Reshoring and Early-Age Building Behavior-Surveyed Performance of RC and PT Flat Slabs in Multi-Story Buildings Implications for Design (PDF)

Surveyed Performance of RC and PT Flat Slabs in Multi-Story Buildings Implications for Design (PDF)

Pdf Summary

The document addresses the surveyed performance of reinforced concrete (RC) and post-tensioned (PT) flat slabs in multi-story buildings, focusing on the implications for design according to various ACI guidelines, particularly ACI 318 and ACI 435. It highlights the discrepancies and shortcomings in current design codes that do not sufficiently address deflection design and the effects of creep, shrinkage, and shoring. The study suggests that for RC systems, a creep multiplier of 3.0 is standard but inadequate, as it neglects shrinkage cracking and shoring effects. Similar issues arise with PT systems due to vague ACI 318 guidelines, leading engineers to apply similar multipliers as those used for RC systems.<br /><br />The comprehensive document proposes improvements in addressing shrinkage and shoring in design, as seen with ACI 435, which provides more complete criteria. For effectiveness, it recommends specific methods for better handling creep and shrinkage. However, the document notes a lack of clear design load combinations for shoring/reshoring effects.<br /><br />Practical design recommendations include verifying material properties through early compression tests and ensuring rebar is included in modeling for detailed crack property calculations. Moreover, the placement of vertical elements is crucial, differing between PT and RC systems for deflection calculations. It's noted that proper shoring/reshoring impacts PT slabs less when following ACI 347 recommendations, unlike RC slabs.<br /><br />Surveyed performance highlighted several stages of PT flat slab projects measuring slab elevations at different construction phases. The findings underline the importance of specific elevation surveys for accurate performance assessments. The document also evaluates methodology, considering camber programs and prediction models like SAFE, which integrate aged modulus of elasticity and ACI 435 methods.<br /><br />In commentaries on ACI 347.2R-17, the document emphasizes needed clarity regarding shoring implications on frame performance. It advises on suggesting design load combinations for better structural design and recognizes challenges in assuming uniform slab stiffness across reshored slabs. The commentary also addresses issues with live load requirements, suggesting they're documented to avoid design missteps. Overall, the document calls for refined criteria to ensure better-performing flat slabs in multi-story structures.

Keywords

reinforced concrete

post-tensioned slabs

ACI 318

ACI 435

creep and shrinkage

shoring effects

deflection design

structural design

slab performance

construction phases