One of the materials which were used for the construction of the Mishkan was shittim wood, from which Benei Yisrael made the planks and beams that constituted the essential structure of the Mishkan, as well as the aron, the shulchan and the outdoor mizbei’ach.
Chizkuni, commenting on Parashat Teruma (25:5), asserts that shittim wood is an especially lightweight species of wood. He proves this point from Sefer Bamidbar (4:31-32), where we read that the nesi’im (tribal leaders) donated twelve wagons for the Leviyim to use in transporting the Mishkan. Eight of these wagons, the Torah tells, were used by the Merari family of Levites, who, as we know from earlier in Sefer Bamidbar (3:35-36), transported all the planks, pillars and beams of the Mishkan. If just eight wagons sufficed for all these, Chizkuni claims, then the wood must have been particularly lightweight. A total of 48 planks were used for the Mishkan, and they were quite large – ten amot long, 1.5 ama wide, and one ama thick. This is in addition to the sixteen wooden beams that ran across the length of the Mishkan. The fact that they were all transported in eight wagons would seem to prove that the wood was especially lightweight.
Further proof to Chizkuni’s theory may be drawn from the Gemara’s discussion in Masekhet Nedarim (38a), where the Gemara cites a tradition that Moshe was a man of considerable physical strength. To prove this claim, the Gemara notes that Moshe is said to have single-handedly spread the cloths of the Mishkan over the wooden planks to erect the Mishkan (“Va-yifros et ha-ohel al ha-Mishkan” – Shemot 40:19). The Gemara then dismisses this proof, suggesting that the cloths, even though they were quite large, were not necessarily heavy. Ultimately, the Gemara proves this point from the fact that Moshe was capable of throwing the two large stone tablets at the time of the sin of the golden calf. Seemingly, as the Rosh noted in his commentary to Masekhet Nedarim, the Gemara could have drawn proof from the Torah’s description of Moshe lifting the large wooden planks of the Mishkan to put them in place (“va-yasem et kerashav” – 40:18). In light of Chizkuni’s comments, however, we may easily answer that since the wood was much lighter than ordinary wood, it did not require special strength to lift the planks, and thus this job which Moshe completed does not indicate that he possessed exceptional physical might.
(See Rav Avraham Albert’s Birkat Avraham, Parashat Teruma)