The Pentagon mistakenly posted plans online for its new office building in Alexandria, Virginia, showing exactly how big a bomb would have to be to destroy the facility. The 424-page Army Corps of Engineers document detailed the bomb-proofing design of the Mark Center near the intersection of Interstate 395 and Seminary Road, into which 6,400 defense workers are expected to move this fall, raising questions about security not only at the new office building but other Defense Department sites. Reuters, which first discovered the document online, reported that the Mark Center was designed to withstand the blast from 220 pounds of TNT-like material if it is detonated outside the Mark Center’s security perimeter, or 55 pounds if the perimeter is breached. That limit is a fraction of the size of the bombs used in attacks against the World Trade Center in 1993 and an Oklahoma City office building in 1995. The bomb-proofing of office buildings varies greatly, a professor of structural architecture at the University of Virginia said. There are no industry standards for such threats. Probably the Pentagons greater concern is that the information in those documents probably applies to lots of other buildings, and that their entire strategy here may be compromised because of the release of something like this,Ã??Ã?Â¢?? said the structural engineer. The plan was removed from the Army Corps of Engineers Web site and from Internet search engines, according to a corps spokesman. The file was dated 2009 and could have been online since then.