Behavior of composite beams with external prestressing in sagging moment regions

Ribeiro, Karla Demoner; Turini, Thiago Tononi; Luchi, Lorenzo Augusto Ruschi e; Calenzani, Adenilcia Fernanda Groberio

doi:10.1590/S1983-41952021000400005

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ORIGINAL ARTICLE • Rev. IBRACON Estrut. Mater. 14 (4) • 2021 • https://doi.org/10.1590/S1983-41952021000400005 copy

Behavior of composite beams with external prestressing in sagging moment regions

Comportamento de vigas mistas com protensão externa nas regiões de momento positivo

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

Prestressed composite steel-concrete structures are scarcely used due to a lack of clear standardized design guidelines and formulations on the subject. The present research aims to present design methodologies for steel-concrete composite beams with external pretension applied via straight tendons. A computer program to perform structural analysis of such beams was developed based on two different methodologies: the first one is presented in ABNT NBR 8800:2008, in which the guidelines for the design of composite beams with compact webs are adjusted to include the effects of the pretension force. The second methodology is extracted from international literature and presents a structural design process based on stress distribution on the beam. Ninety prestressed and thirty non-prestressed beams were analyzed and designed with the aforementioned computer program to evaluate the influence of beam length, degree of symmetry of the steel profile and eccentricity of the pretension force on the mechanical resistance of the beams. It was observed that, although the prestressing force considerably improved resistance to bending, it introduced high compression stresses on the steel profile; hence, the pre-stressing of composite beams is proved efficient only for steel profiles with symmetrical cross-sections.

Keywords:
prestressed composite beams. design methodology. external prestressing. computer program.

Loads	Before Concrete Casting	After Concrete Casting
Weight of the steel beam ( $g_{1}$ )	$γ_{g 1}^{'}$ = 1.15	$γ_{g 1}$ = 1.25
Prestressing force on steel tendons ( $P$ )	$γ_{g 2}^{'}$ = 1.20	$γ_{g 2}$ = 1.20
Weight of the concrete slab ( $g_{3}$ )	$γ_{g 3}^{'}$ = 1.25	$γ_{g 3}$ = 1.35
Serviceability dead load ( $g_{5}$ )	-	$γ_{g 5}$ = 1.35
Serviceability live load ( $q_{6}$ )	-	$γ_{g 6}$ = 1.50

Verification phase	Loads	U.L.S Verification
1^st Phase: Steel Beam	q₁ – Weight of the steel beam	- Compression $(N_{S d 1} \leq N_{R d 1})$ ;
	P – Prestressing force on tendons	- Bending Moment $(M_{S d 1} \leq M_{R d 1})$ ;
	q₃ – Weight of the concrete slab	- Shear force $(V_{S d 1} \leq V_{R d 1})$ ;
	ΔP₄ - Increment of prestressing force due to concrete the weight of the concrete slab	- Flexure-compression Interaction
2^nd Phase: Prestressed steel-concrete composite beam t = 0	Including 1^st phase loads
	q₅ – Serviceability dead load	- Compression $(N_{S d 2} \leq N_{R d 2})$ ;
	q₆ – Serviceability live load	- Bending Moment $(M_{S d 2} \leq M_{R d 2})$ ;
	ΔP₇ - Increment of prestressing force due to serviceability dead load	- Shear force $(V_{S d 2} \leq V_{R d 2})$ ;
$n = \frac{E_{a}}{E_{c}}$	ΔP₈ - Increment of prestressing force due to serviceability live load	- Flexure-compression Interaction
3^rd Phase: Prestressed steel-concrete composite beam t = ∞	Including 1^st and 2^nd phase loads	- Compression $(N_{S d 3} \leq N_{R d 3})$ ;
3^rd Phase: Prestressed steel-concrete composite beam t = ∞	q₉ – Concrete slab shrinkage	- Bending Moment $(M_{S d 3} \leq M_{R d 3})$ ;
$n = \frac{3 E_{a}}{E_{c}}$		- Flexure-compression Interaction

Verification phase	1^st Fase: Steel Beam	2^nd Fase: Prestressed steel-concrete composite beam
Verification phase	1^st Fase: Steel Beam	t = 0 and $n = \frac{E_{a}}{E_{c}}$	t = ∞ and $n = \frac{3 E_{a}}{E_{c}}$
Loads	q₁ – Weight of the steel beam	q₆ – Serviceability live load	q₅ – Serviceability dead load
	P – Prestressing force on tendons	ΔP₈ - Increment of prestressing force due to serviceability live load	ΔP₇ - Increment of prestressing force due to serviceability dead load
	q₃ – Weight of the concrete slab
	ΔP₄ - - Increment of prestressing force due to the weight of the concrete slab
Verifications S.L.S	Immediate deflection: $(δ_{1})$	Short term deflection: $(δ_{2})$	Long term deflection: $(δ_{3})$ .
Verifications S.L.S	Total deflection: $δ_{t o t a l} = δ_{1} + δ_{2} + δ_{3} \leq \frac{L}{350}$

IBRACON - Instituto Brasileiro do Concreto Instituto Brasileiro do Concreto (IBRACON), Av. Queiroz Filho, nº 1700 sala 407/408 Torre D, Villa Lobos Office Park, CEP 05319-000, São Paulo, SP - Brasil, Tel. (55 11) 3735-0202, Fax: (55 11) 3733-2190 - São Paulo - SP - Brazil
E-mail: arlene@ibracon.org.br

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[1] Corresponding author: Adenilcia Fernanda Groberio Calenzani. E-mail: afcalenzani@gmail.com

(1)	$g_{1}$ – Weight of the steel beam
(2)	$P$ – Prestressing force on steel tendons
(3)	$g_{3}$ – Weight of the concrete slab
(4)	$Δ P_{4}$ - Increment of prestressing force due to the weight of the concrete slab
(5)	$g_{5}$ – Serviceability dead load
(6)	$q_{6}$ – Serviceability live load
(7)	$Δ P_{7}$ - - Increment of prestressing force due to serviceability dead load
(8)	$Δ P_{8}$ - - Increment of prestressing force due to serviceability live load
(9)	$P_{9}$ – Concrete slab shrinkage