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How to Design a Girder or Beam - Part 3

Table 1
Weights of Various Building Materials & Composite Structural Items

Item

Description

lbs./sq.ft.

Walls

Wood stud wall: 1/2″ drywall, both sides 4″

8.2

4" lightweight concrete-block wall

20.0

Floors

Plyscore
Hardwood flooring

Joists with hardwood floor
and plyscore subfloor:
2 X 6
2 X 8
2 X 10
2 X 12

1.5
4.0



10.5
11.5
12.0
12.5

Roofing

Shingles:
Wood
Fiberglass-asphalt strip
Plywood sheathing


2.5
2.5
1.5

Windows

For windows and sliding glass
doors constructed of wood frame and
double glazing (insulated glass)

3.5

Ceilings

1/2″ Gypsum Board (Drywall)

2.1

Roof Plank

2″ thick
3″ thick

5
8
 

lbs./linear ft.

Beams & Girders

4 X 8
3 X 10
4 X 10
4 X 12
6 X 10
4 X 14
6 X 12

7.2
6.6
9.1
11.0
13.8
13.0
16.7

Stairs

Weight for a complete simple
stair about 3 feet wide.

300 lbs.

We consult Table 1 – Dead loads for the weights of the various structural members. The 4 X 10 weights 9 lbs. per linear foot; (9 lbs. X 8 ft. = 72lbs.).

Calculations

L.L. + D.L.

=

4,800

D.L. of the girder (9 lbs. X 8')

=

72

Loads for the load-bearing partition:
4,800 lbs. for the floor above
(assuming a second story)
that it is supporting and
500 lbs. for the weight
of the wall itself.

=

5,300

D.L. of non-load bearing partition

=

500

Total L.L. & D.L.

10,672

Table 2 will give us the proper girder for the given loads. We find that a 4 X 14 will support 10,900 lbs. for an 8’ 0″ span. An alternative would be a 6 X 12, which would allow for more headroom. Or we could consider a steel beam. The table for steel beams shows spans greater than 10 feet. However, the I-beam, designated S 6 X 17.25, can support 10,400 lbs. for a span of 10 feet. It is a safe assumption that it will be good for 10,600 lbs. for the smaller span of 8 feet.

Table 2
Safe Loads In Pounds For Wood Beams & Girders

Size

Span In Feet

6

7

8

9

4 X 8

    

4,250

3,780

4 X 10

    

6,820

6,060

6 X 10

11,357

10,804

9,980

8,887

6 X 10
 built-up 

10,068

9,576

8,844

7,878

6 X 12

    

13,300

12,900

8 X 12

    

18,100

17,500

4 X 14

    

10,900

10,400

4 X 16

    

13,300

12,600

Size

Span In Feet

10

11

12

4 X 8

3,400

3,090

2,830

4 X 10

5,450

4,960

4,540

6 X 10

7,997

7,520

6,890

6 X 10
 built-up 

7,086

    

6 X 12

12,100

11,000

10,100

8 X 12

16,500

15,000

13,800

4 X 14

10,100

9,840

9,180

4 X 16

12,100

11,700

11,500

When designing girders and beams, overdesign! The bit of extra lumber, or steel, you throw into the girder or beam comes to pennies when compared to the total cost of a home.