Briefly describe the following adjustments To what design va

Briefly describe the following adjustments. To what design values do they apply? Give the NDS reference for numerical values of adjustment factors. a. b. c. d. Size factor Wet service factor Repetitive member factor Temperature factor

Solution

Q1. Below is a list of the significant adjustment factors that are applied to the reference design values. Not all of the adjustment factors are used on every type of design value. For example, the bending reference design value is adjusted for all the factors below except for the column stability factor (CP) and the buckling stiffness factor (CT). The following slides will explain and show you how to determine each adjustment.

•Repetitive Factor, Cr

  •Temperature (C t)

•Moisture Content, CM

•Size Factor, CF

                        Moisture Content Factors CM

Strength Property

fb

ft

fc

FC(P)

fv

E

Sawn Lumber, Visual or Machine Graded Wet conditions of use MC > 19% Dimension lumber (including Southern Pine

0.85

1.00

0.80

0.67

0.97

0.90

5 in. X 5 in. and larger1

1.00

1.00

0.91

0.67

1.00

1.00

Decking Wet conditions if use all Species except Southern Pinec

0.85

0.67

0.90

Notes: a) When (Fb)(CF) for dimension lumber of all species 1150 psi, CM= 1.0.

b) When (Fc)(CF) for dimension lumber of all species except Southern Pine 750 psi, CM= 1.0; when Fcfor visually graded Southern Pine 750 psi, CM= 1.0. c)

C) For Southern Pine, use Reference design values for wet service conditions

2. Size Factor CF Some more notes regarding the size factor:

•Most Southern Pine has Size Factor included in the baseline design values with the exceptions of those listed on slide 13

•Mechanically Graded Lumber, already included

•Included for visually Graded Timbers (5”X 5”and larger) up to 12”in depth. Beyond 12”in depth, CF= (12/d)1/9 •For decking, except redwood, bending design values are based on 4”thickness. When 2”or 3”thick decking is used, the bending design values, Fb, for all species except redwood shall be multiplied by the following size factors: CF= 1.10 for 2”thick and CF= 1.04 for 3”thick.

3. Temperature Factor, Ct

The temperature factor is not applicable for wood used under 100°F and occasionally heated up to 150°F. Wood increases in strength when cooled and decreases when heated. Prolonged temperatures above 150°F may result in permanent loss of strength. For sustained elevated temperatures up to 150°F, the temperature factor must be applied.

4. Repetitive Member Factor, Cr The reference bending design value may be increased for repetitive members as described below:

•Must have at least 3 members not spaced more than 24”apart

•Members must be used as joists, truss chords, rafters, studs, planks and decking or similar members in contact

•Members must be joined by floor, roof or other load distributing elements capable of supporting the design load

•Not used with decking since it is already considered

•Factor is 1.15

Strength Property	fb	ft	fc	FC(P)	fv	E
Sawn Lumber, Visual or Machine Graded Wet conditions of use MC > 19% Dimension lumber (including Southern Pine	0.85	1.00	0.80	0.67	0.97	0.90
5 in. X 5 in. and larger1	1.00	1.00	0.91	0.67	1.00	1.00
Decking Wet conditions if use all Species except Southern Pinec	0.85			0.67		0.90