鋼筋混凝土角柱之火害行為研究

Abstract

建築物之結構體中，柱為主要之構件，其結構性能關係到整個建築物之安全，因此鋼筋混凝土柱火害行為之研究非常重要，以往之研究均僅針對內柱（四面）火害行為，很少針對角柱（二面）火害行為進行研究。 由於角柱通常比內柱具有較大之偏心距離且有僅二面受到火害高溫之特性，與內柱四面受到火害高溫之模式不同，角柱受火害後，形成混凝土材料非對稱，將使結構行為更複雜，本研究提出一套模擬鋼筋混凝土角柱同時受軸壓、雙向彎矩及非對稱火害之試驗方法，並對於其火害行為進行討論。 本研究試驗及分析後發現： （1）混凝土角柱（30公分×45公分）在相同混凝土壓應力0.2 下，4小時火害延時內之斷面溫度分佈與保護層厚度（5公分、7公分）、混凝土強度（140kgf/cm2、280kgf/cm2）、鋼筋比（2%、3%）及偏心大小（0公分、7.5公分）等變數，並無明顯之差異。此行為與混凝土內柱相同。 （2）火害影響混凝土角柱表面產生裂縫或剝落之因素，按嚴重程度依序為火害延時、保護層厚度、鋼筋比及混凝土強度。 （3）混凝土角柱之火害後極限強度殘餘率比較發現：（a）合力偏心距離大於平衡偏心距後，即合力點位於拉力破壞區，極限強度折減變化明顯，隨著合力偏心距離之增加，極限強度殘餘率由小而大，合力偏心距離越大，極限強度殘餘率越高；合力偏心距離小於平衡偏心距後，即合力點位於壓力破壞區，極限強度殘餘率變化並不明顯；（b）火害延時較長，極限強度殘餘率較低，但火害延時對強度影響較嚴重在前2小時，火害延時2小時後之極限強度殘餘率下降速率將減緩；（c）鋼筋比大者，極限強度殘餘率較鋼筋比小者高，但兩者差異不明顯；（d）保護層大者，極限強度殘餘率較保護層小者低，經4小時火害延時試驗，保護層7公分將使極限強度殘餘率僅餘57%，而保護層5公分則強度殘餘率仍有77%；（e）混凝土強度（280kgf/cm2）高者，極限強度殘餘率較混凝土強度（140kgf/cm2）低者低，經4小時火害延時試驗，280kgf/cm2強度之極限強度殘餘率60.8%，140kgf/cm2強度之極限強度殘餘率極限強度殘餘率76.05%，兩者極限強度殘餘率相差約15%。 （4）混凝土內柱（40公分×40公分）與角柱（30公分×45公分）之火害後極限強度殘餘率比較：（a）混凝土280kgf/cm2以下強度，內柱經2小時火害延時，混凝土強度（248kgf/cm2）極限強度殘餘率約77.5%，混凝土強度（198kgf/cm2）極限強度殘餘率約86%，顯示混凝土強度高者，極限強度殘餘率低，故內柱與角柱在此特性顯示相同之趨勢；（b）內柱經火害延時2小時之極限強度殘餘率約53.9%，火害延時4小時之極限強度殘餘率約39.4%，而角柱在火害延時2小時之極限強度殘餘率約60%，火害延時4小時之極限強度殘餘率約50%，顯示火害延時對內柱極限強度殘餘率影響較角柱明顯；（c）保護層7公分內，內柱之試驗顯示火害延時4小時之強度殘餘率均約64%，沒有明顯之差異，但角柱經4小時火害延時試驗，保護層7公分之極限強度殘餘率約57%，而保護層5公分則強度殘餘率約77%，有明顯之差異，故保護層對角柱極限強度殘餘率影響較內柱明顯。 （5）混凝土內柱（40公分×40公分）與角柱（30公分×45公分）之火害試驗後，其色澤、裂縫及爆裂現象，無明顯之差異。 （6）混凝土內柱（40公分×40公分）與角柱（30公分×45公分）於破壞試驗後，外觀均有類似之破壞情形。

Columns are the most important members of structure. The strengths of the columns in general determine the strengths of the structure. Therefore, the study of the strengths of columns under high temperature is very important. Many studies have investigated the strength of interior (four faces fire loading) columns under high temperatures; however, there has been little research on the strength of corner (two faces fire loading) columns under high temperatures. Corner columns are usually under high biaxial eccentricity than interior columns. Furthermore, during a fire, only the interior two faces of corner columns are exposed to fire in contrast to interior columns of which all four faces are exposed. The material asymmetry of concrete after fire further complicated the behavior of the corner columns. This study investigated the behavior of corner columns under axial loading, biaxial bending and asymmetric fire loading. This study shows： (1)The corner column under the same stress ratio, the temperature distribution across the section is similar to interior columns and is not affected by concrete cover thickness, concrete strength or steel ratio. (2)The factors affecting the initiation of the cracks under high temperature on corner column in the order of importance are fire duration, concrete cover thickness, steel ratio, concrete strength and eccentricity. (3)A further examination on the residual strength rates of the corner columns after fire loading shows：(a)in the tension failure zone, the higher the eccentricity, the higher the residual strength; in the compression zone, the residual strength did not change significant, (b) the longer the fire, the less the residual strength, (c) the higher the steel ratio, the higher the residual strength, (d) the thicker the cover, the less the residual strength, and ,(e) the higher the concrete strength, the less the residual strength. (4)After the fire test, the color, crack and burst, observed from the surface are similar for both interior and corner columns. (5)After the strength test, failure surfaces are similar for both the interior and corner columns. (6)A further examination on the residual strength rates of the interior and corner columns after fire loading shows：(a) the higher the concrete strength, the less the residual strength rate, the interior and corner columns yield same trend, (b) the effect of fire duration on the residual strength rate is more prounced for interior columns, and ,(c) the effect of concrete cover is more important for exterior columns.