Adam Eaton’s solo home run opened the scoring in the first inning, and the Nationals didn’t let up in a 13-0 win over Pittsburgh. (Keith Srakocic/AP)

Fans were still walking over the Roberto Clemente Bridge — black hats on, cheap tickets in their pockets, empty seats waiting inside the ballpark — when the Washington Nationals took over PNC Park.

It took just a few moments, and one loud swing, for the Nationals to fill a dull atmosphere with their own noise after Adam Eaton parked a solo homer into the first rows beyond the right field fence. It accounted for the first run of many in Washington’s 13-0 win over the Pittsburgh Pirates on Monday. And, as a matter of course, it triggered another ­dugout dance party.

The Nationals’ post-homer raves have become tradition, social media catnip and all too common in the past week — if you ask their opponents. They tied a club record with eight home runs against the Milwaukee Brewers on Sunday. They hit four more Monday, three in the first two innings, to bury the Pirates to open a four-game series.

The Nationals are five games back in the National League East and, at 68-56, 12 games over .500 for the first time in two seasons under Manager Dave Martinez. They have scored 13 or more runs in four of their past five contests. Raw power goes a long way.

“It settles everybody down; here we go,” Martinez said of celebrating that early in the game. “And you can tell: They start having fun and loosening up a little bit.”

Now back to the dugout in the top of the first.

Once Eaton homered on the eighth pitch from Pirates starter Trevor Williams, the celebration was waiting for him. A runway was cleared between the front and back stairs. Gerardo Parra, in his pink-tinted glasses, began jumping up and down. So did Aníbal Sánchez, who is supposedly to blame — or praise? — for all the bad dance moves these past few months. Parra and Sánchez, longtime friends from Venezuela, both veterans who don’t play every day, are the ringleaders. Then the hitters take it from there.

Eaton skipped down the steps, hopped toward his teammates and took off his red helmet to use as a prop. He shook the helmet around his body as the yelling got louder and louder. That’s when the home crowd, a sparse one, became subject to the racket. Because the ballpark was so quiet, and already sunk by an early home run, the Nationals’ shouts of “Hey! Hey! Hey!” could be heard in the upper sections, loud and clear.

“If you write a book on it, you’ll become a millionaire. That’d be great,” Eaton said when asked to explain this offensive surge. “No, we don’t know. You just keep putting good at-bats together and kind of feed off each other.”

Matt Adams added a two-run blast later in the first to make it 4-0. He then jumped onto the back of Ali Modami, the Nationals’ ace batting practice pitcher, and Modami ran them through the dance line. Trea Turner cracked a three-run shot in the second — 419 feet to left — and bent to a knee while pointing to the sky, reminiscent of sprinter Usain Bolt. The Nationals kept growing their lead, up to 8-0 by the end of the second and 11-0 after the fourth. And Joe Ross, starting for the Nationals, was cruising until a come-backer struck his right shin.

Ross exited in the fourth, the reason it wasn’t all good for the Nationals on Monday. Pirates first baseman Josh Bell ripped a sinker right back at the 26-year-old right-hander. After it caught him below the right knee, Ross skipped in front of the mound to make a play. Ross retired the first eight batters he faced and was well into a fourth consecutive strong outing. His final, shortened line was 3⅓ innings, three hits, no walks, two strikeouts and a very big bruise.

X-rays were negative on Ross’s shin after the game. Martinez characterized it as a “contusion” and added that Ross will be further evaluated Tuesday morning. Ross has made a strong case to remain in the rotation instead of Erick Fedde once Max Scherzer returns, but his health could make the decision simpler. Javy Guerra relieved him Monday and set down all 10 batters he faced.

“It looks pretty sore, but I’ll be all right,” Ross said. “I wish I could have stayed in the game, but it wasn’t looking very likely.”

This all comes amid leaguewide debate about “juiced” baseballs. Home runs are flying out more than ever. There were 313 in the past week alone, the most ever between a Monday and a Sunday, and 301 in the week before that. But that doesn’t take away from how well the Nationals are swinging. All that matters in the end, and in the pennant race, are that runs and wins pile up.

Eaton smacked his 11th home run of the season and now has one in three consecutive games and four of the past five. Adams reached 20 in his 59th start. Turner is up to 13 despite missing six weeks with a broken right index finger. Asdrúbal Cabrera crushed a two-run homer in the ninth for good measure, and that gave Washington a franchise-record 16 homers in its past three games.

The Nationals’ 43 runs in their past three games are the highest three-game total in baseball this season and a club record. Their 62 runs in their past five are the highest five-game total of the year and the most since a stretch by the New York Yankees in 2007. If Washington’s hitters are benefiting from a fluky ball — and to some measure they are — they are doing so better than any other team right now.

Just ask the Pirates. Or ask those fans on the bridge if they could hear the noise.