Example Problem #1

Example Problem #1

To test your understanding of the various specific gravity definitions, consider the following information about a quantity of softwood lumber that you are going to ship across the country:

Green specific gravity (GSG)	=	0.42
Oven-dry specific gravity (DSG)	=	0.48
Present moisture content (dry basis)	=	40%
Volume of bulk piled lumber at 40% moisture content (G Weight)	=	1,500 ft³
Volume of bulk piled lumber at 0% moisture content (OD Weight)	=	1,300 ft³

Using this information, calculate the shipping weight at 40% moisture content.

Step 1: Calculate the quantity (weight) of oven-dry wood in the shipment:

The wood is at 40% moisture content and occupies 1,500 ft³ of space when bulk piled (no spaces between the boards).

Since wood is at 40% moisture content (above the fiber saturation point) we use the green specific gravity formula and solve for oven-dry weight:

GSG =	OD Weight

	Wt. of an equal volume of water

0.42 =	OD Weight

	1,500 ft³ × 62.4 lb/ft³

OD Weight = 0.42 × 1,500 × 62.4

OD Weight = 39,312 lb

Step 2: Calculate the weight of wood when at 40% moisture content.

Calculation of moisture content is discussed in the module on lumber drying and also in the glossary. To review, dry-basis moisture content is calculated by the following formula:

		[Green weight - Oven-dry weight]
Dry-basis moisture content (%)	=		× 100
		Oven-dry weight

[Green weight - Oven-dry weight]

Dry-basis moisture content (decimal) =

Oven-dry weight

The moisture content formula in the decimal form can be solved for green weight as follows:

Step 1	Moisture content × Oven-dry weight = Green weight - Oven-dry weight
Step 2	(Moisture content × Oven-dry weight) + Oven-dry weight = Green weight
Step 3	Oven-dry weight × (Moisture content + 1) = Green weight

In order to calculate the weight of wood we are going to ship, we simply solve the above formula for green weight, substituting the correct moisture content and the previously calculated oven-dry weight.

39,312 lb × (0.40 + 1) = Green weight

39,312 lb × (1.4) = Green weight

55,037 lb = Green weight_{at 40%}

Forest Products Management Development Institute

		[Green weight - Oven-dry weight]
Dry-basis moisture content (decimal)	=
		Oven-dry weight