High winds, fireproofing, water pressure require special attention for 234-foot, 25,000-ton Block 9 tower

FARGO -- As construction workers begin building downtown's Block 9 high-rise, destined to be the tallest in the city and the second tallest in the state, they're going to start by going down -- way down. "Fargo has a very good bearing layer, but ...
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FARGO - As construction workers begin building downtown's Block 9 high-rise, destined to be the tallest in the city and the second tallest in the state, they're going to start by going down - way down.

"Fargo has a very good bearing layer, but you gotta go pretty deep to get to it," said James Pawlikowski, a senior structural engineer with Skidmore, Owings & Merrill who worked on the design of the high-rise.

The clay soil at the surface can, for most practical purposes, only bear the weight of buildings four to five stories tall. But Block 9 will stand 18 stories tall in the 200 block of Broadway. To bear the high-rise's 25,000 tons plus a little extra, the foundation has to reach 110 feet down - about the height of downtown's Black Building - to hard, compressed clay.

It's one of the many challenges of constructing tall buildings, which are subjected to many of the same forces as smaller buildings but only more so. Besides bearing its own weight, Block 9 will have to resist stronger wind forces both because of its greater surface area and the strength of the wind higher up. It will have to amplify water pressure because it's taller than even the city's tallest water tower. And it will have to contain fires better because it's harder for firefighters to access, those involved in the project said.

With the high-rise being a big construction project in a crowded neighborhood, there's another extra requirement: Builders will have to work hard to minimize disruption to surrounding businesses, according to Keith Leier, project manager with the Kilbourne Group.

"Block 9 is really going to have their hands full," said architect Craig Helenske, who designed what's now the tallest building in the city, the nearby Radisson Hotel. He dealt with all the tall-building challenges that Block 9 must deal with, but he said downtown had far fewer businesses to disrupt back in 1984.

Block 9 Partners, made up of the Kilbourne Group owned by Gov. Doug Burgum and the R.D. Offutt Co., expects the 234-foot high-rise to be completed in fall 2020 at a cost of $117 million. It'll be home to RDO's offices, a hotel with event space, other office space, residential condos and a plaza for public events. A public parking ramp will be built next door as part of the project.

Deep roots

For Pawlikowski, who worked for SOM on Dubai's 2,717-foot Burj Khalifa, the tallest building in the world, Block 9 is a much more down-to-earth affair.

He said the sandy clay soil here reminds him of Chicago, where his firm has its headquarters and where it built, among others, the 1,451-foot Willis Tower that was the world's tallest for 25 years.

Like Chicago, Fargo was once the bottom of a lake that formed at the end of the last ice age. The waterlogged clay can compress a lot under a tall building and lead to a lot of settling.

Beneath the lake bottom is a mix of clay and gravel brought by glaciers during past ice ages and compacted by them, with the deepest layers being the most compact.

Workers will drill 280 holes spread across Block 9's footprint, each deep enough to reach down there, and pour concrete into them. These long columns, called pilings, will transfer the high-rise's weight to the compact clay below and also, through friction, to the clay clinging to the sides of the pilings, according to Leier.

Each piling has a maximum capacity of 200 tons, plenty to spare for the building's weight as well as all the stuff that goes in it.

The depth of Block 9's pilings will be the same as the Radisson's pilings, perhaps not surprising given the hotel is only 27 feet shorter and sits on the next block over.

Helenske said he still remembers drilling the holes for the pilings. Contractors didn't have cameras they could use to inspect the boring in those days, he said, and had to lower a person down there. "I thought about (going), but I said, 'Nah, maybe not!'"

While some tall buildings require pilings resting on solid bedrock - in downtown Fargo the bedrock is deeper than 260 feet - Pawlikowski said that's only true if they're in the same class as the 108-story Willis Tower. There are lots of 70-story buildings in Chicago that go only as deep as the hard clay, he said.

The piling requirement carries such an expense that it's shaped downtown's skyline.

With four or five stories the limit for a building without piling, that's about as high as most downtown buildings get, according to Leier. There are few buildings that are six or seven stories because once developers invest in piling, they'll want more floors and more tenants to recoup the cost.

"Once you've gone that far - that's millions of dollars - you might as well maximize as far as you can take it; you wouldn't want to do that for one floor," said Derek Hoeschen, McGough Construction's general manager in Fargo. His firm is the general contractor for Block 9.

According to Emporis, a database of tall buildings, out of 53 low-rise to high-rise buildings, 13 have six or seven floors. But building very high is also rare. There are only three buildings with more than 10 floors, including the Radisson, Lashkowitz High Rise downtown and the new Sanford hospital.

Stiff spine

Just as important as fighting gravity, tall buildings have to fight the sway brought by strong wind gusts and earthquakes. This swaying can cause load-bearing structures to weaken or buckle, endangering the building.

While there have been a handful of earthquakes near Fargo - the last one a 3.7 magnitude quake north of Sisseton, S.D., in 1995 - the threat is so minuscule that protecting a building from wind will be enough to protect it from quakes.

The Block 9 tower is designed to sway less than an inch in wind gusts of 115 mph, based on building codes that account for the history of strong winds here, according to Leier and Pawlikowski. Building codes don't require protection against tornadoes, which are much more destructive.

To achieve the necessary rigidity, the tower has at its core a hollow concrete shaft with 14-inch thick walls. This stiff spine also houses the stairwells and elevators.

The outer skin of the high-rise is a concrete cage, where the windows are installed, and the space between the cage and the spine is spanned by concrete floor slabs. When the wind blows, each window transfers the force to the slab above and below it, and the slabs transfer the force to the spine.

This contrasts with the design of the Radisson, which has a thinner cross-section. It has a stiff wall on the north side and a stiff tower in the southeast corner, but otherwise uses a similar system of slabs and columns, Helenske said.

A 1984 Forum story about the hotel's construction quoted the general contractor saying the building is designed to sway only 9 inches in a 90-mph wind, which was what building codes required at the time.

Fighting fires

A building's height also imposes challenges for making it safe and livable.

Fighting fires, in general, is tougher in tall buildings because they're harder to evacuate and harder for firefighters to enter. That's why, in general, building codes get tougher as buildings get taller, according to Fargo Fire Marshal Ryan Erickson.

To reduce the risk, tall buildings can be designed to make both easier, as well as buy time by slowing the spread of fire, according to Scott Cherney, an architect with SOM who worked on Block 9.

The high-rise, like many tall buildings, is designed with sprinklers and fire-resistant material to slow down a fire. It has standpipes, essentially in-building fire hydrants, so firefighters won't have to haul hoses from street-level hydrants. And it has stairwells and hallways under slightly higher air pressure to keep smoke out.

One problem with the water supply in tall buildings is the water has to fight gravity. The city of Fargo, like most cities, uses water towers to maintain constant water pressure and that pressure drops significantly in places higher than the level of water in the towers. Block 9 will be about 100 feet taller than the top of the tallest water tower in town.

Leier said the high-rise will use pumps to keep sprinklers, standpipes and other water systems under pressure, with backup generators to ensure reliability.

While a building's appearance isn't a life-or-death issue, architects do strive for a design that fits in with the surroundings.

With most neighboring downtown buildings no more than four or five stories tall, Block 9 was designed to blend in with a five-story "podium" surrounding the tower's west and south sides, where pedestrian traffic is heaviest.

Cherney said the intent was to give the high-rise a "human scale."

Leier said the design with its vertical columns is also meant to evoke the patterns on the side of grain silos, the "skyscrapers of the prairie."

During construction, with tall cranes and truck traffic, there will be little builders can do to blend in. The goal will be to minimize disruption, according to Leier.

One of the ways to do that is to effectively spread the construction site across the city, according to Hoeschen, the McGough manager. Many steel components will be pre-assembled and concrete components pre-cast at factories around the area and trucked in only when workers are ready to use them. There won't be long lines of trucks, because if that threatens to happen, drivers will be asked to wait at an off-site location.

Hoeschen said construction plans allow him to know what's being built and what needs to be delivered down to the hour. That helps the contractors, too, he said. "It's just quicker, less resources, more efficient, less waste as well."