1

Timber, framing andconstructions

References: “Building with Wood” by Rempel“Building the Timber Frame House” by Benson

Heavy TimberWhen exposed to fire, heavy timberdevelops a charcoal layer at the rate ofabout 1/40th of an inch deep per minute offire exposure.The charcoal is actually burnt wood, actingas an insulator to slow down theconsumption of the wood inside.If the wood continued to burn at 1/40 thinch per minute, 1 inch burns in 40minutes, 2 inches in 80, and 3 inches - theburn-through point would take up to 120minutes - TWO HOURS

2

It’s considered Heavy Timberwhen...

• The minimum sizes are…– 6” x 8” for a column supporting a roof– 8” x 8” for a column supporting a floor– 6” x 10” for a floor beam– 8”x 8” for a roof truss member– 4” x 6” for a roof beam– 3” + 1” finish floor decking– 2” roof decking– ...According to the building code

Some additional fine print fromcodes...

• No concealed spaces between ceiling and floor, inwalls, and roof.

• Stair treads & risers, made of 2” thick material• Finished floors stop 1/2” short of exterior walls.• Wood bearing partitions cannot support more than 2

floors and roof• Built up members must be min. 3” thick

3

Code BonusArea per floor

Height

(feet)

Type I

A

Type I

B

Type II

A

Type II

B

Type III

A

Type III

B

Type IV

HT

Type V

A

Type V

B

Group H / A UL 160’ 65’ 55’ 65’ 55’ 65’ 50’ 40’

A1 H UL 5 3 2 3 2 3 2 1

A UL UL 15,500 8,500 14,000 8,500 15,000 11,500 5,500

B H UL 11 5 4 5 4 5 3 2

A UL UL 37,500 23,000 28,500 19,000 36,000 18,000 9,000

•Even as combustible construction, the code recognizes that timber framedbuildings afford occupants more time to evacuate, and firefighters more timeto fight a fire because the massive members don’t burn through and failnearly as quickly as lightwood (stud) framing members.

•Due to this slow burn through (a function of the charring acting asinsulation) the code gives type IV HT construction an additional two floors inheight and twice the square footage than a type V-a construction type for a‘B’ occupancy.

CombustibleNon-combustible

Where did Timberconstruction start?

• No one knows.• Pit dwellings from 1000 AD have

been found on the North Americanand European continents.

• Pit dwellings were simpleconstructions where the earth wasshaped for protection, fallen trees orlimbs lashed together to formoverhead structure, and skins orvegetation placed over the structureto form shelter.

4

Crucks• Cruck frames were a

development of the pithouse. Curved tree trunkswere leaned together andlashed / joined to wall androof framing to provide somelateral bracing.

• The shortage of curved treesbegan to limit the spread ofthese frames acrossEngland and Europe.

Next… braces

• The braced frame wasthe next development oftimber following thecruck. It used curvedbraces between theverticals and horizontalsto provide lateralresistance.

Cruckremnants

5

A heavy skeleton

Ground Sill

Corner Principals

Summer Beam

PostsStuds

Braces

BeamsJoists

PrincipalRafters

KingPost

6x6 StudsA common form of timber framing for smaller structures was to framethe walls with vertical members, raise it, then add roof members toenclose. An early form of the stud wall…only much heavier.

6

Studs at this close spacing have an impressive structural capacity,which goes mostly unutilized.

Bents

• This braced frame is thebasis for mosttraditional timber framesconstructed today.

• The frames are notassembled in place onepiece at a time, butrather as siteprefabricated structuralassemblies calledbents.

7

• The bent is pre-assembled on theground, and with somepeople lifting, somepushing pike poles, andsome pulling with ropes,the bent would be raisedinto place.

Tools for Timber Framing

8

Jobs for all ages

Due to the weight of the members and the special skills neededto layout and cut the joints, frame raisings could employ theolder, more skilled, as well as the younger, stronger laborer.

Minimal refinement

These floor framing members have only been hewed flat on thetop side, the bottom still has bark on it. It took so much labor torefine the logs into building components that only those surfacesabsolutely needing refinement were worked.

9

Raising itself was done with a crew on one side of thebent pulling, and a crew on the other side pushingwith pike poles. The foreman had to keep balancebetween the crews to prevent the bent from beingpulled or pushed over onto the opposing crew.

Spaces between the bents are filled in with joists andpurlins to carry the roof shingles

10

The completed frame is a community achievement

11

Timber framing in our time

Power tools have made thecomponents for the timberframe easier to refine, but themethods for joining andprinciples of construction arelargely unchanged from the1600’s

Here a timberwright lays out aknee brace to be inserted in amortise cut in the column andbeam to give the bentadditional lateral stability.

When complete, the frame is laterally braced in both axisby the knee braces between the columns and beams.

12

Setting the frame requiresSaws for cutting off thetrunnelsRubber bandsSledge hammers &The ‘beetle’ or ‘commander’ (the really big hammer)

These housed dovetail purlins are stockpiled on the site. They will be thepieces that drop into the bents to tie the frame together at the roof.

13

This mortise and tenon connectionis the most common type of jointfound in the timber frame.

Instead of nails or screws, trunnels(tree nails) are used to join the twomembers together.

Usually the tenon is cut shorterthan the mortise (to allow for crossgrain shrinkage)

And the holes for the mortise andtenon don’t line up! (a drawbore)They are offset by an 1/8 inch orso to pull the tenon tight into themorise (the drawbore draws themortise into the tenon)…and keepit there.

This mortise and tenon joint is shouldered (the vertical isnotched to receive the horizontal)

Shouldering puts the full depth of the beam to workresisting shear…so the tenon doesn’t have to do all thework

14

• This mortise and tenonis located at the pointwhere the roof membermeets the column. Thesingle trunnel is allthat’s needed as mostof the load puts the roofmember intocompression.

Today, it’s common tofind the frame beingraised by a craneinstead of pike polesand ropes…(fewersquished timberframers!)

The last bent is beingpositioned, it will be heldin place by the secondfloor beam at a mortise& tenon joint, and by theroof purlins, which use adropped in type ofhoused dovetail.

15

• Even with the crane, the crew struggles to fit the tenoned end ofthe beam into the mortise cut in the bent. That last bent istemporarily tilted away from the frame and braced until all themortise and tenon joints are partially fitted. Then the last bent willbe pulled into vertical with portable winches called “come alongs”

The “come alongs” areconnected to the frame withstrips of inner tube to keep thesteel cable from scarring thetimbers.

Trunnels are partially fitted, andwill be “driven home” with amallet.

16

Driving trunnelshome with amallet

“come along”

Once the trunnels are ‘driven home’ any projectingtrunnel is cut off with a handsaw.

17

The completed frame is sanded and oiled like a finepiece of millwork.

This is the traditional time for the post-frame raisingfeast… I think here it was KFC.

To preserve the presence of the frame on the interior panels that areboth sheathing, insulation and drywall are spiked to the frame.