Basic Timber Engineering Emphasizing GLULAM

… why TIMBER (or Glulam)?

As a building material -

– WOOD - natural, warm, human

– CONCRETE – hard, cold, bulky

– STEEL – strong, but skeleton-ish

Glulam Roof Framing

why? (WOOD) …

As a RESOURCE –

– low embodied energy

– available (all around us)

– Net Carbon `NEGATIVE’ (use ties up global carbon)

Structurally!

– simple calculations (rectangular sections)

– members are relatively STABLE

– construction is simple`drop in’ or `prop in’ place

Simple Framing Members

• Load (and deflection) calculations are …

V = ω L/2 … or ω (L/2 – d)

M = ω L2 /8

Δ = 5 ω L4 / 384 EI

EASY!

Stress calculations …

horizontal shear … fv = (3V/2A)

extreme fiber in flexure… fb = M / S …

S = bh2/6

Deflection … single value E

… and I = bh3/12

… also easy!

Design checks

Horizontal shear against allowable (admissible) shear parallel to grain … … fv ≤ Fv’

Extreme fiber against allowable flexural tension or compression … fb ≤ Fb’

Deflection under applied load against allowable prescribed by Code (or Owner) … Δ ≤ 0.003 L

Allowable Stress Design (ASD)

• Current practice for wood design in USA (and Malaysia)

• Allowable stresses derived from `design values’ dictated by Code (Malaysian Standard) and adjusted for service conditions

• Design values already incorporate Factors of Safety (FOS)

current practice …. USA

Strength Design (SD) / Load and Resistance Factor Design (LRFD) … (Limit States) design values for wood are derived from the ASD design values …

Published in `Dual Format’ but one set of values.

… no actual advantage to using a limit states approach (for wood) except to better distinguish the loads.

Design Example – Using Malaysian Glulam

DESIGN EXAMPLE

• Members Span 24 ft (7315 mm), Spacing 4 ft (1219 mm)

• Occupancy Live load 40 psf (1.9 kPa), Dead load 10 psf (0.5 kPa).

• Use D35 Stress Class Malaysian Glulam: σ m, ll = 11 N/mm2, etc.

Design for Bending (Flexure)

• ω = area load x trib widthω = (1.9 kPa + 0.5 kPa) x 1.22 m = 2.8 kN/m … ωself weight = 0.2 kN/m (est.)ω TL = 3.0 kN/m

• M=ωL2/8=M = 3.0 kN/m (7.315 m)2/8 = 20.1 kN-m = 20.1 (10)3 N-m = 20.1 (10)6 N-mm

• Trying 100mm x 300mm

S = bh2/6 = (100)(300)2/6 = 1,500,000 mm3

Design for Bending (Flexure)

• Calculating applied stress

σ m, a, ll = M/S = 20.1(10)6 N-mm / 1.5(10)6 mm3 = 13.4 N/mm2

• Design Check: is σ m, a, ll ≤ σ m, adm, ll ?

• σ m, adm, ll ?

Allowable Bending Stress

• Issues– load duration– wet or dry service– temperature– load sharing? – horizontal or vertical glulam (`laminating effect’)– curvature (and/or taper)– depth (volume) … flexural tension– stability … flexural compression

Corresponding Adjustment Factors

– load duration (CD or K1)– wet or dry service (CM or K20)– Temperature (CT )– load sharing (Cr or K2)– horizontal or vert. glulam (Design Values or K11 or

K17)– curvature and/or taper (CC or K24)– depth (volume) … flexural tension (CV or K6)– Stability (CL or Rx or MY Rx)

`Normal’ Load Duration

• EXAMPLE … use Occupancy Live …

• Occupancy Live … USA … CD = 1.00

• MS 544 … K1 = depends

– medium term … 1.25 – short term … 1.5

• This example … use 1.25

Horiz. Glulam (Laminating Effect)

• Table 6, MS 544 …

… assume 9 lams @ 33 mm ea. …

• K11= … about 1.42

(Depends on lam size ↔ number of)

Depth (Volume) Effect

• K6 = 1.00 … (since h = 300 mm)

Note: … this is the so-called `standard depth’.

This depth should be one that appears commonly in framing and easily manufactured (and easily tested).

(Construction Industry ↔ Manufacturer)

(Research ↔ Code ↔ Design)

Beam Stability …

• Beam Depth/Breadth Ratio …

300/100 = 3 … deemed stable by MS Code

– So, … stability is not an issue! (CL = … = 1.00)

Sweet!

So, (back to the) Design Check

• is σ m, a, ll (= 13.4 N/mm2)≤ σ m, adm, ll ?

• σ m, adm, ll = σ m, ll K1 K20 K11 Ct K2 (K6 or CL)

• =11.0 N/mm2 (1.25)(1)(1.42)(1)(1)(1.0 or 1.0)=… 19.5 N/mm2

… is σ m, a, ll (= 13.4 N/mm2)≤ σ m, adm, ll (= 19.5 N/mm2) ?

YES! … GOOD! …

Unity Check

• Alternate way to express this design check:

Is the ratio of the applied to the admissible less than or equal to one (`unity’)? …

Is …

YES!

2m, a ,ll

2m,adm,ll

13.4 N/mm 0.69 1.00?19.5 N/mm

Check Deflection/Serviceability

• Obtain Deflection Limit (Allowable) …

• Calculate Deflection (Sag) under appropriate load case

• Check Calculated versus Allowable

Deflection, cont.

• Δ = 5wL4/384EI

– … Simple! … most of us know this equation already (from M.O.M.)

– E needs adjusted (per MS Code)

– Deal with shear deflection (or not)

– I = bh3/12 … (easy!)

eee

E = E adm, ll = E ll K20 K16 Ct K7

K20 = 1 ( … not wet)

K16 = 1.07 (Table 6 MS 544 … laminating effect)

Ct = 1 ( … not hot)

K7 = 1 ( … no load sharing)

E adm, ll = 10,000 N/mm2 (1.07) = 10,700 N/mm2

• Calculated deflection …

Δ = 5(2.3 N/mm)(7315 mm)4 / 384 (10,700 N/mm2)(225,000,000 mm4) … = 35.6 mm

• Allowable … 0.003 L = 0.003 (7315 mm) = 22 mm …

• Check: … is calculated = 36mm ≤ allowable = 22mm? … NO, NOT GOOD!

• Remedy: make deeper! (363mm …WORKS!)

Other Design Checks

• Horizontal Shear – Shear Parallel to Grain … generally doesn’t control on longer spans –does pass design checks in this example.

• Bearing (at Supports) – generally dealt with by calculating a required area for bearing and specifying min. bearing length at support.

• And …

Simple Anchorage …

• Beams typically anchored (held in place) at supports by simple through bolts!

– Resisting incidental horizontal forces or movements

– Resisting uplift forces

simple

Beams

• SHORT Spans – design for shear

• MEDIUM Spans – design for bending (flexure) and deflection

– Simple Beams … deflection first, then check flexure

– Continuous Beams … flexure, then check deflection

• LONG SPANNING BEAMS – design for deflection

What the designer must know …

SAWN TIMBER … what is AVAILABLE

GLULAM … what is makeable(manufacture-able)

• Glulam Structures (per se) …

– Glulam provides unique, noticeable structure … arched religious structures, public centers, gymnasiums, etc.

• Structures Highlighting Glulam

– Glulam used for necessary structure provides architectural – aesthetic appeal … big exposed beams … curved, straight, etc.

• Glulam Framing

– Glulam simply provides the necessary framing

– Exposed, or not

(From a Manufacturing Point of View)

• Custom …– arches, curved, tapered, curved and tapered, etc.

– unlimited shapes and sizes possible

– We (designers) will be directing the manufacturer what to make (species, layup, size, shape, curvature, appearance, etc.)

(from a manufacturing point of view)!

• Stock …

– Manufactured efficiently and economically in large volumes to suit framing needs of construction industry

– We (designers) will be specifying standard sizes and grades … beams that are `already made’ … (and stocked at distribution yards)

Stock Glulam

• Specify (select) from members that are the most economically available

– Species, strength, and stiffness

– Often regionally dependent

– Required appearance grade

Glued laminated timber beams (and columns) may be used simply as `drop in place’ (or `prop’ in place) framing members …

Designs are generally statically DETERMINATE.

Connections are simple: primarily by direct bearing for gravity loads; through bolts to resist anchorage and uplift conditions.

GLULAM!

• Smaller Sections (than sawn timbers) ... for the same loads and spans

• Larger sections available

• Longer lengths … definitely!

• Straight, curved, or cambered

how large?

Beam(s) shown …

• 14-3/8 in. wide (365 mm)

• 58.5 in. deep (1486 mm)

• 72 ft long (22 m)

• … and some have been manufactured to twice that depth and twice that length!

… 1.7 km of lam material

BEAMS! STRONG BUT SIMPLE

• DESIGN … is simple.

• CONSTRUCTION … is simple.( … though may require a crane!)

• LOOK … is simple … ... yet STRONG, STABLE.

CUSTOM … ARCHED, CURVED, TAPERED MEMBERS

• not quite as easy to design …

– non-linear flexure stresses due to curvature

– lamination pre-stress due to curvature

– shear and bending stress interaction where tapered

– But still in many cases statically determinate

• not quite as easy to manufacture

– curved jigs and/or presses– non-uniform lamination thickness for tapered

members

– will be reflected by higher unit costs

… BUT!

Still quite do-able!

Oh, WHAT ABOUT FIRE!

• Sawn and Glued Laminated Timbers perform well in FIRE situations

• Charring of outer surfaces shields inner wood from further burn

• Remaining section and wood strength typically accommodate necessary design loads during fire allowing evacuation

Fire Performance of Wood

Cold Wood

Heated Zone

Char Layer

Fire Performance of Wood

Zion Baptist Temple-Chicago

Zion Baptist Temple-Chicago

Fire Performance of Wood

• … for smaller members or where additional fire rating / protection is required, additional laminations may be manufactured into the beam (or column) section.

Fire-Rated Lay-up

302

A

Standard Layup

B

B

A

C C

B

A

A

B

302

One-hour Layup

GLULAM … for MALAYSIA!