In D.C., the law is a maximum fo 3" for a folder and I do not believe that fixed blades are legal to carry. Do not try to conceal your knofe, but sticking it in your pocket is not consdered concealment. But having a clip showing is not, per se, considered to be "brandishing" as it is in some jurisdictions.
The law in federal buildings is that "pocket knives" with blades of less than 2.5" are legal pursuant to 18USC930. But that is a civilian federal regulation, so it probably does not apply to military bases. As Morimotom suggested, I would check with the base on what they allow.
BTW, DO NOT try to carry any blade into the US Capitol, the Supreme Court, the White House, or the Library of Congress. They have their own laws. And I understand that the Smithsonian Institution is now banning all blades and scissors.