Moadim).
12. “LAmishkan” Rabbi Oshioh asked (M.R. 35:4), "Why doesn't it say 'V'asisa es hakroshim Mishkan,' since the beams were the actual structure of the Mishkan?" He answers that this is to indicate that the Mishkan and the future Batei Mikdash are collateral, "LAMASHKON, for a collateral." If the Bnei Yisrael will sin, the Batei Mikdash will be destroyed.
13. Another Gemara (Sukkah 45b) It also teaches that the kerashim are eternal. Why were the beams eternal and not the other vessels? In the desert a constant refrain was “let us return to Egypt.” Such a threat compromises the “gift” of the various donations for the Mishkan. However, the wood for the beams was an inheritance from Yaacov already deeded completely to the Mishkan.
14. The Gemara also offers the explanation (Chizkiyah citing R. Yirmiyah citing R. Shimon b. Yochai) “Mitzvos must be done as the article grows” (learned from standing beams the method of growth) such as Esrog.