Cromatina y compartimentalización del núcleo

7.1 General 50

7.2 Reference Design Values 50 7.3 Adjustment of Reference Design

Values 50

7.4 Special Design Considerations 52

Table 7.3.1 Applicability of Adjustment Factors for

Prefabricated Wood I-Joists ... 51

7

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7.1 General

7.1.1 Scope

Chapter 7 applies to engineering design with pre-fabricated wood I-joists. Basic requirements are pro-vided in this Specification. Design procedures and other information provided herein apply only to prefabricated wood I-joists conforming to all pertinent provisions of ASTM D 5055.

7.1.2 Definition

The term “prefabricated wood I-joist” refers to a structural member manufactured using sawn or struc-tural composite lumber flanges and wood strucstruc-tural panel webs bonded together with exterior exposure ad-hesives, forming an “I” cross-sectional shape.

7.1.3 Identification

When the design procedures and other informa-tion provided herein are used, the prefabricated wood I-joists shall be identified with the manufacturer’s name and the quality assurance agency’s name.

7.1.4 Service Conditions

Reference design values reflect dry service condi-tions, where the moisture content in service is less than 16%, as in most covered structures. Prefabricated wood I-joists shall not be used in higher moisture service conditions unless specifically permitted by the prefabri-cated wood I-joist manufacturer.

7.2 Reference Design Values

Reference design values for prefabricated wood I-joists shall be obtained from the prefabricated wood I-joist manufacturer’s literature or code evaluation re-ports.

7.3 Adjustment of Reference Design Values

7.3.1 General

Reference design values (Mr, Vr, Rr, EI, (EI)min, K) shall be multiplied by the adjustment factors specified in Table 7.3.1 to determine adjusted design values (Mr', Vr', Rr', EI', (EI)min', K').

7.3.2 Load Duration Factor, C

_D

(ASD Only)

All reference design values except stiffness, EI, (EI)min, and K, shall be multiplied by load duration fac-tors, CD, as specified in 2.3.2.

7.3.3 Wet Service Factor, C

_M

Reference design values for prefabricated wood I-joists are applicable to dry service conditions as

speci-fied in 7.1.4 where CM = 1.0. When the service condi-tions differ from the specified condicondi-tions, adjustments for high moisture shall be in accordance with informa-tion provided by the prefabricated wood I-joist manu-facturer.

7.3.4 Temperature Factor, C

_t

When structural members will experience sustained exposure to elevated temperatures up to 150F (see Ap-pendix C), reference design values shall be multiplied by the temperature factors, Ct, specified in 2.3.3. For Mr, Vr, Rr, EI, (EI)min, and K use Ct for Fb, Fv, Fv, E, Emin, and Fv, respectively.

PREFABRICATED WOOD I-JOISTS

7

Table 7.3.1 Applicability of Adjustment Factors for Prefabricated Wood I-Joists

ASD

only ASD and LRFD LRFD only

Format Conversion Factor Resistance Factor

Load Duration Factor Wet Service Factor Temperature Factor Beam Stability Factor Repetitive Member Factor

K

F



Time Effect Factor

M

r'

= M

r

x C

D

C

M

C

t

C

L

C

r

K

F

0.85 

V

r'

= V

r

x C

D

C

M

C

t

- - K

F

0.75 

R

r'

= R

r

x C

D

C

M

C

t

- - K

F

0.75 

EI

^'

= EI x - C

M

C

t

- - - - - (EI)

min'

= (EI)

min

x - C

M

C

t

- - K

F

0.85 - K

^'

= K x - C

M

C

t

- - - - -

7.3.5 Beam Stability Factor, C

_L 7.3.5.1 Lateral stability of prefabricated wood I-joists shall be considered.

7.3.5.2 When the compression flange of a prefabri-cated wood I-joist is supported throughout its length to prevent lateral displacement, and the ends at points of bearing have lateral support to prevent rotation, CL=1.0.

7.3.5.3 When the compression flange of a prefabri-cated wood I-joist is not supported throughout its length to prevent lateral displacement, one acceptable method is to design the prefabricated wood I-joist compression flange as a column in accordance with the procedure of 3.7.1 using the section properties of the compression flange only. The compression flange shall be evaluated as a column continuously restrained from buckling in the plane of the web. CP of the compression flange shall be used as CL of the prefabricated wood I-joist.

Prefab-ricated wood I-joists shall be provided with lateral sup-port at points of bearing to prevent rotation.

7.3.6 Repetitive Member Factor, C

_r

For prefabricated wood I-joists with structural composite lumber flanges or sawn lumber flanges, ref-erence moment design resistances shall be multiplied by the repetitive member factor, Cr = 1.0.

7.3.7 Pressure-Preservative Treatment

Adjustments to reference design values to account for the effects of pressure-preservative treatment shall be in accordance with information provided by the pre-fabricated wood I-joist manufacturer.

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7.3.8 Format Conversion Factor, K

_F

(LRFD Only)

For LRFD, reference design values shall be multi-plied by the format conversion factor, KF, provided by the prefabricated wood I-joist manufacturer.

7.3.9 Resistance Factor,  (LRFD Only)

For LRFD, reference design values shall be multi-plied by the resistance factor, , specified in Table 7.3.1.

7.3.10 Time Effect Factor,  (LRFD Only)

For LRFD, reference design values shall be multi-plied by the time effect factor, , specified in Appendix N.3.3.

7.4 Special Design Considerations

7.4.1 Bearing

Reference bearing design values, as a function of bearing length, for prefabricated wood I-joists with and without web stiffeners shall be obtained from the pre-fabricated wood I-joist manufacturer’s literature or code evaluation reports.

7.4.2 Load Application

Prefabricated wood I-joists act primarily to resist loads applied to the top flange. Web stiffener require-ments, if any, at concentrated loads applied to the top flange and design values to resist concentrated loads applied to the web or bottom flange shall be obtained from the prefabricated wood I-joist manufacturer’s lit-erature or code evaluation reports.

7.4.3 Web Holes

The effects of web holes on strength shall be ac-counted for in the design. Determination of critical shear at a web hole shall consider load combinations of 1.4.4 and partial span loadings defined as live or snow loads applied from each adjacent bearing to the oppo-site edge of a rectangular hole (centerline of a circular hole). The effects of web holes on deflection are negli-gible when the number of holes is limited to 3 or less per span. Reference design values for prefabricated wood I-joists with round or rectangular holes shall be

obtained from the prefabricated wood I-joist manufac-turer’s literature or code evaluation reports.

7.4.4 Notches

Notched flanges at or between bearings signifi-cantly reduces prefabricated wood I-joist capacity and is beyond the scope of this document. See the manufac-turer for more information.

7.4.5 Deflection

Both bending and shear deformations shall be con-sidered in deflection calculations, in accordance with the prefabricated wood I-joist manufacturer’s literature or code evaluation reports.

7.4.6 Vertical Load Transfer

Prefabricated wood I-joists supporting bearing walls located directly above the prefabricated wood I-joist support require rim I-joists, blocking panels, or other means to directly transfer vertical loads from the bearing wall to the supporting structure below.

7.4.7 Shear

Provisions of 3.4.3.1 for calculating shear force, V, shall not be used for design of prefabricated wood I-joist bending members.

STRUCTURAL

In document Lic. Camila Vázquez Echegaray (página 31-38)