LVL Handbook EUROPE

2. LVL STRUCTURES IN FLOORS, WALLS, ROOFS AND IN SPECIAL APPLICATIONS

LVL joist floors can be built on a wooden, steel or concrete

frame or walls. The 1:8 width to depth ratio of rigid LVL-P

joists is ideally suited to the structural depths of floor con-

structions even with long spans. The recommended minimum

width of supports and LVL floor joists is 45 mm in order to

provide proper support for decking panels.

Floor elements can be prefabricated off-site. This saves

time on the construction site, reduces waste, and improves

quality as the elements can be made in dry factory conditions.

An element floor usually has a slightly higher material con-

sumption as the edge joists that seal the elements at the sides

form double joists in the completed floor structure.

Floor deflections and vibrations are not only dependent on

the floor joists themselves, but on the decking on the joists and

on the blockings installed for transverse bracing perpendicular

to the span of the floor. Due to the rigidity of typical decking

panels, a joist spacing of c/c 400 mm is recommended. The

spacing should not exceed 600 mm to avoid excessive deflec-

tion of the panels in the perpendicular direction to the joists.

It is also recommended to glue the decking panels to the joists

with polyurethane adhesive for best results. Depending on the

gluing conditions (on-site or off-site) and country-specific re-

quirements, at least half of the advantage of the composite ef-

fect of gluing can be utilized in structural calculations.

Blockings between the joists reduce deflection under point

loads, but they need to be fixed well to the joists to give the

desired transverse stiffness and to avoid creaking in the long

term. The best improvement can be achieved by fixing the

Figure 2.4.

LVL-P joist floor members and details.

1. Floor joist

2. Rim board

3. Blockings at the centre of a span

4. Blockings at support

5. Trimmer beam

6. Staircase opening, trimmer connection

7. Joist hanger connection

blockings to a transverse tension flange board installed under

the floor joists, see Figure 2.6.

Multi-span structures reduce deflection and enable longer

maximum span lengths compared to single-span structures.

However, if the floor joists are continuous between separate

rooms more conservative span lengths are recommended as

people are more sensitive to vibrations originating from spaces

external to the space that they are occupying.

Floor vibration is controlled in the design based on: 1)

deflection under point 1kN load when the lowest natural fre-

quency

is higher than 8-9 Hz depending on country-specific

design rules and 2) acceleration when f1 is 4,5-8 Hz. It is rec-

ommended to design joist floors to

> 8 Hz, because achiev-

ing the requirements of the acceleration criterion requires the

addition of considerable extra weight to the structure. For more

information on floor vibration design, see subsection 4.3.7.

Residential floors within apartments can be light, with-

out acoustic or fire resistance requirements, in which case de-

flection under 1kN point load is the governing requirement.

Intermediate floors between apartments require protective

cladding underneath the floor and extra mass on the top of

the floor, which usually make the fundamental natural fre-

quency

> 8-9 Hz the most critical requirement. Figures 2.7

and 2.8 gives the maximum span lengths for different joist

sizes. Span lengths up to ~6 m can be achieved with normal

floor thicknesses, but with the biggest joists over 8 m spans

are possible.

LVL Handbook Europe