Timber Frame Water Mill Reconstruction Project, UK Part 1

Written by Guest Author, Gavin Sollars, Timber Frame Carpenter, UK

The village is the token and pride of England; there are usually found in it vestiges of earlier life – cottages, manor-houses, farm-houses, with buildings of more or less historic interest; and who should understand them, their origin, their peculiarity of structure, better than the local carpenter?

Walter Rose, The Village Carpenter 
The reconstruction job site as seen from across the River Test. Notice the green netting installed to prevent things from falling into the environmentally-sensitive river.

Introduction 

I became acquainted with Stan early in 2019 when I decided to look at buying an Ootsuki Nomi. During my search I became skeptical of many of the Japanese tools that are widely available to the European market and, after a lot of research, I came across Stan’s contact details and sent him an email. Stan took a great deal of time to discuss with me what really motivated my purchase, the kinds of things I should take into consideration when looking at Japanese tools and went into detail about the intricacies of Japanese craftsmanship. The information he freely provided was invaluable, and with his help I feel I made a very good choice, and now have a tool that will serve me for my entire career.

I recently updated Stan with some pictures of buildings I had worked on and he asked if I would be willing to share them with the readers of his blog. The overall aim of these ramblings is to describe to you (who Stan calls his “Gentle Readers”) the general outlines of the reconstruction of an 18th century traditionally-jointed timber frame structure in a beautiful area of the English countryside in the summer of 2019. I hope that this article will give you an understanding of the work that was undertaken and also the enthusiasm I have for this archaic variety of building craft and carpentry as a whole. 

Project Background

Timber framing in the UK has enjoyed a resurgence in popularity over the last 30 or so years, with quite a number of small to medium sized specialists in the craft building new ‘post and beam’ style buildings that emulate the traditional frames of the past. These companies are mixing time-honoured design details and timber framing techniques with more modern methods of production: chiefly circular saws and portable chain mortisers to rough out the work. This deep understanding and appreciation of historic building vernacular made my employer (The Green Oak Carpentry Co.) well placed to undertake the reconstruction of this Project.

The Mill prior to 2018 fire

The company was awarded the contract to reconstruct the building as close to the original as possible.

The Project is situated to the North of the Test Valley in Hampshire County in Southern England on the banks of the beautiful River Test, famous for trout fly fishing and “gin-clear waters.” The original waterwheel powered a grain mill. It was later converted to paper production, and even later housed a generator serving the nearby Manor House. Unfortunately, the original structure was completely destroyed by fire in early 2018.

At 21 metres (68.8’) long and 5.7 metres (18.7’) wide, the main structure is situated on a small island created by a man-made diversion in the River Test. The river flows from the north and is then diverted via a sluice gate to the right. The river then widens into a pool and bubbles quickly along the west side of the building. The diversion along the east flank drives the turbine, and passes underneath a wing that links the main structure back to the existing dwelling and also houses the turbine and mill workings. 

Historical & Structural Considerations

The original building was “listed” with the Historic Buildings and Monuments Commission for England confirming the historical importance of the building on the one hand while placing restrictions on how any work to the building can be performed on the other. What remained of the original building after the fire was still subject to the Commission’s rules and regulations, of course. It once featured shorter posts sitting on top of a brick wall from approximately first floor height. In the wake of the fire the surviving outer brick walls were deemed too structurally unsound to bear a load – however, due to the walls being “protected” under UK law, they had to be preserved. To get around this issue we installed two outer plates running around the perimeter of the building with the lower of the two (D – below drawing) supported on metal brackets (C) connected to the back of the timber Jowl posts (B) by lag bolts. The full weight of these two plates, as well as the softwood stud wall with conventional insulation and weatherboarding is carried by these brackets transferring the load to the jowl posts (B).

A detail drawing (drawn by myself) of the steel bracket, showing how load was removed from the fragile existing wall. The drawing also explains the interplay between our frame and the other elements in the building.

Design & Construction Details

Framing work started in July 2019 with a team of eight carpenters framing the bulk of the structure over a period of five weeks in workshops off-site. A team of four transported the fabricated components of the timber frame to the jobsite, assembled and raised the frame, framed the hips and valleys, then fitted the common rafters and cut and assembled the jack rafters.   

Constructed entirely from European green oak, the structural frame is very utilitarian by design and lacks the aesthetic details like the curved braces typical in many historic timber structures in the UK. Nonetheless, it has some nice detailing that might not be obvious at first glance. 

The main posts (wooden structural columns) are mostly jowl posts (aka “gunstock posts”) that flare at the top with tenons that fit into both the top plate (beam running along the top of and connecting the exterior posts) the column and tie beam. Historically, jowl posts were cut from the flaring grain of the base of a tree. These butts were often quartered and each post placed in the building adjacent to its sibling. I believe that this is a similarity historic English carpentry shares with its Japanese counterpart.

Here you can see the cross frame construction 

The dominant style of cross frame (or bent in North America) features a bridging beam (the large beam that spans the first floor and carries the common joists), a tie beam which spans the top plates. This beam stops the wall frames from spreading under the load from the roof. And then a simple truss design consisting of two vertical studs and an upper collar with short stub ties jointed horizontally between the principle rafters and studs. The purlins (the members that run the length of the roof) are ‘clasped’ between the stub tie and the principal rafter.  

The main roof frame is comprised of bridled common rafter pairs, a pre-Georgian (prior to 1714) hip gable at one end and a standard gable at the other. A pre-Georgian hip is the English name for a hip rafter that is usually square in section and canted so that one edge is in the plane of either the gable or the main roof. Hip roofs were historically framed in this way until carpentry methods changed and more of a ‘hip board’ set plumb with jacks pitched onto the sides became the preferred method of hip framing. 

The pre-Georgian hip gable. The effect of canting the hip rafter in this way means that the jack rafters pitched from the two walls have a side cut angle on top and a square cut on the edge, but the jack rafters on the gables have a more complex (and comically named!) ‘nuns crutch’ or ‘lip cut’.

The adjoining wing has a wider span and a higher apex to the main building, and the roof meets the main roof in a valley. These valleys are similarly canted into the plane of the roof like the hips. In the same way the hips produce two different jack rafter cuts so does the valley. You’ll notice that on the main building there is a square jack cut and on the linking building the jacks have a compound cut onto the valley rafter. After running into the valleys, two small hip rafters pick up the opposite slope of the main roof. All of these angles were found using the framing square. 

The junction between the wing and the main building looking back down the east wall.

The main building is split into two clear halves; one which is vaulted floor to ceiling, and the other which has two floors of joists.      

Green Oak Timbers

What sets this type of carpentry apart from other woodworking is its use of timbers that are rough-sawn and often of irregular dimensions, requiring an understanding of how to deal with imperfections. For example, timbers are often significantly out of square, and dimensions only approximate: according to the European standard allowable tolerances are +9mm ~ -3mm in section. 5mm of deflection per metre is also allowed. These significant irregularities complicate the carpenter’s job.

Moisture contents can be in excess of 60% in fresher felled stock and during the summer months the warmer weather can cause problems with drying and shrinkage – we often keep our stacks covered with hessian cloth in an effort to shade the timbers and reduce warpage and cracking. 

Timber framing using this challenging material teaches a carpenter much about the nature of wood as a living thing, the characteristics of the timbers, how they are likely to behave and what can be asked of them.  

The Layout Process

Carpenters have developed various layout systems over many centuries to overcome the difficulties of working with irregular timbers. In my company we use lofting, for example. According to this method, we draw out entire layups on the floor and align the outside edges of each member to these lines with any sectional irregularities placed on the inside of the building, whilst any crowning (deflection or bowing) is oriented upwards and outwards.

Once the layout is drawn full-scale on the lofting floor, the timbers are placed on blocks in alignment with their corresponding grid lines on the floor. The various members are then laid one over the other so carpenters can accurately mark out lengths and scribe the shoulders of the joints using levels and plumb bobs. 

The plumb bob is an ancient tool that you are doubtless familiar with – its importance in carpentry cannot be overstated. Plumb bob scribing, or ‘scribe rule’ layout, is a difficult thing to describe without actually seeing it done. What it boils down to is using a perfect reference in a less than perfect situation. By sighting down the plane of the timber by eye and comparing it at the same time with a plumb line a carpenter can gauge to what degree that face is out of plumb and then accurately replicate that plane on the shoulder of the intersecting timber. The shoulders of tenons and widths of mortices are carefully marked using this technique.

Once a frame is cut and fitted back together, a plumb bob can be used to accurately transfer and mark additional details up from the floor. In the case of this watermill I used a plumb bob to mark the theoretical positions of the purlin housings on each truss. The result was that, regardless of the amount of deflection or variation in thickness of each principal rafter, the purlin housing was maintained in a consistent level position down the entire length of the building.

A plumb bob being used to mark the principal rafter to tie beam joint. On the floor you can see the layout lines. We mark out each individual frame in a building full size on the floor with chalk lines, and then set up the timbers to those lines. This plumb bob belonged to my great grandfather who was also a carpenter. 

Frames are generally made up of wall frames (running the length of a building) and cross frames (spanning a building). Many of the timbers are therefore used repeatedly in multiple layups. In the case of main posts they are first framed into the exterior walls of a building. Once the walls are completed they are framed in the ‘cross frame layups’. To ensure that the posts return to the right height and rotation that they were in during their previous layups, we use scratched datums (often a set distance from the top of top plate) and rotation marks that allow us to wedge the timber to return it to plumb or level. 

Ensuring that a designated point on each timber is plumb and level is essential, particularly for those in multiple layups. It guarantees that a timber has been returned to the correct plane each time it is fitted up so that when it is stood up and connected to multiple beams, the rotation of each individual shoulder scribe is correct.   

Once the bulk of the main structure is framed we laid-up floor joist and ‘cogged’ the tie beams into the top plates. Traditionally tie beams were dovetailed into the plates but because the shrinkage of dovetails (green oak, remember) tends to cause the building to spread, it is now more common to see a simple cog used. A cog also slightly outperforms a dovetail in green timber when under tension.

Assembly and Erection at the Job Site

The building enclosed two concrete pads differing in elevation by about 200mm (8”). The base of each post therefore was designed to accommodate this change maintaining the design elevation of the building. This and other variables made laying-out of the building one of the most challenging I have ever been involved in.


This photo shows the “jowl posts” and the boom of the spider crane as assembly work is underway.

The only access to the site was a track through a field at the rear of the building and a small trackway and bridge over the river too narrow for a mobile crane to cross. The solution we employed was to bring in and set up a small spider crane in various places inside the building. The limitations of this machine required us to be very methodical about the assembly sequencing to ensure we didn’t obstruct subsequent lifts. 

Space was at a premium. Without a large area to unload the piles of timber, we had to unload the timbers and other materials in the field behind the property and then use a remote-controlled tracked carrier to ferry timber across the narrow access bridge.  

And, to throw one more element into the mix, the river is an extremely well-protected ‘Site of Specific Scientific Interest’, meaning we had to be very careful when cutting roof members to prevent sawdust from drifting into the water course. The scaffolders installed netting around the entire perimeter to prevent anything from falling from the scaffold. We also did the majority of our cutting away from the edge of the scaffold on a plywood deck we laid on the joists.

As the building began to take shape its scale became more apparent. At nearly nine meters high, it’s an impressive structure. 

Lessons Learned

I took away a lot of lessons from this building project such as managing levels on-site, and the importance of every trade singing off the same song sheet, as it were.

We had issues with the initial layout of the structure as it became clear that the structural steel that effectively served as the starting point for everything above it had not been installed at the correct elevation. The work was delayed while we sorted out this problem.

I also learned valuable lessons about effective joint placement. Because of the space constraints mentioned above, we were forced to erect the structure by lifting and placing each cross frame and then linking it back with its purlins. However, because the scarf joint landed on the wrong side of each truss, every time we craned a purlin into position, it was left temporarily unsupported at one end. These decisions were admittedly made early on before any proper site visits were made. A proper method statement was in place, of course, but the experience taught me that starting with the end in mind is important when planning.

I hope that you, Gentle Reader, gained some insight into the work that I am involved with and found it an interesting read. If you would like further information about historic timber framing in the UK, I recommend a small book titled Discovering Timber Framed Buildings by Richard Harris. 

– Gavin Sollars

Gavin Sollars hard at work

Thank you for your reading this article. It is rare to find a craftsman like Gavin with the skills and inclination to write about his work and a willingness to share it freely with others. Gavin did not write this to promote his company, but if you like this sort of thing as much as we do, please visit his company’s website and sign up for their newsletter.

In the next post in this series Gavin will outline the construction of the roof frame. Please stay tuned.

YMHOS

If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google or incompetent facebook and so won’t sell, share, or profitably “misplace” your information. Stick a needle in my eye.

https://covingtonandsons.com/2020/09/12/timber-frame-water-mill-reconstruction-project-uk-part-2-framing-the-roof/

Other Posts in Timber Frame Water Mill Reconstruction Project Series

Part 2 – Framing the Roof

The Sewari Kerf

If the only tool you have is a hammer, it’s hard to eat spaghetti

The David Allen

You may have noticed sawkerfs cut into the sides of the peeled cedar logs in the pictures in my earlier post about Sotomaru nomi and wondered “what the heck!?” I know that was my reaction the first time I saw similar slits many years ago.

“Sewari Kerf” sounds a bit like the phrase for hello in Thai, but trust me, I know the difference. “Sewari” 背割りtranslates from Japanese to “back split.” Nothing to do with drafty pants.

Notice the sewari kerf in the upper surfaces of the peeled cedar beams in this picture. This will be oriented upwards in the structural frame.
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Three peeled cedar logs joined for a structural frame. Oriented upside down in this photo. Notice the sewari kerfs and the minor cracking. This degree of cracking is acceptable.

You will also notice some narrow cracks in the peeled logs in the photo above. These are natural shrinkage cracks that always occur in timbers that contain the tree’s heart. If sewari had not been cut into the peeled cedar logs in the photos above, the cracking would not be hairline, but would be as wide and wandering and ugly as a politician’s morals.

A sewari kerf provides a predetermined location for shrinkage stresses to collect resulting in a more attractive and structurally sound post or beam. It also makes it possible to use these shiny peeled cedar posts and beams with less waste while achieving a more refined, orderly, reliable appearance.

Allow me to digress a bit while the ink dries.

The Japanese consumer places high value on uniformity of appearance even in natural materials. This is also why Japanese ladies will pay $120 for a perfect musk melon as a gift for someone knowing it won’t taste any better than a less-beautiful but still expensive $20 melon. Both giver and receiver understand and appreciate the sentiment inherent in such a gift beyond the melon’s taste.

A Father’s Day gift in a wooden box
 (102058)
Peeled cedar logs awaiting purchase at the Meibokuya warehouse. These exposed structural timbers are expensive and will be wrapped in foam and cardboard and plastic before shipping.

The point: A natural product is made to look more uniformly natural by eliminating all natural defects. Makes perfect sense, right? Welcome to Japan.

Another aspect of this cultural peculiarity can be seen most in the traditional Japanese garden, if you have eyes to see it. Tremendous time and effort and money is spent constructing and maintaining a miniature representation of the natural universe in a small space. In this case, not uniformity but exaggerated naturalness is the goal. While the ostensible goal is the appearance of natural growth and random placement of features, there is not a single natural or random thing to be found in a Japanese garden, except perhaps the water in the pond. A beautiful art form to be sure. A triumph of design and patience. But about as natural as most movie actresses nowadays.

Kutsura Rikyu Garden

The sewari kerf too is not natural, but it helps nature appear both more natural and more uniform. It is also better for the environment. Did someone just say “Poppycock?” Ah. In that case, let us reason together, Gentle Reader.

The sewari makes it possible to cut square posts and beams from smaller diameter trees at less cost and with less waste. Indeed, without the sewari, many millions of small trees that would otherwise be clear-cut to make room for roads, infrastructure, and development, then chipped and tossed aside on the forest floors of Japan to return to the soil and atmosphere, can instead be used for construction lumber.

This wasteful activity is common throughout the entire world and has a tremendously harmful impact on the atmosphere, soil erosion, and water quality. Sewari is an environmentally-friendly way to make more efficient use of the world’s most environmentally-friendly building material.

Please encourage wood producers and governments in your area to develop and employ better ways to use and maintain forests, because neither thoughtless harvesting focused solely on profits, nor abandoning forests to burn and rot and release particulate and chemical contaminants into the atmosphere and destroy animals and their habitats in the process, is responsible stewardship. We need the building materials, oxygen, and carbon dioxide entrapment capabilities of forests now more than ever. Bambi needs a home and dinner too.

I see the ink has dried so I will step down from my soapbox now (Oops, I almost tripped and broke my fool neck!).

Back to the subject of this post, please take a gander at the photos below of two square construction-grade Akita Cedar posts, both with hearts in their centers. The one on the left does not have a sewari cut, but it does have a nasty collection of shrinkage cracks. Ugly, oh sooo ugly. The one on the far right has a sewari cut, but only a couple of tiny shrinkage cracks. If you had a choice, which one would you buy? Which one do you think is more dimensionally stable? Which one is a more efficient use of natural resources?

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A beautiful Hinoki post with sewari cut and dimensioned to final width and thicknes and marked with location and orientation in the building. Notice how the kerf is narrow near the center and widens towards the timber’s exterior. This began as a simple sawcut with parallel sides, but as the wood dried and shrunk, the kerf spread into a V shape. The timber’s dimensions distorted accordingly, so this timber was trued to this final dimension after the shrinkage and internal stresses calmed down.

Japanese building codes, especially those governing wooden construction, have changed a great deal since the Great Hanshin Earthquake of 1995 mandating metal connectors in tension loading, and metal plates spanning many connections in wooden structural frames. To accomodate the sewari kerf, manufacturers of these structural connectors have developed extra-wide plates that span the kerf, with screws and nail holes offset from the centerline. The point I am trying to make in my meandering way is that sewari is now an integral but hidden part of public and private life in Japan.

Perhaps the sewari kerf looks unsightly. In the case of posts, the carpenter will orient it away from view as far as is reasonable. In the case of beams, he will orient the kerf upwards out of sight.

Sometimes, after the wood has reached equilibrium moisture content and internal shrinkage stresses have calmed down, the carpenter will glue a strip of wood into the sewari kerf to fill it. Sometimes this strip makes the member look better, sometimes it makes it look worse, especially when it pops out and flops around. What do you think?

In the top left-hand image a sawkerf is made in wet wood. As the wood shrinks, the kerf opens and the walls inside the kerf often develop a curvature. These surfaces must be trimmed straight after the wood reaches equilibrium moisture content. A wedge-shape strip of wood is then glued into the kerf. All four sides of the wood are then trued. This is an extreme example.
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リフォーム改修用語 背割り埋め

In conclusion, I would like to add a few points of clarification and a real-world example from my long-list of screw-ups.

Sewari doesn’t usually add strength, but it makes it possible to use less than ideal timbers, and to process those timbers, including reducing the moisture content to allowable limits, in a shorter period of time and with much less waste than would otherwise be possible. This is a big deal if you care about conservation of natural resources. It’s also a big deal if you are concerned about the cost of materials.

So long as the kerf can be oriented away from view, the appearance of timbers with sewari is a heck of a lot better than those without. And have you ever noticed how customers will look aghast at wandering, gradually widening shrinkage cracks in a large timber post or beam imagining that it will cause the member to eventually fail? Of course you have.

I did one job in Nevada, the driest State in the USA, using many large and long square timber posts that developed shrinkage cracks after they were installed. The cracks alarmed the Client so badly they insisted they would not occupy the building unless we installed metal straps at three heights around the posts to ensure they wouldn’t explode, when in truth the timbers were not expanding but rather shrinking, and the cracks did not impact the posts’s strength or resistance to buckling to any significant degree.

But if we had cut a sewari kerf into those posts immediately when they arrived at the hot dry desert jobsite, the amount of shrinkage would not have been less, but that shrinkage would have concentrated at the kerf and not caused the Client to make illogical, pointless and expensive demands. On the other hand, if cosmetics had been a priority, the Client would have been right to object to those ugly cracks, not that straps would have made any difference.

So I put it to you, Gentle Reader, did we save money on that job by not taking the time to cut sewari kerfs and consequently being forced to spend money and time fabricating and installing silly metal straps to resolve the Client’s complaints later, invalid though they may have been? I think not.

Go forth and do better, my son!

YMHOS

kitayamasugi-5
A well-managed cedar forest in Japan

If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google or incompetent facebook and so won’t sell, share, or conveniently and profitably “misplace” your information.

A Cool Coat

A few weeks ago I posted an article about seismic dampers used on a high-rise building currently under construction near my office in Marunouchi Tokyo (a 3 minute walk from Tokyo Station). I pass this same jobsite on foot several times a week and take the occasional snapshot. I have other construction sites ongoing, but no high-rise buildings right now, and none that non-disclosure agreements will allow me to share with you. So this is a good opportunity to introduce you to some lesser-known details about major construction work in Tokyo as seen from the sidewalk without risk of offending any clients.

Please notice the gentleman in the orange uniform and big boots in the picture above. I have never met him before, but judging by the color of his uniform, he’s an employee with Obayashi Corporation, one of Japan’s largest and arguably most competent general contractors. I have done a lot of work with this company and respect it a great deal.

Sir Norman Foster, a famous British architect and the designer of Apple’s Campus 2 in Cupertino, California once said that Obayashi Corp is the world’s best general contractor. I tend to agree. And I say this as someone that used to work for two of Obayashi’s competitors in Japan, and who has also worked with many other contractors around the world. If you have visited the Boulder Dam near Las Vegas, Nevada recently, you probably drove over Obayashi’s bridge spanning the gorge.

Anyway, please notice that this erstwhile young man is he wearing what looks like a thick coat all puffed up like a marshmallow on a sunny day in mid-August in 37℃ (98°F) temperatures in the shade and 76% relative humidity? Is he loco, Cisco?

Setting aside the somewhat inelegant safety boots and rolled trouser cuffs which do not help the fashion statement his ensemble is making, you will notice a round white grill on his coat near his elbow. There is an identical grill on the opposite side of the coat you can’t see.

If you haven’t already guessed, the two round grills are actually battery-powered fans pulling outside air into the coat and pushing it out at his collar and wrists cooling our young contractor as he labors diligently in the heat.

Makita tool cordless fan jacket
A two-fan cool coat

These “fan coats” are very popular in Japan. They can make a big difference so long as one can perspire adequately and are credited with saving many construction workers from heat stroke and even death in hot months.

They also come in kiddie sizes and many colors.

Makita makes a “Cordless Fan Jacket” that is sold on Amazon overseas for a lot more than it costs in Japan. Instead of two fans, it has a single fan at the back. I have not used the Makita product and can’t endorse it.

The Makita Cordless Fan Jacket. Notice the high, stiff collar directing moving air over his neck for better cooling, and the fan unit at his back. The cord is leading to the angle grinder he is using, not to the fan.

I hope the weather in your neck of the woods is always balmy with cool breezes in summer so a coat like this is never useful. In the meantime, I’m just waiting for someone to develop steel-toed boots with cooling fans. (ツ)

YMHOS

If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” Your information will remain confidential (we’re not evil Google or incompetent facebook).

Seismic Damping

The more one gardens, the more one learns; And the more one learns, the more one realizes how little one knows.

Vita Sackville-West

This post is definitely different from my previous ones. It will deal with buildings and earthquakes. It has nothing to do with wood or woodworking tools.

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Toranomon Hills Business Tower (foreground)

In my day job I am an executive with a large international Real Estate and Construction Project Management company in Tokyo, Japan. Without going into details that might violate non-disclosure agreements, my job involves managing all aspects of real estate acquisition and leasing, as well as the design, procurement, and construction of commercial and industrial projects. Mostly for non-Japanese Clients.

The photo above is one project I am involved in on behalf of a Client.

Tokyo is an expensive place to set up operations, and the real estate and construction processes are especially confusing for foreign companies. Ergo, the need for me and my teams.

My educational background is structural engineering, focused on seismic-resistant design. All of my Clients are very concerned about the earthquakes Tokyo experiences almost daily. There will be several magnitude 4~5 quakes here each year. This tends to keep people focused.

My point is that earthquakes are a constant threat taken seriously and for good reason. Accordingly, to one degree or another as building codes and Owners require, all buildings incorporate aseismic design features.

I have worked on buildings with expensive full-blown base isolation using rubber bridge bearings and hydraulic dampers similar to giant automotive shock absorbers, and other systems designed to dissipate damaging earthquake forces, but what I would like to show you today is a “slip-joint brace damper” just installed at a building near my office located in Marunouchi near the Imperial Palace (not the building in the perspective rendering above).

Notice how deep the beams are, and how thick the steel is. Although they don’t show up well in the photo, the columns too are massive. Much heavier than is typical in Western structures. I love Japanese structural steel!

Slip-joint Brace Dampers (white), Tokyo

I am not involved in this high-rise building, but a contractor I have used in the past named Obayashi Corporation 大林組 is the General Contractor. I was able to snap this picture of Obayashi’s jobsite while walking to my office from the subway station last week during a rare moment when the front gate was open and nothing was in the way.

The white columns and diagonal braces are the key to this seismic damping system.

The white paint is a fire-resistant intumescent coating designed to protect the steel from heat during a fire. Structural steel is very weak when exposed to fire, much more so than wood or concrete, so this sort of protection, while expensive, is necessary. The rest of the structural steel will be sprayed with a thicker, less-expensive and less-durable fire-resistant coating of one variety or another.

The diagonal braces in the photo are basically two steel plates bolted together face to face. The bolt holes are slotted to allow the bolts and plates to slip past each other when subjected to a certain amount of force.

The plates and bolts are contained in a steel pipe filled with high-friction oil to prevent the brace from buckling, prevent corrosion, and ensure the coefficient of friction between the plates/bolts remains constant for many decades in the future.

As the ground moves during an earthquake, the building moves with it, and the structural steel sways. The rectangular opening framed by connected beams and columns changes shape, becoming longer or shorter in the diagonal directions. Braces resist this “racking” movement.

As the plates and bolts in these dampers slip past each other, a great deal of friction is created converting the earthquake’s energy to heat, slowing down the racking motion, and controlling the harmonic vibration of the entire building.

Image result for racking forces

Although fixed-length braces are common in lighter, shorter structural frames, they are not usually a good thing in large structural steel frames because they tend to behave erratically and fail suddenly. This can be inconvenient.

The steel frame must be made strong enough without fixed-length bracing to absorb these forces without failing anyway to make a reliable structure. But if the frequency of the building’s movement back and forth and side to side begins to match that of the ground, then something called “resonance” can occur potentially doubling the forces acting on the building, forces powerful enough to suddenly and violently bend, break and topple the building. This can be inconvenient.

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The center image employs the damping braces as in the 2nd photo above.

An alternative to this sort of damping brace system is the more expensive “Base Isolation System”

So why would anyone use an expensive system like base isolation?

Base isolation allows the entire building, from its base up, to move opposite the ground motion in the horizontal direction, reducing the induced sway, racking, and damage to its interior and systems and equipment stored inside. This level of protection is necessary if the building must continue to function uninterrupted immediately after a large earthquake. Hospitals, R&D centers, Data Centers and other sensitive buildings with lots of expensive equipment that must be kept running no matter what are often worth the cost of base isolation systems.

But in the case of an office building like the one in the photo, the owner decided some interior damage, and the business interruption repairs would entail after the earthquake, would be acceptable.

The photos below show two components of the typical base-isolation system.

Base isolation bearings being tested. Very stiff in the vertical direction, but flexible in the horizontal direction. Two bearings are being tested in this photo, but they are never stacked like this in actual installations
Related image
Hydraulic damper combined with base isolation bearings in the background to form a base-isolation system. The building’s entire weight is supported on these bearings. The dampers keep the building’s movement from getting out-of-hand, like shock absorbers on an automobile.

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YMHOS

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