Lecture 4 пдк

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04
LECTURE 4
Connections of Timber
Structure Elements
How to join boards, beams, and logs securely and reliably with glue, bearing
joints, dowels, and steel ties
COURSE
DESIGN OF TIMBER
STRUCTURES
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AGENDA
Lecture Outline
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General Provisions
Glued Connections
Bearing and Notch Connections
Classification of connection types and their flexibility
(deformation)
Joining boards and plywood with glue, finger-joint
connections
Joining compressed logs and beams at nodes, design
conditions
04
05
Dowel Connections
Tension Ties
Joining with steel, wood, and plastic dowels, placement
rules
A brief overview of steel ties and glued-in rods
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SECTION I
General Provisions
and Classification
Understanding how and why we join timber structure elements
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WHY DO WE NEED CONNECTIONS?
Reasons for Joining Elements
To increase length — we join short logs together end to end to form a long beam.
To increase cross-section size — we combine several boards to get a section with enough thickness.
To form a node — where elements intersect at one point, transferring force from one part to another.
Connection Type
How It Works
Glued
Designed for shear
Frontal bearing / notch
Transfers compressive force
Dowel
Works in bending
Glued-in steel rod
Works in withdrawal, push-through, and shear
Metal elements
Works in tension
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FLEXIBILITY
Why is some “give” in a connection useful?
Every connection type except glued ones develops some elastic and plastic
deformation during use.
What does “ductile” behavior mean?
This is mostly beneficial: the connection works “ductilely” — it doesn't snap
suddenly, but yields gradually.
A material doesn't break instantly under load — it first
Extra gaps and looseness from imprecise assembly should be minimized.
warning sign in the tool or structure.
A connection should be tight yet flexible — this makes it reliable and longlasting.
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bends slowly, and only then fails. This gives an early
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NUMBERS
Maximum Allowable Deformation
At full load, in millimeters
Connection Type
mm
Solid-section wood, perpendicular-to-grain bearing
3
Glued (laminated) section, bearing
2
Via dowels (except glued-in)
2
Frontal notches and bearings (end-to-end)
1.5
All types of metal plates
1
Glued-in rods, tension perpendicular to grain
0.5
Glued-in rods, tension along the grain
0.25
Glued connections have no deformation at all. At partial load, values decrease proportionally to the force.
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DESIGN PRINCIPLE
More, “weaker” dowels are more reliable
Using few but strong dowels is risky: if one hits a wood defect (a knot), the
remaining dowels must carry the extra load.
So it's better to use more dowels, each with a smaller design capacity.
This spreads the load evenly across several points, so the failure of one dowel
doesn't bring down the whole connection.
1 defect
if lost — neighboring dowels don't lose
capacity
N pieces
each with low capacity, many dowels
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SECTION II
Glued
Connections
Turning boards, beams, and plywood into a single section that resists tension and
compression
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WHY DO WE GLUE?
What is glue used for?
Used to glue wood, plywood, and fiberglass material along its length, width,
and thickness.
Steel or fiberglass rods, washers, and plates can also be glued into wood.
Glued surfaces must always be planed smooth.
≤33 mm
Maximum board thickness after planing in straight
elements
≤42 mm
Allowable thickness for boards with longitudinal kerf
cuts
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FIGURE 1
Types of Glued Connections
a — gluing boards face-to-face and edge-to-edge; b, d — details of the finger-joint connection; c — scarf-jointing plywood with a long tapered splice; e — fingerjoint connection of structural elements.
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RULES
Requirements for Width and Angle
When gluing boards to each other or to plywood at a 90° angle, the width should not
exceed 100 mm.
At a 30–45° angle — a width of up to 150 mm is allowed.
≤100 mm
If two or more boards are needed across the width, the seams between rows must
overlap (be offset).
Width, when glued at 90°
When making a finger-joint connection, the angle between the axes of the joined
elements must be at least 104°, and the “teeth” should only emerge on the edges of
the glued package.
≥104°
Smallest angle in a finger-joint connection
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MATERIALS
Types of Glue
Glue Group
Main Property
Application Area
Resorcinol-based, phenol-resorcinol
Resistant to moisture and weathering
Outdoor structures exposed to the elements
Carbamide-melamine
Moderate moisture resistance
Enclosed, moderately humid rooms
Carbamide (urea)
For dry conditions, cost-effective
Indoor, dry-room structures
Epoxy
Bonds wood to metal or fiberglass
Mixed-material connections (wood-metal)
The specific brand is chosen based on the structure's service conditions, strength requirements, and economic efficiency.
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SECTION III
Bearing and Notch
Connections
Ways to join compressed elements made from logs and beams at a node
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FIGURE 2
Schemes of Bearing and Notch Connections
• a, b — frontal bearing with a steel plate and a steel dowel
• c, d — single-tooth notch (in beams and logs)
• e — double-tooth notch
• f — bearing through a cushion pad
• g, h — one-sided and two-sided notch schemes
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DESIGN CONDITIONS
Checking for Crushing and Shear
The force transmitted through a bearing or notch is checked against two conditions: crushing at the contact plane, and shear of the wood.
For single-tooth notches and cushion-pad bearings, both of the following conditions must be satisfied:
Nc ≤ Fcr · Rcrα
Nc · cosα ≤ Fsh · Rsh,avg
Crushing condition
Shear condition
Fcr — design crushing area; Fsh — design shear area; Rcrα — design crushing resistance at an angle; Rsh,avg — average design shear resistance.
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A MORE COMPLEX CASE
Double-Tooth Notch
In a double-tooth notch the force is transmitted at two levels, so the notch is checked for overall crushing and for shear at each tooth
separately.
Nc ≤ (Fcr + F'cr) · Rcrα
Overall crushing condition
≥1.5h
Smallest length of the shear plane (lsh)
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≤10hn
Largest length of the shear plane
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MINIMUM VALUES
Minimum Notch Depth (hn)
≥2 cm
≥3 cm
≤h/4
≤h/3
In beams
In logs
At truss and bracing-system nodes
In all other cases
h — the height of the element in which the notch is cut.
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SECTION IV
Dowel
Connections
Joining timber elements with steel, aluminum, fiberglass, and oak dowels
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MATERIALS
What is a dowel made of?
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02
03
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Steel
Aluminum, fiberglass
Oak, birch
Nails, screws
As a rod, bolt, nail, or screw — for larger
elements (beams, logs)
Used in chemically aggressive
environments or where dielectric
properties are needed
As a flat (plate) dowel — for assembling
laminated beams
In boards and beams; screws are used to
fasten thin metal and plastic sheets
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FIGURE 3
Dowel Connection Schemes
• a, b — symmetric, double- and multi-shear
• c, d, e — asymmetric, single-shear
• f, g — asymmetric, double-shear
• h, i, j — symmetric connections with thin metal or plastic plates
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RULES
Tie Bolts and Hole Precision
To prevent elements from spreading apart, at least 25% of the dowels at a joint must
be tie bolts.
Holes must match the dowel diameter exactly — for a tight fit.
For screws, the hole should be 2–4 mm smaller than the screw's smooth shank.
Blind bolts screw into the wood to a depth of at least 5 times their diameter.
≥25%
Minimum share of tie bolts (at least 3 pieces)
≥5d
Blind bolt penetration depth
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FIGURE 4
Placement of Dowels and Nails
• a — rods in two rows
• b — staggered in round elements
• c — rods and nails in four rows
• d — nails in diagonal rows
• e, f — nails in a staggered pattern
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SPACING
Minimum Spacing Between Dowels
d — dowel diameter; s1 — along the grain, s2 — across the grain, s3 — from the edge to the axis
Material
s1
s2
s3
Steel rod, bolt, screw
7d
3.5d
3d
Nails
15d
4d
4d
Aluminum, fiberglass rod
6d
3.5d
3d
Oak rod
5d
3d
2.5d
Values apply where the element thickness exceeds 10 times the dowel diameter; for thinner elements the spacing is slightly reduced.
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FORMULA
How Do We Calculate the Number of Dowels?
nd = N / (Tmin × ns)
• N — design force
• ns — number of design shear planes per dowel
• Tmin — the smallest design bearing capacity of a dowel in one shear plane
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FORMULA
Shear Check for a Plastic Dowel
Tsh = (π · d² / 4) × Rsh
• d — dowel diameter
• Rsh — design shear resistance of the dowel material
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CONCEPT
What Does a Dowel's Bearing Capacity Depend On?
A dowel's bearing capacity in one shear plane is determined by whichever happens first: crushing of the wood under the dowel, or bending
of the dowel itself.
Calculations are done separately for symmetric and asymmetric connections, because when the middle and outer elements differ in
thickness, the force distribution changes too.
A steel dowel's capacity is calculated with a different formula than a fiberglass dowel's — the materials have different stiffness.
When the force hits the dowel axis at an angle, the bearing capacity is reduced by a special correction coefficient.
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FIGURE 5
Design Charts
• a — chart of the kn correction coefficient for asymmetric connections
• b — chart of the kα coefficient based on the angle between the force and the
dowel axis (for steel, aluminum, fiberglass, and oak dowels)
These charts simplify the calculation — the needed coefficient can be read directly
from the chart instead of computed from a formula.
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SPECIAL TYPE
Flat (Plate) Dowels
Straight-grain flat dowels made of oak or birch are used to join several logs or beams into one bending element (at most 3 pieces).
In elements up to 150 mm wide the slot is cut all the way through; wider elements get a half (blind) slot.
The slots alternate on the two faces of the element.
T = 0.75 × bpl
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Bearing capacity of one plate dowel in pine and spruce
elements, kN
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SECTION V
Tension Ties and
Glued-In Rods
Covered in more detail in the next lecture
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BRIEF OVERVIEW
What Are Steel Tension Elements For?
Ties, rods, and clamps made of round steel are used in the tension rings of arches and frames, in the tensioned elements of trusses, and
in anchors.
Glued-in steel rods are widely used — they work in tension, push-through, and bending.
We'll look at this connection type in more detail, with calculation formulas, in the next lecture.
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SUMMARY
What Should You Remember?
• Five main types: glued, bearing/notch, dowel, glued-in rod, and metal-element connections.
• Flexibility is useful: every connection except glued ones bends slightly, which makes them ductile and reliable.
• More, weaker is better: so that one defective dowel failing doesn't bring down the whole connection.
• Two checks — crushing and shear: bearing and notch connections are checked against exactly these two conditions.
• Spacing and depth rules: the placement of dowels and notches must not fall below the minimum values in the table.
Next lecture: full calculation methods for steel ties and glued-in rods
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