IN CASE you were wondering about the extravagant new office building the Senate is constructing for itself, you should understand that it's the computers' fault. All those computers around the country send mail to the Senate. That calls for more staff. And that, in turn, calls for more space.A child of 10 could have figured it out. As Sen. Richard Schweiker (R-Pa.) explained last week, that is why the Senate needs a third office building with elegant paneling, top-floor restaurant, "physical-fitness facility" (an indoor tennis court) and senatorial suites with 16-foot ceilings (for storing taller stacks of mail). Apropos of the gym, anyway, which will be the Senate's third, columnist Mary McGrory suggested in The Washington Star the other day a more plausible motive. She thinks the Senate may well be planning to field an Olympic team.

It was on Schweikerian terms, however, that the Senate beat down Rhode Island Republican Sen. John Chaffee's impudent idea that the huge office complex under construction should be stopped. And there were other reasons offered for pressing on. Economy in government, for example. As Sen. Milton Young (R-N.D.) explained, the structure is "too elaborate" and "too expensive" - but "we have gone so far now" that abandoning it would cost well over $50 million, far too much to waste in an inflationary time.

Critics will reply that the girders went up only last spring - oddly enough, right after Sen. Chafee first proposed stopping it. But the plan has - indisputably - been gaining steam for two decades. The Senate Office Building Commission was thinking about a third complex before the second (Dirksen) edifice was done. To understand what force such thoughts can gain, all that you need to know is that the commission has a self-evident mission, three extremely senior members - Chairman John J. Sparkman (D-Ala.) and Sens. Edmund S. Muskie (D-Maine) and Howard W. Cannon (D-Nev.) - and a long record of working hand-in-glove with the Capitol architect.

You won't find speeches by those elders in the Congressional Record of Aug. 4. They were silent when Sens. Chafee, John Danforth (R-Mo.) and a few others made the heretical suggestion that Senate staffs might be pruned. Sen. Dale Bumpers (D-Ark.) was left to stand up for the building during the debate. But it's no wonder that only three senior Democrats - Sens. Claiborne Pell (R.I.), William Proxmire (Wis.) and Jennings Randolph (W. Va.) - voted with the up-starts when Sen. Chafee's motion was buried, 45 to 29.

That isn't all. Appropriations Chairman Warren Magnuson (D-Wash.) finished off the day with an extra lesson in how the Senate works when something really precious is at stake. First he chatted a bit about how the new restaurant should be for the public, not for senators.(He din't mention who should use the gym.) Then he offered a two-part motion that was rapidly adopted, 65 to 13. One part puts a $135-million ceiling on the building. That is a bit more than the architect currenly plans to spend. The second part appropriates $54,853,000. And that, added to the $85,147,000 approved some years ago, allows the office building commission to finish and furnish its new palace, barring overruns, without any more disruptive debates.

This sort of thing tends to make taxpayers write protest letters to their senators. But our advice in this case is: Don't bother.More mail might make the building even bigger. Besides, it's clear what answer you will get - the same one Sen. Chafee and his friends got. Ring Lardner summed it up in a story: "'Shut up,' he explained."